Chinook Salmon (Oncorhynchus tshawytscha): COSEWIC assessment and status report 2018 : page 1

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Wildlife species description and significance

Name and classification

Class: Actinopterygii
Order: Salmoniformes
Family: Salmonidae
Latin binomial: Oncorhynchus tshawytscha (Walbaum)
Designatable Unit: See DU section

Common species names:
English - Chinook Salmon, Spring Salmon, King Salmon (Scott and Crossman 1973)
French - saumon Chinook
First Nations – tyee, sac'up, k^wex^we, k'utala, keke'su7, po:kw' (Ducommun 2013); ntitiyix, sk'elwis (Vedan 2002), t'kwinnat or quinnat (Scott and Crossman 1973)

Morphological description

Chinook Salmon are the largest of the Pacific salmon (Netboy 1958), reaching over 55 kg in weight (Bailey pers. comm. 2018). In addition to their distinctive size, Chinook Salmon adults (Figure 1) differ from other Oncorhynchus species by: (1) the presence of small black spots on both lobes of the caudal fin (also occurs in Coho Salmon O. kisutch); (2) black gums at the base of the teeth in the lower jaw; (3) a pointed lower jaw; (4) a large number of pyloric caeca (>100) (McPhail and Lindsey 1970; McPhail and Carveth 1994; Bailey pers. comm. 2018); and (5) large otoliths. Chinook Salmon also differ from all but Coho Salmon in terms of their variable flesh colour, which ranges from white through different shades of pink, to red (Healey 1991; Lehnert et al. 2016).

Chinook Salmon fry and parr are distinguished by the presence of parr marks extending well below the lateral line, the deepest of which are deeper than the vertical eye diameter (McPhail and Carveth 1994). The adipose fin is normally unpigmented in the centre, but edged with black (Dahlberg and Phinney 1967). The anal fin is usually only slightly falcate, and the leading rays do not reach past the posterior insertion of the fin when folded against the body. The anal fin has a white leading edge, but the adjacent dark line present in Coho Salmon is absent. However, juvenile characteristics are highly variable, so proper identification often requires otolith measurements (M. Trudel, pers. comm.) or meristic and pyloric caeca counts (Healey 1991).

Population spatial structure and variability

Information about population spatial structure and variability is addressed in the following section about Designatable Units.

Designatable units delineation

The Designatable Units (DUs) reviewed in this report have been approved by COSEWIC. Using methods based on both COSEWIC guidelines ("Appendix F5: Guidelines for Recognizing Designatable Units") and work by Fisheries and Oceans Canada (DFO) ("Conservation Units for Pacific Salmon under the Wild Salmon Policy" (Holtby and Ciruna 2007) and "Canada's Policy for Conservation of Wild Pacific Salmon" (DFO 2005a)), the southern British Columbia (BC) populations of Chinook Salmon were subdivided into 28 DUs based on geographic distribution, life-history variation, and genetic data. DU designation is detailed in "COSEWIC Report on Proposed Designatable Units for Chinook Salmon Oncorhynchus tshawytcsha in Southern British Columbia" (COSEWIC 2015).

Both the Wild Salmon Policy (WSP) Conservation Units (CUs) and COSEWIC DUs share the same fundamental approach: they are both oriented toward maintaining genetic variability at the wildlife species level. Therefore, isolation and distinctiveness of populations are important in both cases. Based on the similarities of underlying rationale and process for developing CUs and DUs, the existing CU definitions are used as a starting point for DUs. The similarities and differences between these delineation types are outlined below.

The goal of the WSP is to preserve the ability of the fish to adapt to local stresses by maintaining genetically diverse spawning (reproducing) populations. Thus, the protection of 'pattern' (groups of populations) as well as 'process' (habitat integrity and connectedness for those populations) is required (Moritz 2002). The definition and documentation of CUs is used for protection of pattern. A Conservation Unit is defined as a group of wild salmon sufficiently isolated from other groups (that is, discrete groups) that, if extirpated (destroyed), is unlikely to recolonize naturally in an acceptable timeframe (Holtby and Ciruna 2007). CUs are defined by studying ecology, life-history, and biochemical genetics to identify evolutionarily significant units (populations that are reproductively isolated and/or exhibit adaptive variation from larger populations). The aim of the CU is to (when possible) maintain genetic diversity by preserving one or more subpopulations within each local population so the population does not become closed. A key difference between CUs and DUs is the inclusion of “wild” in the definition of CUs, as COSEWIC DUs could include enhanced populations provided they have a neutral or positive effect on fitness of the wild populations. It should be noted that there is considerable evidence that hatchery-origin fish have a detrimental effect on the fitness of wild Chinook Salmon populations (DFO 2017, in press).

Designatable Units are defined as "discrete and evolutionarily significant units of the taxonomic species", where "significant" means that the unit is important to the evolutionary legacy of the species as a whole and if lost would likely not be replaced through natural dispersion”. This statement aligns with the definition of a CU (a group that is sufficiently isolated from other groups that, if extirpated (destroyed), is very unlikely to recolonize naturally), with the exception that the DU does not dictate a timeframe for recolonization or replacement.

The codes D1, D2 and D3 are used to describe the three criteria for discreteness.

Discreteness may be based on one or more of the following criteria:

1. genetic distinctiveness including inherited traits (including life history or behaviour) and/or neutral genetic markers (including DNA microsatellites)
2. natural differences in geographic range (such that local adaptation is likely)
3. occupation of differing eco-geographic regions that reflect historical or genetic distinction

The codes E1, E2, E3 and E4 are used to describe the four criteria for evolutionary significance.

Evolutionary significance of a discrete population is determined by one or more of the following criteria, each of which can be considered a measure of evolutionary significance:

1. evidence that the discrete population is markedly genetically different
2. persistence of the discrete population in a unique ecological setting that is likely or known to have given rise to local adaptation
3. evidence that the discrete population is the only surviving natural occurrence of a species that is only found elsewhere as an introduced species
4. evidence that loss of the discrete population would result in an extensive gap in the range of the species in Canada

In general, the criteria used to define CUs and DUs share enough similarities that it was possible to use the work from Holtby and Ciruna (2007) to define DUs. Discrepancies between CU and DU designation methods mean there is not a direct translation from CU to DU for all cases, and some DUs will contain multiple CUs. Because of these differences, the methods (see below) for defining DUs is slightly different than previously documented for CUs.

Methods

Designatable Unit (DU) designation methods are adapted from Conservation Unit (CU) designation methods (Holtby and Ciruna 2007) and involve separating different populations of Chinook Salmon using a hierarchy of characteristics to determine distinctiveness and evolutionary significance. Specifically, geographic differences are applied to demonstrate distinctiveness while life-history characteristics are applied to demonstrate evolutionary significance.

For comparison, the CU designation methodology is visualized in Figure 2 and the DU methodology is illustrated in Figure 3.

The step-wise method shown in Figure 3 can be summarized in three steps that distinguish populations by:

• ecotypology (geographic region supported by genetics data)
• life history variant
• migratory/spawning timing differences

Distinctiveness (ecotype/Joint Adaptive Zones)

The first DU division occurs at the geographic level based on ecotypic regions called Joint Adaptive Zones (JAZ) that meet the criteria for discreteness of populations (D2, geographic range) and (D3, eco-geographic regions). Joint Adaptive Zones represent distinct geographic ranges that are described by the intersection between a Freshwater Adaptive Zone (FAZ) (Figure 4) and a Marine Adaptive Zone (MAZ) (Figure 5) where local adaptation is likely (Table 1, Figure 6). Short-hand codes used to describe the MAZ and FAZ are shown in Table 2 and Table 3 respectively. There are 39 JAZ defined in BC, and 34 of these contain Chinook Salmon; 19 of the Chinook Salmon-bearing JAZ are in southern BC (DFO 2013a).

Salmon population presence within a JAZ is based on criteria defined in Holtby and Ciruna (2007). To determine which sites in each JAZ are relevant to defining Chinook Salmon DUs, a list of Annual Escapement Water Bodies was created using data from DFO's New Salmon Escapement Database System (NuSEDS). Sites that meet one or more of the following criteria are considered to contain Chinook Salmon:

• the site has five or more entries in NuSEDS from 1950-2006
• genetic Chinook Salmon samples have been obtained from the site (Chinook Salmon genetic samples were available for 312 sites)
• records of spawn timing for Chinook Salmon are available in NuSEDS
• local knowledge and/or public consultation in the area confirmed the presence of Chinook Salmon

The sites in NuSEDS were geo-referenced against the JAZ map. Joint Adaptive Zones that did not appear to include any Chinook Salmon habitat were not considered further in this process, as per Holtby and Ciruna (2007) (Figure 7).

Genetic data support the use of JAZ to characterize southern BC Chinook Salmon DUs. First, Chinook Salmon demonstrate extreme fidelity to natal spawning grounds (Bentzen et al. 2001). Second, Chinook Salmon from a given JAZ tend to have a high degree of relatedness and reproductive isolation from other groups.

Spawning ground fidelity is demonstrated through data from redd sampling and a captive rearing program. Chinook Salmon from redds in the same reach of the river were more related to each other than they are to salmon from redds further away in the river, implying within-river substructure and fine-scale homing (Bentzen et al. 2001). Because of this extreme fidelity, the geographic range can be divided on a finer scale than would be typical for species that are less philopatric to natal areas than Chinook Salmon (Sockeye Salmon (O. nerka) can exhibit similar homing capabilities).

Genetic differences from microsatellite array data support the use of JAZ to classify DUs. Differences in neutral genetic structure for all genetic samples were assessed using Cavalli-Sforza and Edwards (1967) chord distance (CSE) calculated using 10 or more microsattelite loci for Chinook Salmon and fixation index (Fst) trees drawn using an unrooted neighbour-joining clustering algorithm. The resulting dendrograms (Figure 8) were compared with the JAZ to determine the likelihood that more than one distinct subpopulation is present.

Genetic samples were available for 312 sites in Canada. In most cases, demonstrated by very high Fst values, the streams within a given JAZ grouped together. Based on Fst pairwise exact tests of population differentiation (with 8,128 paired comparisons) performed using GENEPOP (Raymond and Rousset 1995), 99.8 percent of pairwise distances were statistically significant with α=0.05 (note that GENEPOP does not apply a Bonferroni correction (Goudet et al. 1996)). This genetic difference is further supported by a bootstrap analysis using the program PHYLIP (Felsenstein 2004) to build consensus trees across loci to assess how reliably a tree topology can be produced from the data at hand (Figure 9). These bootstrapped datasets were analyzed the same way as the single dataset, but data were re-sampled 1,000 times to generate random datasets from the original dataset.

Evolutionary significance (life history variants/spawn/run timing)

Evolutionary significance of a population is based on demonstrated local adaptation (E2) and on the likelihood that the loss of a population would lead to a gap in the geographic range (E4). Two attributes of Chinook Salmon meet these criteria: (1) life-history variants; and (2) spawn/run-timing differences.

Life history variants

Chinook Salmon are unique among species of Pacific salmon in the wide variety of strategies they exhibit at all life stages, including: variation in the age at which juveniles disperse from their natal streams; length of freshwater, estuarine and ocean residence; ocean distribution; and age/timing of the spawning migration (Brown et al. 2013a). Differences in timing of when Chinook Salmon go to open water lead to distinct life-history variants that do not readily interbreed, even though the Chinook Salmon population in a given geographic area is often a mixture of types (Healey 1991, 2001). Originally framed dichotomously as stream and ocean behavioural types, stream-type Chinook Salmon were understood to spend a year in freshwater before migrating to the ocean, and return to freshwater early to spawn farther inland. Ocean-type Chinook Salmon were understood to migrate to the ocean a few weeks to months after emerging from gravel, and return late to spawn a short distance inland.

In recent decades, it is considered more accurate to frame Chinook Salmon life-history strategies along a continuum ranging from ocean-type to stream-type. Variations within the ocean-type and stream-type behavioral forms have been identified based on the geographic origins of the fish and the resulting conditions to which they have adapted (Brannon et al. 2004). Many local adaptations have resulted in atypical return timing and freshwater rearing strategies (Healey 1991; Waples et al. 2004). Ocean-type Chinook Salmon can rear in freshwater for up to six months post-emergence, while in some systems, stream-type Chinook Salmon may only remain in freshwater for a few weeks. This variation is critical for Chinook Salmon persistence as it spreads risk across many different strategies in the face of variable climatic conditions (Bradford and Taylor 1997).

Spawning Chinook Salmon populations at opposite ends of the strategy continuum are generally geographically separated to a considerable degree (there are exceptions) (Bailey pers. comm. 2018). Ocean-type life-history variants dominate all runs south of the Alaskan border except, the Yakoun River on Haida Gwaii (the Queen Charlotte Islands), the Fraser River, and the Columbia River. Stream-type life-history variants make an important contribution to runs south of the BC-Alaska border/Dixon Entrance. Wherever the two ends of the continuum are sympatric, stream-type variants are found more frequently in headwater spawning areas and ocean-type variants occur more frequently in downstream spawning areas (Rich 1925; Hallock et al. 1957; Healey and Jordan 1982).

Evidence suggests that ocean-type and stream-type variants, have different racial origins (Healey 1991; Waples et al. 2004). Waples et al. (2004) note that within the Columbia River, the two lineages behave essentially as two different species with little evidence of gene flow, despite co-migrating through large areas of riverine and ocean habitat, and in some cases, spawning in adjacent systems (alternative interpretations suggest that ocean-type and stream-type Chinook Salmon south of the Upper Columbia River Basin share the same lineage – see Moran et al. 2013). Similar situations also exist within the Thompson basin of the Fraser River (Fraser et al. 1982; Candy et al. 2002; Bailey unpublished data). Various authors (for example,, McPhail and Lindsey 1986, Healey 1991, and Waples et al. 2004) have postulated that the two lineages may have arisen from different glacial refugia ('Beringia' in the north and 'Cascadia-Columbia' in the south), and then become locally adapted since the last ice age. Within the Fraser River CUs there is much diversity in terms of freshwater rearing, ocean distributions, and return run-timing. Both stream- and ocean-type variants are represented, and many of the interior Fraser CUs are thought to be descended from Beringia-origin stream-type variants. Other populations on the South Coast, excluding some of the Fraser River stock groups, are thought to be of Cascadia-Columbia origin, and appear to be resident in waters over the continental shelf (Brown et al. 2013a).

Each JAZ was examined for information regarding the presence of different life-history variants. If both types were present, two separate DUs were defined within that JAZ. Since some genetic 'straying' is known to occur (Waples et al 2004; Walter et al 2009), when genetic information was available, it was used to confirm that the populations in the DUs were not interbreeding.

Spawn/run timing

Spawn timing is the time of year when sexually mature individuals complete migration, reach spawning grounds, and reproduce. Chinook Salmon sexual maturation can occur during the first to the seventh year (Bailey pers. comm. 2018). The most common age at maturity varies among populations and females generally have an older average age at maturity than males (Quinn 2005; DFO unpublished data). Spawn timing can precede actual spawning activity by weeks, or even months for some individual spawning populations. Genetics and environmental factors appear to be the primary determinants of these characteristics for individual populations (Quinn 2005).

Beacham and Murray (1990) consider spawn timing for Chinook Salmon and other salmon species to be evidence of local adaptation and timing differences are thought to limit the capacity for interbreeding (Waples et al. 2004). Within BC, peak 'in-migration' timing for northern Chinook Salmon populations generally occurs from July to September, and for southern populations from April to September (Bailey pers. comm. 2018). Within a given river system, multiple populations may co-exist, each with different spawning times and occupying specific reaches of the river (Parken et al. 2008). The diversity of timing strategies demonstrates the specificity of thermal requirements for hatching and emergence of fry, as well as the need to synchronize these requirements with other environmental factors such as food availability and hydraulic conditions.

Typically, spawn timing is expressed as run-timing in the literature. For example, to estimate run-timing from spawn timing, Holtby and Ciruna (2007) use a linear regression model.

mean DOY spawning = 161.4 + 0.482 median DOY migration

r²adj = 0.595
SEest = 20.75
F_1,49 = 74.4; P < 0.0001

Run timing is the time at which adult Chinook Salmon begin their return migration to natal streams. Waples et al. (2004) provide standardized run-timing definitions (Table 4) that are used to classify southern BC Chinook Salmon populations (Parken et al. 2008).

The New Salmon Escapement Database System (NuSEDS) records run-timing as specific dates. These dates are available for multiple sites in some JAZs. If there are sufficient observations for a given population in more than one run-timing categorization, the average Day-of-Year (DOY) of the different spawn timing groups is calculated and compared for statistical significance using an ANOVA. If the difference is found to be statistically significant, the groups are separated into different DUs. DUs that contain only one run-timing group are simply classified according to JAZ name.

Summary of overall approach

In this analysis, populations were first distinguished by ecotype (that is, the JAZ), then if required, they were distinguished by run-timing. For each life-history variant in a given JAZ, available information was reviewed for spawn/run-timing differences. If more than one run-timing group was present for a given life-history type in a given JAZ, each run-timing group was defined as its own DU. When available, genetic data were compared to confirm that the Chinook Salmon from DUs with different run-timing were not interbreeding. While gene flow is not large among Chinook Salmon DUs, it does occur (Waples et al. 2004; Walter et al. 2009).

Results

The DU designations established using the methods described above are summarized below and in Table 5. Corresponding CU designations are also included in Table 5 to illustrate the key similarities and differences in results using each approach.

Distinctiveness - geography (ecotype)

Based on ecotype, of the 19 JAZ identified in southern BC, seven contained a single discernible population (Boundary Bay–Georgia Straight, Fraser Canyon–Georgia Straight, Upper Fraser River, Lower Thompson–Georgia Straight, South Coast–Georgia Straight, South Coast–Southern Fjords, East Vancouver Island–Southern Fjords). The remaining 12 JAZ contained multiple populations based on evolutionary significance characteristics (life-history types, run-timing, and/or spawn timing groups), and required further analysis.

Evolutionary significance - life history

Four of the 12 JAZ with multiple population groups contained both ocean and stream life-history variants. Two of these four, South Thompson–Georgia Straight (STh-GStr) and Homathko–Southern Fjords (HK-Sfj), were each found to contain two distinct DUs, one stream-type population and one ocean-type population. The remaining two JAZ required further analysis. In the Lower Fraser River–Georgia Straight (LFR–GStr) JAZ, the populations corresponding to each type of life-history variant were both found to have further divergences. In the East Vancouver Island–Georgia Straight (EVI–GStr) JAZ, the stream-type population was found to be unique, while the ocean-type population had further divergences.

Evolutionary significance - spawn/run timing

Four of the 12 JAZ with multiple population groups contained multiple run-timing groups. In Lower Fraser River-Georgia Strait (LFR–GStr), ocean-type populations occurred with run-timings in both fall and summer, and stream-type populations occurred with run-timings in both spring and summer. The Middle Fraser River–Georgia Straight (MFR–GStr) JAZ only contained stream-type variants but these had run-timings in the spring, summer, and fall. Similarly, North Thompson River–Georgia Straight (NTh-GStr) contained only stream-type variants, but with separate runs in the spring and summer. EVI–GStr ocean-type variants were observed to have runs in both the summer and the fall.

These populations were separated into different DUs based on run-timing. When available, Day-of-Year (DOY) spawn timing data were also analyzed to determine if the runs required further separation. However, only 12 populations contained more than one spawn timing categorization, and only four had a sufficient number of observations in more than one category (Holtby and Ciruna 2007). Evaluation led to the further division of two groups into five separate DUs: the LFR–GStr stream-type summer run, which was found to have two different spawn timing groups, and the EVI ocean-type fall run, which was found to have three different spawn timing groups.

Comparison to CU designation

Although the WSP CU designations differ from the DU designations defined in this report, they share some similarities. The greater emphasis placed on neutral loci genetic differences by the WSP CU methodology compared to the DU process led to a larger number of CUs (Figure 10).

Special significance

Chinook Salmon are one of five anadromous and semelparous species of Pacific salmon native to North America (Healey 1991). Chinook Salmon constitute a key component of natural ecosystems, being an important food source for other piscivorous fish and for certain marine mammals. For example, the fish is a very important prey species for 'resident' fish-eating killer whales (Orcinus orca) in southern British Columbia (BC), whose survival has been linked to Chinook Salmon abundance on the west coast (Ford and Olesiuk 2012). In Georgia Strait during summer months, nearly 80 percent of DNA sequences found in killer whale fecal samples are from Chinook Salmon (Ford et al. 2016). The fish species is also highly significant to First Nations and Métis in BC as a cultural symbol and source of food (Brown et al. 2013a; COSEWIC 2014), and represent an important target species for recreational and commercial fisheries in BC (Brown et al. 2013a).

The distribution of Chinook Salmon on Vancouver Island, Sunshine Coast, and the Fraser River overlaps with the traditional territory and interests of more than 170 different First Nations and Tribal Councils (COSEWIC 2014). Chinook Salmon are a culturally defining species and often the most highly desired fish amongst Nations across the species' range. Salmon (including Chinook Salmon) form an important foundation of tribal cultures with economic, nutritional, cultural, and spiritual significance. In addition to providing a highly desired food source, Chinook Salmon have been used both historically and currently for sale or trade, various ceremonies, and as the basis of many stories related to First Nation origins (COSEWIC 2014).

Chinook Salmon is also a species of high economic and recreational value along the entire western Pacific coast from California to Alaska as well as elsewhere in the world where this fish occurs naturally, or has been successfully naturalized (Brown et al. 2013a). The history of human interaction with Chinook Salmon is long and extensive in the western Pacific and as a consequence, significant fisheries targeting this species have evolved in all regions. These fisheries occur as regulated commercial and recreational activities in marine inshore and offshore areas, as well as in tidal and non-tidal portions of river systems. Fishing activities involve a variety of gear types with the principal capture methods relying on hook-and-line gear (troll and recreational fisheries) and gill and seine net gear. In marine areas, Chinook Salmon are also caught as by-catch in certain fisheries, for example, pollock fisheries in the Gulf of Alaska. In certain freshwater systems, traditional capture methods are still employed by First Nations. Fishery participants include many First Nations, licensed commercial fishers who are First Nations and non-First Nations in origin, and members of the general public (citizens of BC and visitors to BC) who fish for recreational enjoyment or as fishing guides for the purpose of earning income (Brown et al. 2013a).

Distribution

Global range

Spawning populations of Chinook Salmon are distributed from northern Hokkaido (Japan) to the Anadyr River (Russia) on the Asian coast, and from central California to the MacKenzie River (Northwest Territories, Canada) along the North American coast (McPhail and Lindsey 1970; Major et al. 1978; Bailey pers. comm. 2018) (Figure 11). The species may be establishing new populations at higher latitudes (for example, Arctic regions of Alaska – see Dunmall et al. 2013), possibly due to global warming and other climatic changes (Heard et al. 2007). Recent evidence for range expansion comes in the form of annual catches of adult Chinook Salmon by subsistence fisheries near Point Barrow, and the collection in 2004 of four adult Chinook Salmon in Ublutuoch River, a tributary stream near the mouth of the Coville River, Alaska (Heard et al. 2007). Occasional records exist of Chinook Salmon captures in the Canadian Arctic; however, there are no current records of continuous spawning (McLeod and O'Neill 1983; Stephenson 2006; Irvine et al. 2009; Dunmall et al. 2013).

Chinook Salmon have also been introduced by humans into areas beyond their natural range. Successful transplants have established spawning populations of Chinook Salmon in New Zealand (McDowall 1994), and in the Great Lakes and tributary streams (for example, Lake Michigan, Lake Superior, Lake Ontario) (Carl 1982). Chinook Salmon were also transplanted to Chile, and landlocked populations rapidly established in Chilean and Argentinian rivers (Becker et al. 2007).

Canadian range

Chinook Salmon are native to rivers along the entire west coast of Canada, and may also be found in rivers on the Canadian Arctic coast (McLeod and O'Neill 1983; Stephenson 2006; Irvine et al. 2009; Dunmall et al. 2013) (Figure 12, Figure 13). McLeod and O'Neil (1983) reported recovering a single specimen from the Liard River in the upper Mackenzie River drainage, and Hart (1973) cited an unpublished report of 13 specimens from the Coppermine River. Chinook Salmon also occur in the Okanagan River, between McIntyre Dam at the outlet of Vaseux Lake (near Oliver, BC) and the north basin of Osoyoos Lake near the border with Washington State (COSEWIC 2006).

Chinook Salmon are characterized by high plasticity and life-history variability, so it is not surprising that the species may be responding to warming climatic conditions in Arctic environments by expanding its range into new regions, especially into the Beaufort Sea drainages of North America (McLeod and O'Neill 1983; Stephenson 2006; Irvine et al. 2009; Dunmall et al. 2013).

Extent of occurrence and area of occupancy

Information about extent of occurrence and area of occupancy for southern BC Chinook Salmon is presented for each Designatable Unit (DU), starting on pg. 73 of this report.

A comprehensive source of distributional data does not currently exist for southern BC Chinook Salmon populations. Definition and application of quantitatively rigorous measures of population distribution have been covered extensively for Sockeye Salmon populations in the Fraser River (see de Mestral Bezanson et al. 2012). Similar methods should in principle be developed and applied to Chinook Salmon where possible. As an interim proxy for distributional metrics for each CU, Brown et al. (2013a) provide the number of watersheds, the total watershed area, and the length of known Chinook Salmon spawning habitat. As discussed, CU delineations are defined by life-history (that is, run-timing), genetics and ecotypology and are limited to Canadian freshwater systems (Holtby and Ciruna 2007). Since each DU is based on adapted CU metrics, marine areas are not directly included in DU delineations. However, marine ranges are indirectly considered through the ecotypology component, where each CU is assigned a freshwater adaptive zone and marine adaptive zone which, combined, make up a Joint Adaptive Zone (JAZ=FAZ+MAZ). Direct inclusion of marine areas would result in very large extents of occurrence and areas of occupancy and would preclude comparisons across DUs.

Habitat

Habitat requirements

Chinook Salmon spawning occurs from near tidal influence to between 1,000 (ocean-type) and 3,000 (stream-type) kms upstream near river headwaters (Diewart 2007). Chinook Salmon require an average of 16-24 m² of gravel per spawning pair (Burner 1951). Successful incubation requires stable flows that are adequate to supply enough oxygen, but not so high as to cause gravel movement or streambed scour. The substrate must be small enough to enable the fish to move it for redd construction, and large enough to allow sufficient through-flow for the incubating eggs and later for the developing alevins. Good subgravel flow is a key factor driving the choice of redd sites by all Chinook Salmon; this is because the relatively large size and small surface-to-volume ratio of Chinook Salmon eggs makes them sensitive to reduced oxygen levels. Provided the conditions of good subgravel flow are met, Chinook Salmon will spawn in a broad range of water depths, water velocities, and substrates (Scott and Crossman 1973; Healey 1991; Diewart 2007). This apparent need for sufficient subgravel flow may mean that suitable Chinook Salmon spawning habitat is more limited in most rivers than superficial observation might suggest (Healey 1991). In terms of thermal conditions, spawning Chinook Salmon require cooler water temperatures than those that can be tolerated during the adult migration. The optimum temperature range for egg and hatchling survival is 5-15°C (Leitritz and Lewis 1976; Van Vleck et al. 1988; McCullough 1999; Diewart 2007). If held constant, the upper and lower temperatures for 50 percent pre-hatch mortality of Chinook Salmon are 16°C and 2.5-3.0°C, respectively (Alderdice and Velsen 1978). The upper lethal temperature for Chinook Salmon fry is 25.1°C (Scott and Crossman 1973), although anecdotal evidence suggests this may be higher since Chinook Salmon fry were observed in 2017 feeding at 25°C in Coldwater River (Bailey pers. comm. 2018). Stock-specific differences in thermal tolerance may also occur (Perry et al. 2013; Plumb and Moffitt 2015).

Chinook Salmon rearing occurs in freshwater (streams, lakes), estuaries, and the ocean. In freshwater, juvenile abundance tends to be highest in shallow waters with low velocity and small substrate particle size, although individuals occur over a wide range of substrate types, water depths, and velocities (Chapman and Bjornn 1969; Everest and Chapman 1972). Older, larger fish tend to prefer higher velocity habitats and greater depths. Chinook Salmon rarely occur in still water or where velocity is greater than 30 cm/s (Murphy et al. 1989). Water temperatures of 10-14°C provide suitable rearing conditions (Scott and Crossman 1973; Van Vleck et al. 1988; McCullough 1999). Temperatures in excess of 18°C will disrupt juvenile migration to the sea (Yates et al. 2008).

While in freshwater, juvenile Chinook Salmon feed primarily on invertebrate species, including adult and larval insects. Optimal substrate for maintaining a diverse invertebrate population includes a combination of mud, gravel, and rubble. A pool:riffle ratio of about 1:1 appears to provide an optimal mix of food-producing and rearing habitat for Chinook Salmon in streams. Healthy, natural streamside vegetation is important for maintaining temperatures, controlling erosion and sedimentation, and supplying food items that are an important component of stream-type Chinook Salmon diets. Additionally, freshwater rearing habitat must have water of sufficient quality and quantity (Diewart 2007). Increasing evidence suggests that, in both winter and summer, groundwater and hyporheic water are important moderators of stream temperature and can create thermal refugia for stream-type Chinook Salmon (for example, protection from anchor ice formation) (Bailey pers. comm. 2018).

Coastal estuaries provide an environmental transition zone, extensive opportunities for feeding and growth, and refuge from predators for rearing Chinook Salmon. As environmental transition zones, brackish estuaries allow juvenile Chinook Salmon an opportunity to acclimate from freshwater to saltwater and between waters of differing temperatures. These habitats provide substantial opportunities for feeding, and typically have higher food productivities than adjacent ocean or freshwater areas. Estuaries may thus offer the opportunity for enhanced growth and therefore, larger size at ocean entry, which may translate to higher marine survival (Quinn 2005). Another role of estuaries is to provide refuge from predators. The higher turbidity often associated with estuarine areas limits the ability of visual predators to key on salmon juveniles. The extensive aquatic vegetation associated with estuaries also provides important structural cover (Diewart 2007). These benefits are likely more important to ocean-type Chinook Salmon, since stream-types are larger when they enter the ocean and do not spend much time in the nearshore environment.

Chinook Salmon are thought to require productive nearshore marine habitats, and survival during the period of early ocean residence may influence total production (Brown et al. 2013a). Chinook Salmon generally remain in sheltered, nearshore environments for varying periods depending on factors such as food availability, competition, predation, and environmental conditions. Coastal areas provide a rich habitat with opportunities for feeding and growth. Throughout this period, kelp and other shoreline vegetation provide an important refuge from predators as well as a productive environment for plankton, a major dietary component for juvenile Chinook Salmon (Williams 1989; Healey 1991; Diewart 2007). Therefore, the health of coastal ocean ecosystems plays a key role in the production of Chinook Salmon stocks.

As they grow and mature, Chinook Salmon disperse widely throughout the North Pacific where they eat mainly small fish (primarily Herring and sandlance), with crab larvae, squid and large zooplankton also contributing to their diet. While migration patterns and other aspects of their marine ecology remain poorly understood, ocean residence is recognized as a very important component of the life cycle of all Pacific salmon. During their time at sea, Chinook Salmon migrate varying distances while increasing in size and acquiring the energy reserves required for reproduction. While distribution patterns vary among years and stocks, all stream-type Chinook Salmon utilize coastal and offshore habitats during a period of rapid growth that is critical to reproductive success (Diewart 2007).

Adult Chinook Salmon generally require access to their home spawning grounds to successfully reproduce at a sufficient level of fitness. Strays can reproduce successfully outside their natal streams, but may have lower fitness. Features such as human-made dams, beaver dams, waterfalls, or rock/mud slides that block upstream migration can limit access to spawning areas and impact production (Diewart 2007; Bailey pers. comm. 2018). Suitable adult homeward/upstream migration conditions are limited to areas and seasons where water temperatures are generally lower than 19°C (Yates et al. 2008). Adult Chinook Salmon stop migration and seek temperature refuges when water temperatures exceed 22°C (Alexander et al. 1998). Adult survival and the viability of unspawned eggs decline at temperatures greater than 16°C and less than 3°C (Van Vleck et al. 1988). If conditions such as high water temperature or extreme flows (high or low) are encountered when spawners arrive at their river of origin, fish will hold in the vicinity of the river mouth waiting for conditions to improve. This delay in river entry can adversely affect survival and spawning success as fish may be exposed to predation from marine mammals (Diewart 2007).

Habitat trends

Habitat trends are discussed in detail within each DU's chapter. This section outlines general factors affecting freshwater and marine habitat.

Freshwater

The status of all Pacific salmon is closely linked to the availability of productive freshwater environments. Human-induced impacts have greatly reduced or eliminated historically accessible habitat and/or resulted in direct mortality of juvenile salmonids. Adverse impacts occur from water withdrawals, construction of dams (for power generation or water diversion) that limit fish passage or entrain/harm migrating fish, and degradation of habitat through industrial, agricultural and urban usage (Raymond 1988; Myers et al. 1998). Water quality is negatively affected by aquatic pollution (for example, agricultural runoff, chemicals from industry), altered movement of sediments from terrestrial to aquatic environments (for example, via road construction for forestry), and channelization/erosion leading to the loss of deep water refugia (Groot and Margolis 1991; Bailey pers. comm. 2018). Additionally, modification of natural flow regimes has resulted in a range of adverse impacts, including: increased water temperatures; changes in fish community structures; and depleted flows necessary for migration, spawning, rearing, flushing of sediments from spawning gravels, gravel recruitment and transport of large woody debris (NOAA Fisheries 2014a). The physical features of dams such as turbines and sluiceways also increase mortality of both adults and juvenile salmonids. The infrequent attempts in southern BC to mitigate adverse impacts of these structures have to date rarely met with success.

Marine

Scientists have long recognized the ocean's importance to salmon population dynamics (for example, Pearcy 1992; Beamish 1993 Schindler et al. 2013). Chinook Salmon spend most of their life history, and gain more than 95 percent of their weight, while at sea. Two time periods are believed to be especially important: (1) the spring and summer months immediately after smolt outmigration; and (2) the first winter at sea (Beamish and Mahnken 2001). During winter, Chinook Salmon endure long periods of low forage and must rely on stored energy accumulated during the growing season for survival. As a result, conditions that cause changes in migration timing can lead to matches or mismatches with important prey resources and predators that ultimately translate into varying growth opportunities and differences in survival (for example, Scheuerell et al. 2009; Holsman et al. 2012).

Natural and human-induced changes in the physical ocean environment (for example, temperature/climate change) can affect salmon directly via physiological processes, as well as indirectly through impacts to the surrounding biological environment (for example, food chain). For example, ocean conditions that benefit spiny dogfish (Squalus acanthias) and River Lamprey (Lampetra ayresii) in the Strait of Georgia lead to increased predation pressure on young Chinook Salmon (Beamish and Neville 2000). Studies of ocean conditions have focused on a range of temporal and spatial scales from large-scale phenomena and indices like the Pacific Decadal Oscillation (Mantua et al. 1997; Hertz et al. 2016b), the Arctic Oscillation Index, the North Pacific Index, the North Pacific Gyre Oscillation, and the Bering Sea Pressure Index (Scheuerell 2012; Hertz et al. 2016b; Malick et al. 2017) to direct measurements of regional physical conditions like sea surface temperature (Mueter et al. 2002 for Chum (Oncorhynchus keta), Sockeye (O. nerka), and Pink Salmon (O. gorbuscha)). Scheuerell (2012) found evidence that sea-level pressure and, to a lesser extent, sea temperature may contribute to some of the temporal trends observed in Chinook Salmon recruits-per-spawner among Arctic-Yukon-Kuskokwim Chinook Salmon stocks. Hertz et al. (2016b) found that Chinook Salmon smolt survival off the west coast of Vancouver Island can be linked to large-scale climate variability through feeding ecology. While associations between survival and large-scale climate indices do not provide a mechanistic explanation for which specific ocean processes are causing variation in survival, they do suggest that broad-scale changes in the environment are affecting the suitability of ocean conditions encountered by juvenile Chinook Salmon during their early marine life phase (Schindler et al. 2013).

Human-induced impacts on marine ecological processes also likely contribute to changes in ocean conditions that affect Chinook Salmon growth and survival. For example, a number of Chinook Salmon populations are supplemented (enhanced) by hatchery fish, and there is evidence that enhancement poses risks to natural populations. However, the effects of enhancement vary from DU to DU, and they are poorly understood because of data limitations. In the Bering Sea, hatchery production and distribution of other salmon species (Pink Salmon, Chum Salmon, Sockeye Salmon) in Chinook Salmon foraging areas may create conditions of increased competition for food (Myers et al. 2010; Ruggerone and Irvine 2018). Competition at sea can lead to reduced growth and survival, and potentially to lower reproductive potential among survivors (Ruggerone and Nielson 2009; Ruggerone and Agler 2010; Schindler et al. 2013). Industrial-scale marine fisheries can act as both competitor (for example, reduction of key fish prey densities) and predator (for example, salmon bycatch), thereby altering the productivity, community structure and dynamics through large removals of target species (Schindler et al. 2013). Finally, human-induced climate change is a significant factor affecting marine temperature regimes.

Biology

The general biology of Chinook Salmon is well documented in North America. The following sections draw heavily from Healey (1991) and Myers et al. (1998).

Life cycle and reproduction

The generalized life history of Pacific salmon involves incubation, hatching, and emergence in freshwater, freshwater rearing, migration to the ocean and subsequent initiation of maturation, and return to freshwater for completion of maturation and spawning (Myers et al. 1998). Juvenile rearing in freshwater can be minimal or extended; some male Chinook Salmon mature in freshwater, thereby forgoing emigration to the ocean (e.g 'jimmies', Bailey pers. comm. 2018; Johnson et al. 2012). Maturation occurs between the ages of one and seven, but is typically achieved by about age five (DFO 2008). Like many Oncorhynchus species, Chinook Salmon are semelparous (that is, they die after spawning once) (Healey 1991).

Within this general life-history strategy, a continuum exists between two behavioural forms – ocean-type and stream-type. Life history variants occur along this continuum that express a range of tactics in both freshwater and ocean phases (see Life History Variants). Generalized life-history strategies of Chinook Salmon and the range of tactical variation within each behavioural type are illustrated in Figure 14 (from Healey 1991).

It is important to note that for Chinook Salmon that tend toward either end of the behavioural form continuum, competing views exist regarding the distinctiveness of genetic lineages (see Waples et al. 2004; Beacham and Withler 2010; Moran et al. 2012; Braun et al. 2015). The study of maternal lineage through mitochondrial DNA is one potential way to resolve these differences (Bailey pers. comm. 2018).

Generally, stream-type Chinook Salmon spend one or more years as fry or parr in freshwater before migrating to sea, perform extensive offshore oceanic migrations, and return to their natal stream in the spring or summer, several months prior to spawning. Occasionally, males of this form mature 'precociously' without ever going to sea (Johnson et al. 2012).

Ocean-type Chinook Salmon migrate to sea during their first year of life, normally within three months after emergence from the spawning gravel, spend most of their ocean life in coastal waters, and return to their natal stream in the fall, a few days or weeks before spawning (Healey 1991). Migration timing is not always correlated with spawning timing as the latter requires Chinook Salmon access to spawning grounds, which can in turn depend on freshet timing and suitable stream temperatures.

These life-history variations are thought to represent adaptation to uncertainties in juvenile survival and productivity within particular freshwater and estuarine nursery habitats. Chinook Salmon appear to have evolved a variety of juvenile and adult behaviour patterns that serve to spread the risk of mortality across years and across habitats (for example, Stearns 1976; Real 1980). Disastrously high mortality in any particular year or habitat can thus be ameliorated (Healey 1991). Risk is also mitigated by the fact that Chinook Salmon have a variable maturation schedule with spawning occurring between ages 2-5 for ocean-type life-history variants and between 3-7 for stream-type life-history variants (Bailey pers. comm. 2018).

In preparation for spawning, the female Chinook Salmon digs a depression in the gravel of the stream bottom by performing vigorous swimming movements on her side near the bottom. Gravel and sand thrown out of the depression accumulate in a mound, or tailspill, at the downstream margin of the depression. Once the nest is complete, the female deposits a group or "pocket" of eggs in the depression which are fertilized by one or more males; she then moves to a spot immediately upstream and repeats the process. The material removed by digging in the new site covers the fertilized eggs in the downstream depression, thereby protecting them from predation and from being washed away by the scouring action of the river or stream (Diewart 2007). Over the course of one to several days, the female deposits four or five such egg pockets in a line running upstream, enlarging the spawning excavation in an upstream direction as she does so. The total area of excavation, including the tailspill, is termed a "redd" (Healey 1991). Redds vary in size (area) and depth depending on flow velocity and coarseness of the spawning gravels (Vronskiy 1972; Neilson and Banford 1983; Healey 1991). Stream-type Chinook Salmon typically build smaller redds in coarser gravels than do ocean-type Chinook Salmon (Burner 1951; Diewart 2007).

Females defend their redds for a period of days to weeks, with the average length of residence declining throughout the spawning season (Healey 1991). In the Morice River (upper Skeena drainage), females remained on redds between 4 and 18 days (Neilson and Geen 1981), and in the Nechako River (upper Fraser drainage) they defended redds for 6 to 25 days (Neilson and Banford 1983). Both the Morice River and Nechako River populations are mainly stream-type fish, although the presence of ocean-type Chinook Salmon has been established with scale analysis.

Considerable variation in fecundity exists both within and between different Chinook Salmon populations, and from year to year (McGregor 1922, 1923; Healey and Heard 1984; Myers et al. 1998). In a study of 16 different Chinook Salmon populations, female fecundity ranged from fewer than 2,000 eggs to more than 17,000 eggs (Healey and Heard 1984). Fecundity was significantly correlated with female size in all but one of the populations examined, but size explained only 50 percent or less of the variation between individuals within a population; a great deal of individual variation remained to be explained. Healey and Heard (1984) speculated that this high variation may reflect an uncertain trade-off between egg size and egg number in the overall fitness of Chinook Salmon populations. In a stable population, fecundity is ultimately sufficient to result in an average production of one adult female spawner for each female spawner in the parent generation (Healey 1991).

The survival of eggs in undisturbed natural redds appears to be quite high. Vronskiy (1972) reported survival of 97 percent to hatching, and Briggs (1953) reported 90 percent survival to the eyed stage and 82 percent to hatching. Neither author dealt with losses due to scouring or siltation (Healey 1991). The length of time required for the eggs to incubate is partially dependent on water temperature; in general, the lower the water temperature, the longer the incubation period required. Alderdice and Velsen (1978) identified the time to 50 percent hatch as about 159 days at 3°C and 32 days at 16°C. Since no natural population incubates to a constant temperature, it is more useful to convert these values to Accumulated Thermal Units (ATUs), thus the ATUs required for 50 percent hatch range from 477-512 (159x3 = 477; 32x16 = 512). Upon hatching, the alevins move varying distances within the spaces between the gravel particles depending on gravel size (Diewart 2007). The newly hatched fish have an attached yolk sac that provides nutrition. Towards the end of incubation in the spring, alevins move up through the gravel to emerge as fry. This process occurs at night which helps to minimize predation and generally coincides with the complete absorption of the yolk sac. The survival of Chinook Salmon eggs from spawning to emergence as fry varies widely between systems and years and is influenced by stream flow, water temperature, dissolved oxygen, gravel composition, spawning timing, and spawner density. Studies suggest that survival to emergence averages about 30 percent (Healey 1991).

Physiology and adaptability

Seaward migration is regarded as one of the most demanding and physiologically challenging phases of the salmon life history, and represents a complex interplay between physiology and behaviour (Miller et al. 2009). After emergence in freshwater, Chinook Salmon fry feed and grow from a few months to two years before migrating to the ocean as smolts. During migration, salmon experience extreme changes in their environment (for example, salinity, temperature, olfactory cues, flow). Smolts undergo profound physiological changes in their transition from freshwater to salt water. They then spend the next one to seven years growing and maturing at sea (mini-jacks and mature parr are exceptions that do not spend a full year at sea). Mature adults return to their natal streams to complete sexual maturation and spawn (National Wildlife Federation 2002; NOAA Fisheries 2014b). Returning stream-type Chinook Salmon adults must maintain their ion balance, without feeding, in the osmotically rigorous freshwater environment for several months before spawning (Healey 1991).

Prior to their run upriver, Chinook Salmon once again undergo significant physiological changes. Fish swim by contracting longitudinal red muscles and obliquely oriented white muscles. Red muscles are used for sustained activity, such as ocean migrations. White muscles are used for bursts of activity, such as bursts of speed or jumping (Kapoor and Khanna 2004). As they enter the estuary of their natal river Chinook Salmon are faced with two major metabolic challenges: (1) to supply energy suitable for swimming the river rapids; and (2) to support maturation of the sperm and eggs required for the reproductive effort ahead. The water in the estuary receives the freshwater discharge from the natal river. Relative to ocean water, this has a high chemical load from surface runoff. Miller et al. (2009) found evidence that as the salmon encounter the resulting drop in salinity and increase in olfactory stimulation, two key metabolic changes are triggered – a switch from using red muscles for swimming to using white muscles, and an increase in the sperm and egg load. Pheromones at the spawning grounds trigger a second shift that further enhances reproductive loading (Miller et al. 2009).

Chinook Salmon produce the largest eggs of all Pacific salmon (Diewart 2007). Physiological and ecological factors have been identified that may limit the potential minimum and maximum egg sizes, 0.12 and 0.47 g, respectively (Quinn and Bloomberg 1992). A recent study by Einum et al. (2002) suggests salmonid egg oxygen consumption does not increase at a greater rate with increasing egg mass and available egg surface area for oxygen diffusion.

Water percolation through spawning gravels is essential for egg and alevin survival, a requirement that can be severely compromised by siltation of spawning beds (Healey 1991). Shelton (1955) concluded that survival to hatching was greater than 97 percent at percolation rates of at least 0.03 cm/s, but that emergence was 13 percent or less from small gravel when percolation rates were less than 0.06 cm/s. Much higher emergence rates (87 percent) were recorded for Chinook Salmon in large gravel with adequate subgravel flow.

Chinook Salmon exhibit a high degree of life-history variation, as evidenced by variability in the duration of freshwater and saltwater rearing stages, age at maturation, spawning habitat requirements, and rearing habitat requirements. The high degree of life-history variation suggests a high degree of adaptability in the species (Healey 1991). However, there is considerable debate as to what degree this variability is the result of local adaptation or the general plasticity of the salmonid genome (Ricker 1972; Healey 1991; Taylor 1991).

Adaptability is also suggested by the level of success achieved with hatchery transplantation. Chinook Salmon have been produced in hatcheries in North America for more than a century, with hatchery outplants introduced to a wide range of rivers with and without native Chinook Salmon populations (Myers et al. 1998). The species has also been successfully introduced into highly novel environments, including the Canadian Great Lakes system and New Zealand rivers. However, there is considerable concern about the apparently low fitness of many hatchery outplants and the impacts this may have on naturally spawning populations (Berejikian and Ford 2003).

Dispersal and migration

Upon emergence from spawning gravels, Chinook Salmon fry swim and/or are passively displaced downstream by flow (Healey 1991). A large downstream movement immediately after emergence is typical of most populations (for example, Lister and Walker 1966; Bjornn 1971; Reimers 1971; Healey 1980b; Kjelson et al. 1982), and is probably a dispersal mechanism that helps distribute fry among the suitable rearing habitats (Healey 1991; Myers et al. 1998). As a result, Chinook Salmon fry often rear in non-natal streams, underscoring the importance of these streams as habitat despite the fact that they are not spawning streams (Scrivener et al. 1994). In larger rivers, Chinook Salmon fry migrate more at the river edges than in high velocity waters near the centre of the channel and, when the river is deeper than about 3 m, they prefer the surface (Mains and Smith 1964; Healey and Jordan 1982). These observations provide further support for the idea that downstream movement of fry is not entirely passive displacement controlled by water velocity, but that some active behaviour of the fry helps direct the migration (Healey 1991). Distance of migration to the marine environment, stream stability, stream flow and temperature regimes, stream and estuary productivity, and general weather regimes have also been implicated in the evolution and expression of specific migration timing (Myers et al. 1998).

For populations that spawn close to tidewater, downstream dispersal carries fry to estuarine nursery areas; in others, it serves principally to distribute the fry among suitable freshwater nursery areas (Healey 1991). Downstream dispersal occurs mainly at night, generally concentrated around midnight, although small numbers of fry may move during the day (Healey 1991). Fry dispersal is normally most intense between February and May, and occurs earlier in more southern populations. South Thompson Chinook Salmon appear to disperse later in the summer (July-August) (Beamish et al. 2010). The timing of the peak can vary substantially from year to year in the same system, and there is also tremendous daily variation in abundance. The causes of annual and daily variation in the downstream dispersal are not well understood (Healey 1991), but may be related to the timing of high discharge events (Mains and Smith 1964; Healey 1980b; Kjelson et al. 1981; Irvine 1986).

In addition to discharge, both intra- and interspecific interaction may serve to stimulate the downstream dispersal of young Chinook Salmon. Reimers (1968) observed lateral displays, chasing, fighting, fleeing, and submission behaviours among juvenile fall Chinook Salmon in stream tanks and in natural stream populations, whereby the agonistic behaviour of one or a few dominant fish apparently stimulated the downstream movement of subordinate fish. Taylor (1988) also reported aggressive behaviour among juvenile Chinook Salmon fry, and between Chinook Salmon and Coho Salmon fry; stream-type Chinook Salmon were more aggressive than ocean-type Chinook Salmon. For stream-type Chinook Salmon, dispersal and seaward migration can be related to resource allocation and dispersal to overwintering habitat, but it is part of the natural life history (Myers et al. 1998). Patterns vary significantly depending on rearing locations (Bradford and Taylor 1997). If suitable overwintering habitat such as large cobble is not available then the fish will tend to migrate downstream (Bjornn 1971; Hillman et al. 1987). Additionally, Stein et al. (1972) observed that juvenile Chinook Salmon grew more slowly in the presence of juvenile Coho Salmon than they did on their own, and speculated that interaction with Coho Salmon may influence the downstream movement of Chinook Salmon.

In the southern half of the Chinook Salmon's range, following close on the heels of fry outmigration, many fry migrate seaward as fingerlings between April and June of their first year (Healey 1980b, 1982; Kjelson et al. 1981, 1982). For ocean-type variants, this migration may occur any time between immediately post-emergence and ~150 days post-emergence; however, the majority move seaward in 60-90 days (hence the term 'underyearling'). For some stocks, passing through large lakes is required to get to sea (for example, Mabel, Mara, Shuswap, Little Shuswap, and Kamloops lakes). The fingerlings migrate downstream throughout the day, but most do so at night (Mains and Smith 1964; Lister et al. 1971). Ocean-type Chinook Salmon are also known to use lakes during rearing (Brown and Winchell 2004; Roseneau 2014), and make extensive use of estuaries prior to seaward migration.

Stream-type variants typically delay migration until the spring following their emergence (hence the term 'yearling') and sometimes wait for an additional year (Healey 1983). Yearling 'smolts' normally migrate seaward in the early spring (April to July), sometimes preceding and sometimes intermixed with the main migrations of fry and fingerlings (Healey 1991). Yearling migrants appear to be less nocturnal than underyearlings, although on average more smolts move at night (Meehan and Siniff 1962; Major and Mighell 1969).

For all life-history variants, the rate of downstream migration appears to be both time and size dependent. Larger Chinook Salmon travel downstream faster than smaller Chinook Salmon, and the rate of migration increases as the season advances (Healey 1991). Downstream travel rates may also be positively related to river discharge (Bell 1958; Raymond 1968), but there has been no systematic study of the triggers (Healey 1991).

Limited data are available concerning the ocean migration of wild stream-type variants; they apparently move quickly offshore and into the central North Pacific, where they make up a disproportionately high percentage of the commercial catch relative to ocean-type variants (Healey 1983; Myers et al. 1987; Trudel et al. 2011; Trudel and Hertz 2013). Coded-wire tag returns have shown that Chinook Salmon from British Columbia, Washington, and Oregon migrate as far west as 160°-175°W longitude (Dahlberg 1982; Wertheimer and Dahlberg 1983; Dahlberg and Fowler 1985; Dahlberg et al. 1986). Stream-type variants perform extensive offshore oceanic migrations before returning to their natal river in the spring or summer, several months prior to spawning. Ocean-type variants migrate to sea during their first year of life, but spend most of their ocean life in coastal waters before returning to their natal river in the fall (Healey 1991). This migration timing is not always correlated with spawning timing as the latter requires Chinook Salmon access to spawning grounds, which can in turn depend on freshet timing and suitable stream temperatures (Bailey pers. comm. 2018).

Transplantation studies and recoveries of marked hatchery fish from ocean fisheries provide evidence of a genetic basis for ocean distributions. Chinook Salmon stocks follow predictable ocean migration patterns based on “ancestral” feeding routes (Brannon and Setter 1987). The productivity of various ocean regions (for example, West Vancouver Island) has been correlated with the degree of wind-driven upwelling (Bakun 1973, 1975). Upwelling brings cold, nutrient-rich waters to the surface, resulting in an increase in plankton and ultimately salmon production (Beamish and Bouillon 1993). Ocean migration patterns represent an important form of resource partitioning and are important to the evolutionary success of the species (Myers et al. 1998).

The availability of coded-wire tag recoveries in recent years has resulted in more detailed stock-specific information on the marine distribution of Chinook Salmon (Beamish et al. 2011a,b; Trudel et al. 2009; Tucker et al. 2011, 2012; Weitkamp 2010). Juvenile Chinook Salmon are found throughout the Strait of Georgia from the surface to 60 m depth from June through to November. Smolts of the stream-type variants generally enter the ocean the earliest in the year (March-May) (Trudel et al. 2007). Ocean-type Chinook Salmon variants from the South Thompson region of the Fraser River enter as late-ocean migrants in July-August (Barraclough and Phillips 1978; Healey 1980a, 1991; Healey and Groot 1987; Beamish et al. 2003). Smolts of the stream-type variants enter the ocean in April-May. These smolts do not remain near shore, but move into the deeper areas of the Strait (Healey 1980a, 1991). Beamish et al. (2011a) found that a proportion of variants of both ocean-type and stream-type life-history strategies spend approximately 3-5 months in the Strait of Georgia, but not all the fish leave. For example, some ocean-type Chinook Salmon overwinter in the Strait as evidenced by fish in their second year of ocean residence caught in the Strait of Georgia sport fishery (Brown et al. 2013a).

Declining abundance of ocean-type and stream-type variants observed in the Strait of Georgia in June/July is likely a result of mortality within the strait combined with migration out of the strait. Chinook Salmon mortality in the Strait of Georgia is thought to be quite high, ranging from 70-92 percent for wild fish (Beamish et al. 2011). There is also evidence that some Strait of Georgia Chinook Salmon stocks may have specific and refined distribution during their early marine period. For example, South Thompson River Chinook Salmon occur on the west coast Vancouver Island in the fall and remain in the region over the winter months (Tucker et al. 2011). These fish disperse further north as they get older (Tucker et al. 2011; PSC-CTC 2012a). Lower Fraser River ocean-type Chinook Salmon appear to migrate off the west coast of Vancouver Island later than South Thompson River Chinook Salmon, and are rarely found north of Vancouver Island (Tucker et al. 2011; PSC-CTC 2012a). In contrast, Cowichan River Chinook Salmon rear primarily in the coastal waters around the southern islands (the "Gulf Islands") of the Strait of Georgia (also referred to as the "Salish Sea") (Beamish et al. 2011a, 2011b). The catches of this stock remain high in the region from May through to September. This stock is rarely identified (based on DNA analysis) from other areas of the Strait of Georgia (Brown et al. 2013a).

On the west coast of Vancouver Island, Chinook Salmon migrate to sea as smolts of ocean-type variants in May-June (Healey 1991) and remain on the west coast of Vancouver Island for nearly a year before migrating north along the continental shelf (Trudel et al. 2009; Tucker et al. 2011). They are found primarily on the shelf and in inlets within the 200m depth contour. In the fall and winter of their first year at sea, most stocks can be found between their ocean entry point and Quatsino Sound, at the north end of Vancouver Island. For instance, Robertson Creek Chinook Salmon are located from Barkley Sound to Quatsino Sound, whereas Marble River Chinook Salmon are distributed exclusively within Quatsino Sound during their first year at sea (Trudel et al. 2012a). Mortality rates have not been quantified for the early marine residence period off the west coast of Vancouver Island, although the overall marine survival of Robertson Creek Chinook Salmon appears to be related to the availability of energy-rich prey (Trudel et al. 2012b; Hertz et al. 2016a). During winter, mortality rates for Marble River Chinook Salmon range from 60 percent to 90 percent depending on the year (Trudel et al. 2012a). The factors contributing to this mortality are currently unknown, but do not appear to be related to size, growth, or energy accumulation (Middleton 2011; Trudel et al. 2012a).

Coastwide, Chinook Salmon remain at sea from one to six years (more commonly two to four years) (Myers et al. 1998). Adult and subadult southern BC Chinook Salmon range as far north as Cook Inlet in Alaska, with the majority of the recoveries in Southeast Alaska and the west coast of Vancouver Island (Weitkamp 2010; PSC-CTC 2012a). Generally, adult Chinook Salmon may make the return migration to their natal river mouth during almost any month of the year (Snyder 1931; Rich 1942; Hallock et al. 1957) (southern BC Chinook Salmon are an exception). There are, however, typically one to three peaks of migratory activity, and the timing and number of these peaks varies among river systems. For northern river systems, a single peak of migratory activity during June appears typical, although peaks may occur from April to September (Bailey pers. comm. 2018; Brady 1983; Vronskiy 1972; Yancey and Thorsteinson 1963). Further south, runs can peak anytime between April and September (Bailey pers. comm. 2018). Returning to the "home stream" provides a mechanism for local adaptation and reproductive isolation (Myers et al. 1998).

The upstream migration of mature Chinook Salmon occurs mainly during daylight hours, at least for the ocean-type variants (Neave 1943). A few fish do, however, migrate upstream at night (Healey 1991).

Interspecific interactions

Chinook Salmon rearing in freshwater feed on terrestrial insects, crustacea, chironomids, corixids, caddisflies, mites, spiders, aphids, corethra larvae, and ants (Scott and Crossman 1973; Healey 1991). Insects are predominant during this phase, providing up to 95 percent of the freshwater diet in all seasons, with adult chironomids comprising 58-63 percent of the food items taken (Becker 1973). The basic Chinook Salmon diet is similar to that of Coho Salmon (O. kisutch), Steelhead Salmon (O. mykiss), and other stream-dwelling salmonids (Mundie 1969; Chapman and Bjornn 1969).

The food habits of Chinook Salmon in estuaries vary considerably from estuary to estuary, and from place to place within a given estuary (Healey 1991). Food items include chironomid larvae and pupae, crab larvae, harpacticoid copepods, Daphnia, Eogammarus, Corophium, and Neomysis (Dunford 1975; Northcote et al. 1979; Levy et al. 1979; Levy and Northcote 1981). As Chinook Salmon grow larger, small fish (for example, juvenile Herring (Clupea pallasii), sticklebacks (for example, Gasterosteus aculeatus), Chum Salmon fry (O. keta) also become important in the diet (Goodman 1975; Healey 1980b; Levings 1982).

Young Chinook Salmon in the marine environment eat mainly fish (particularly Herring), with invertebrates like pelagic amphipods, squids, shrimp, euphausiids, crab larvae, and insects comprising the remainder of their diet (Scott and Crossman 1973; Healey 1980a; Hertz et al. 2016b). Subadult Chinook Salmon (27 to 72 cm in length) in the Qualicum River area of the Strait of Georgia have been reported to feed on Chum Salmon fry, larval and adult Herring, Sand Lance (Ammodytes hexapterus), and euphausiids (Robinson et al. 1982). Fish dominate the diet of adult Chinook Salmon, especially Herring (Reid 1961; Prakash 1962); other food fish include Sand Lance, pilchards/sardines, and sticklebacks (Pritchard and Tester 1944). Invertebrate taxa form a relatively small component of the ocean adult diet, although there is considerable regional (and seasonal) variation in diet composition (Healey 1991). Coast-wide data suggest that the importance of Herring and Sand Lance in the adult diet increases from south to north, whereas the importance of rockfishes (Sebastes sp.) and anchovies (Engraulis mordax) decreases (Healey 1991).

Chinook Salmon and Coho Salmon reside sympatrically in many streams and rivers that are tributary to the North Pacific Ocean (Taylor 1991). There is some evidence that agonistic behaviours occur between Chinook Salmon and Coho Salmon fry (Taylor 1988), and that competition for resources between these species may occur (Stein et al. 1972). In a controlled setting, Stein et al. (1972) observed that juvenile Coho Salmon apparently dominated Chinook Salmon and grew faster in sympatric groupings in stream troughs. When alone in the troughs, however, Chinook Salmon were able to grow as rapidly as Coho Salmon. Also in a controlled setting, Taylor (1991) reported that Coho Salmon behaviourally dominated Chinook Salmon and outnumbered them in upstream channels where food was introduced. However, in natural stream settings where Chinook Salmon and Coho Salmon were sympatric, the two species used different habitats – Coho Salmon preferred slow, deep 'pool' areas whereas Chinook Salmon preferred faster, shallow 'riffle' areas. Chinook Salmon made greater use of pool habitats. While Coho Salmon may socially dominate Chinook Salmon in pool habitats, differences in habitat preference between the species that have developed during sympatric evolution probably minimize the extent to which Coho Salmon influence the duration of freshwater residence by Chinook Salmon (Taylor 1991).

Spiny Dogfish (Squalus acanthias) and River Lamprey (Lampetra ayresii) have been identified as major predators of juvenile Chinook Salmon in the Strait of Georgia (Beamish and Neville 2000). Southern resident killer whales also have a strong preference for Chinook Salmon throughout much of the year, especially in the lower Georgia Strait (Ford and Ellis 2005). Hake (Merluccius productus), Mackerel (Scomber japanicus), Sea Lions (Zalophus californianus, Eumetopias jubatus), Harbour Seals (Phoca vitulina), White-sided Dolphins (Lagenorhynchus obliquidens), and Humpback Whales (Megaptera novaeangliae)are also known predators of salmon in the marine environment (Riddell et al. 2013).

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