Narwhal (Monodon monoceros) COSEWIC assessment and status report: chapter 7

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Population Sizes and Trends

A good estimate of the initial size of the Baffin Bay and Hudson Bay narwhal populations cannot be generated from historical harvest data (Mitchell and Reeves 1981; Reeves 1992a). Consequently, it is not possible to assess whether historical hunting activities have depleted population sizes, and estimates or indices of abundance from surveys after 1975 are the only benchmarks for future population assessments.

Until 1996, estimates of population size were limited to methods that only estimated a portion of the population (Innes et al. 1996). They were based on aerial surveys of areas of aggregation and seldom accounted for narwhals that were submerged beyond view (availability bias), missed by observers because of ice or poor visibility (perception bias), or simply outside the survey area (DFO 1998a, 1998b). They also did not distinguish between mature and immature animals, so changes in the population structure cannot be assessed, and may significantly underestimate young-of-the-year which are more difficult to see and identify from the air (Richard et al. 1994). Estimates from the aerial photographic surveys are typically higher than those obtained from simultaneous visual observations (Innes et al. 2002; P. Richard, pers. comm. 2002).

More data are required before survey estimates can be accurately corrected for diving animals. Recently, Heide-Jørgensen et al. (2001) and Innes et al. (2002) applied correction factors for availability and perception bias but noted the need to develop more precise correction factors for availability bias. Land-based observations (Born et al. 1994) and dive data from small samples of tagged whales suggest that less than 50% of the animals in a population may be visible at the surface at any one time (Heide-Jørgensen and Dietz 1995; Heide-Jørgensen et al. 2001; Laidre et al. 2002). Survey estimates are sensitive to violation of the assumption that narwhals are visible and recognizable to a particular depth, typically 5 m.

Baffin Bay Population

Aerial surveys of the pack ice of western Baffin Bay in May 1979 estimated that there were 34,363 (±SE 8282) narwhals at the surface (Koski and Davis 1994). This survey covered about 2/3 of Baffin Bay but missed West Greenland waters and Smith Sound where narwhals also occur in May. The population estimate did not correct for perception bias. If narwhal wintering in Baffin Bay show preference for the edge of the coastal shelf, as recent work by Dietz et al. (2001) suggests, it may be worthwhile to revisit these data and recalculate these estimates.

In August 1984, an aerial photographic survey of Eclipse Sound, Admiralty Inlet, Prince Regent Inlet and Peel Sound estimated that there were 17,991 (90%CI 14,724-21,258) narwhals at the surface (Richard et al. 1994). This survey covered important areas of summer aggregation for narwhals, but did not cover the entire known summer range of the Baffin Bay population in Canadian waters (Figure 4). The number of Baffin Bay narwhals that summered in the waters of West Greenland in 1984 is unknown, but land-based observers counted 4,043 narwhals passing their observation point in Inglefield Bay on 18 August (Born 1986).

A systematic aerial survey of belugas in Prince Regent Inlet, Barrow Strait and Peel Sound was conducted from 31 July to 3 August 1996 (Innes et al. 2002). Visual observations were made in the offshore areas and aerial photographs were taken in the concentration areas. Narwhals were also counted during the same survey. The visual estimates were corrected for animals missed by the observers, sightings without distance estimates, and for submerged animals to produce an estimate of 45,358 narwhals (95% CI = 23,397-87,932). The submergence correction was obtained from retrievable time-depth-recorders deployed on four narwhals in Creswell Bay and Tremblay Sound (Laidre et al. 2002) and assumed that narwhals were visible to a depth of 5 m.

Estimates by Innes et al. (2002) of the number of narwhals at the surface of Barrow Strait and Prince Regent Inlet (14,474, CV = 0.24) compare well with those obtained by Smith et al. (1985) (11,142, CV = 0.09), considering that the latter was not corrected for animals missed by the observers (perception bias). Likewise, estimates of the number of narwhals at the surface of Prince Regent Inlet (12,324, CV = 0.25) and central Peel Sound (1,891, CV = 0.55) compare well with those obtained by Richard et al. (1994) (Prince Regent Inlet 9,754, CV = 0.18; Peel Sound 1,701, CV = 0.17; CV from Innes et al. 2002), considering that the latter was not corrected for perception bias. Nevertheless, the statistical power to detect a trend, if there is one, is low given the large variance in the estimates.

Several other systematic surveys have covered smaller portions of the summer aggregation areas (Fallis et al. 1983; Smith et al. 1985; Dueck and Riewe 1986; Dueck 1989) but these data are not useful for estimating the overall population size or trend (DFO 1998a). The same is true for a number of systematic aerial surveys of migrants in the spring (Finley 1976; Johnson et al. 1976; Davis et al. 1978; Koski 1980a, 1980b), fall (Stepney and Wooley 1976; Koski 1980b; Koski and Davis 1980), and winter (Heide-Jørgensen et al. 2002); of non-systematic surveys (Hay and McLung 1976); and of land-based observations in the spring (Tuck 1957; Greendale and Brousseau-Greendale 1976).

Without recent and complete survey information, and with uncertainty about hunting mortality and natural mortality, it is not possible to provide a quantitative estimate of the size of the Baffin Bay narwhal population, or of that portion of the population that enters Canadian waters. Earlier reports concluded that there was no evidence of population decline (Strong 1988; Remnant and Thomas 1992; Thomsen 1993) and that the “narwhal hunt may be sustainable” (SWG 1997). In fact, although the Canadian population is obviously still large as evidenced by the 1996 survey (Innes et al. 2002), the true size and trend of the Baffin Bay narwhal population are unknown. There has been concern about the summering grounds of narwhals hunted in the fall in West Greenland, but recent information suggests that they are not from Canada. Removals due to hunting appear to have risen in recent years (Table 1; Figures 5 and figure6), and have increased the urgency of improving knowledge of the Baffin Bay population (JCNB/NAMMCO 2001). To address this need, DFO undertook surveys of the population’s summering grounds in Arctic Canada in August 2002 and 2003 (P. Richard pers. comm. 2004).

There is no clear consensus among Inuit hunters of a population trend in the Baffin Bay population (Stewart 2001). In 1992, most hunters from Qikiqtarjuaq and Clyde River believed that, despite year-to-year variation, the narwhal population in their area had increased since the 1960s or 1970s (Remnant and Thomas 1992). Most hunters from Pond Inlet, Arctic Bay, Resolute, Igloolik, Hall Beach, and Grise Fiord believed the narwhal population in their area to be stable despite year-to-year variations and changes in local seasonal distributions (Remnant and Thomas 1992; Stewart et al. 1995). However, interpretation of this information is confounded by changes in the local seasonal distribution of narwhals.

Hudson Bay Population

Richard (1991) conducted systematic visual and photographic aerial surveys of narwhals in the Repulse Bay area between Roes Welcome Sound and Lyon Inlet, north of Southampton Island in March 1983 and July of 1982, 1983, and 1984. These surveys included the major known summering concentrations of the Hudson Bay narwhal population. The July 1984 photographic survey was repeated in August 2000, with the addition of northern Lyon Inlet and Foxe Channel (P. Richard, pers. comm. 2002). Without correcting the results of either survey for submerged animals, or the latter survey for persistent fog or animals that may have occupied Wager Bay (Gonzalez 2001), the narwhal population was estimated at 1355 (90%CI = 1000-1900) animals in 1984 and 1780 (90%CI = 1212-2492) animals in 2000.

At least 50% of the whales seen in Hudson Bay are believed to be submerged when surveyed. In North Baffin Bay the correction factor may be 2.6 times the number counted based on the proportion seen at the surface (38%, SE = 9%, Innes et al. 2002). The current Hudson Bay population likely numbers about 3,500 animals in summer after correcting for submerged individuals. While the 2000 survey result is preliminary, it suggests that the population has not decreased between surveys (1984 and 2000), despite concern over heavy exploitation of the population in 1999 (Table 2; Figure 6). These hunting levels are not believed to reflect ongoing harvest levels, and should not be interpreted to have been sustainable.