Fin whale (Balaenoptera physalus) COSEWIC assessment and status report: chapter 7
Biology
Reproduction
Information on the reproductive biology of whales is derived primarily from animals taken during commercial whaling (Lockyer 1984). Fin whales reach sexual maturity at 5 to 15 years of age for both sexes (Perry et al. 1999) with the average reported as 6–7 years for males and 7–8 years for females (Aguilar 2002). The average length at sexual maturity in northern hemisphere fin whales is 17.2 m (Mitchell 1974, Ratnaswamy and Winn 1993).
Conception and calving are believed to occur in winter, likely in low latitudes (Mizroch et al. 1984, Reeves et al. 2002). After a gestation period of 1 –12 months, calves are born at an average length of 6 m. Average length at weaning is about 11.5 m, at approximately 6–7 months of age (Omura 1950, Gaskin 1976, Ratnaswamy and Winn 1993). Females generally undergo a 6-month resting period after weaning a calf. Agler et al. (1993) calculated a mean interbirth interval of 2.71 years (n=13), and estimated a potential interval of 2.24 years.
Pregnancy rates have been estimated at between 38% and 50% of adult females (Aguilar 2002). Agler et al. (1993) estimated the gross annual reproduction rate of fin whales at 8% based on photographic identification.
Survival
An adult natural mortality rate for fin whales has been estimated at 4% (Doi et al. 1970, Lockyer and Brown 1979, Ratnaswamy and Winn 1993). There appears to be no information on survival rates for calves and juveniles.
Possible sources of natural mortality include predation by killer whales (Orcinus orca) (Vidal and Pechter 1989) or certain species of sharks (Connor 2000). However, there are no conclusive reports in the literature of successful predation. Considering the speed and size of fin whales, attacks would likely succeed only when old, ill or immature animals are targeted (Perry et al. 1999, Aguilar 2002).
Lambertsen (1986) estimated that 90–95% of fin whales in the North Atlantic carry heavy loads of the giant nematode Crassicauda boopis. Such loads could be pathogenic, resulting in renal inflammation and, in extreme cases, kidney failure and death (Lambertsen 1992, Perry et al. 1999).
Species characteristics and physiology
Fin whales attain 95% of their maximum body size at 9–13 years of age (Aguilar 2002). The amount of ossification of the vertebral column has been used to estimate physical maturity at approximately 25 years of age in both sexes (Aguilar and Lockyer 1987). Average lengths of adult fin whales range from 24 m in the northern hemisphere to 27 m in the southern hemisphere. Adult females can be up to 2 m longer than adult males (Lockyer and Waters 1986, Ralls and Mesnick 2002, Reeves et al. 2002). The average weight reported for adults ranges from 40–50 tonnes in the northern hemisphere to 60–80 tonnes in the southern hemisphere (Jefferson et al. 1993, Aguilar 2002). These weight estimates come from commercial whaling data with corrections for fluid loss during flensing (Lockyer 1976, Gambell 1985). Maximum life span may be as high as 100 years (Gambell 1985).
Fin whales rarely raise their flukes on a terminal dive and they only occasionally breach or strike the sea surface with their flukes or flippers. The strong mother–calf bond terminates upon weaning. There is no evidence for long-term social bonds. Fin whales sometimes travel in groups of 2–7 animals, though larger, ephemeral aggregations occur in areas of high productivity (Aguilar and Lockyer 1987).
Migration
It is generally assumed that most fin whales migrate between foraging grounds in high latitudes and calving/breeding grounds in lower latitudes (Macintosh 1965, Sergeant 1977). However, fin whale movements do not appear to be so simple.
In the North Pacific, Pike (1950) observed that some, mostly young, fin whales appeared to spend the summer feeding off British Columbia. Recent surveys have observed summer fin whale feeding in both shelf-edge and on-shelf waters (J.K.B. Ford, personal communication).
Acoustic monitoring of seafloor hydrophone arrays from September 1991 to August 1992 (Moore et al. 1998) showed that fin whale calls occurred year-round at the hydrophone closest to Canada (Site 5, near 45° N). Call counts at this site peaked in February and May, when counts were virtually absent at stations farther south. Additionally, a period of increased call intensity at Site 5 in July-August preceded by about one month a similar pulse of calls at the next station south.
Watkins et al. (2000) did not find evidence of large-scale migratory activity in the North Pacific based on acoustic signals. However, the signal used (termed “20-Hz pulses”) has been associated with male breeding displays (Watkins and Schevill 1979). This confounds the interpretation that fin whales do not migrate over large distances in the North Pacific.
Evidence exists for an age-structured migration in the North Pacific (Gregr et al. 2000), and in the eastern North Atlantic, where Aguilar (1987) and others have reported a migration order of pregnant females leading, followed by males and resting females, and then lactating females and juveniles. However, Agler et al. (1993) found the summering ground arrival times of females with calves did not differ from the arrival times of other individuals in the Gulf of Maine.
In the Atlantic, it has been suggested that the Newfoundland and Nova Scotia stocks move southward in the winter, with the Newfoundland stock moving into the summer grounds of the Nova Scotia stock and the Nova Scotia stock moving further south (Kellogg 1929, Allen 1971, Mitchell 1974). Fin whales also occur in the Gulf of St. Lawrence throughout the summer, ranging into the St. Lawrence estuary (Edds and MacFarlane 1987, Kingsley and Reeves 1998). The Gulf Stream may influence the latitudinal distribution of habitats, making higher latitudes suitable as wintering grounds, thereby accounting for shorter north-south movements (Aguilar and Lockyer 1987).
Year-round observations in areas such as eastern Nova Scotia (Brodie 1975), the Mediterranean (Notarbartolo-Di-Sciara et al. 2003) and the Gulf of California (Tershy et al. 1990) suggest that not all individuals within a population complete a “full” migration (Mitchell 1974, Tershy and Wiley 1992). Aguilar (1987) also suggested that animals may occasionally remain at higher latitudes throughout the winter, or at lower latitudes throughout the summer.
Fin whales have been described as highly vocal animals, with acoustic activity increasing markedly from late August through fall and again in the mid-winter on the edge of the Scotian Shelf and just offshore. This could be indicative of southward movement in the fall and northward movement in late winter (Clark 1995).
Walker et al. (1992) found statistically significant seasonal associations between fin whale sightings and areas of low geomagnetic intensity and gradients, suggesting that fin whales may use the earth’s magnetism to guide migration.
Diet composition
Fin whales forage on a variety of species. Generally in the northern hemisphere they eat small invertebrates, schooling fishes and squids (Jefferson et al. 1993, Bannister 2002). The available information supports the assertion by Gambell (1985) that fin whale diet is as much a function of availability as preference.
In the North Pacific, the diet dominated by euphausiids (70%) followed by copepods (25%) with some fish and squid (Kawamura 1980). Flinn et al. (2002) examined records of stomach contents for fin whales taken in British Columbia and found similar results.
In eastern Canadian waters, fin whales consume primarily euphausiids and capelin, with euphausiids occurring more frequently early in the year and the capelin proportion increasing later in the summer (Sergeant 1966). Capelin appears to dominate the diet off Newfoundland (Mitchell 1975, Brodie et al. 1978, Whitehead and Carscadden 1985), while in the Bay of Fundy euphausiids dominate the diet once concentrations become available in surface waters (Gaskin 1983). Fin whales in the St. Lawrence estuary presumably take advantage of the high local concentrations of euphausiids and the associated schools of capelin (Simard and Lavoie 1999). They feed alongside humpback whales on euphausiids and schooling fish, although fatty acid analysis confirms that fin whales occupy a higher trophic position (Borobia et al. 1995). Fin whales have also been observed feeding on herring off Nova Scotia (H. Whitehead, unpublished data).
Inter-specific interactions
Due to the global overlap in range and diet with other baleen whales, inter-specific competition is likely (Aguilar and Lockyer 1987). Mixed groups of fin and blue whales are common and hybrids occur with surprising frequency (Bérubé and Aguilar 1998). In the Bay of Fundy and off Newfoundland, fin and humpback whales have been observed foraging in the same general areas (Whitehead and Carlson 1988, Katona et al. 1993). Fin whales have also been associated with right whales in the lower Bay of Fundy (Woodley and Gaskin 1996) and on the Scotian Shelf (Mitchell et al. 1986). Whitehead and Carlson (1988) noted the possibility of interference and exploitation competition between humpback and fin whales when foraging on capelin.
To some extent, large baleen whales, consequent to their depletion by whaling, may have been “replaced” in the ecosystem by ecologically equivalent finfish stocks (Payne et al. 1990). Trites et al. (1999) suggested that in the Bering Sea some species of fish are significant competitors of whales.
Adaptability
The ability to include small schooling fish in their diet shows that fin whales have some flexibility in their feeding strategy. This may allow the species to better adapt to reductions in particular prey items (e.g., euphausiids) than the more stenophagous blue whale, but perhaps not as well as the more generalist sei whale.
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