Sei whale (Balaenoptera borealis) COSEWIC assessment and status report: chapter 7

Population Sizes and Trends

Historically, sei whales were never as abundant in the North Atlantic as in the North Pacific (Horwood 1987). However, there are no recent abundance estimates or population trends available for sei whales in either ocean. The best population estimates (still cited today) are based primarily on catch per unit effort data from commercial whaling.

Tillman (1977) estimated the entire (1974) North Pacific population at between 7,260 and 12,620 individuals, and estimated a pre-exploitation population of 42,000 animals. However, Nemoto and Kawamura (1977) suggested that the sei whale’s range in the North Pacific was extended by the depletion of blue and fin whales. Combined with the earlier depletion of North Pacific right whales, this could have allowed populations of sei whales to increase prior to their becoming a target species. Thus, any pre-exploitation estimates for this species may be inflated.

Estimates for the entire North Atlantic range from 4,000 (Braham 1991 cited in Perry et al. 1999) to 12-13,000 (Cattanach et al. 1993). However, the estimate from Braham (1991) is considered imprecise (Perry et al. 1999), while that provided by Cattanach et al. (1993) combines Mitchell and Chapman's (1977) estimate for western Atlantic waters (2,248) with the results of a 1989 ship-based survey of Icelandic and adjacent waters (10,300 animals, CV=0.268).

Mitchell and Chapman (1977) estimated the size of the Nova Scotia stock using mark-recapture and census data. The mark-recapture analysis estimated the stock at 1,393 – 2,248 animals. The census study estimated the Northwest Atlantic population at 2,078, with minimums of 870 for the Nova Scotia stock, and 965 for the putative Labrador Sea stock.

A CeTAP (Cetacean and Turtle Assessment Program) estimate of 253 animals between Cape Hatteras, North Carolina and Nova Scotia, on the continental shelf and shelf edge, was derived using data from aerial surveys conducted from 1978 to 1982 (Waring et al. 2001). This estimate, when corrected for dive time and probability of detection on the track line, is approximately the same as Mitchell and Chapman's (1977) mark-recapture estimate (Waring et al. 2001). The CeTAP data were also used to estimate a maximum population of 2,273 animals in U.S. Atlantic waters (Mizroch et al. 1984). However, these estimates are no longer considered reliable, nor are any of the pre-exploitation estimates for the North Atlantic (Perry et al. 1999).

Historically, few sei whales were caught in the Northwest Atlantic prior to the last fishery off Nova Scotia between 1966 and 1972, which took a total of 825 animals. Sei whales were not the preferred species in the Northwest Atlantic (Mitchell and Chapman 1977, Tonnessen and Johnsen 1982). On the Pacific coast, at least 4,002 sei whales were taken by coastal stations in British Columbia between 1908 and 1967, with the majority taken after 1955 (Gregr et al. 2000).

Today, the sei whale is considered rare in California waters, and a recent (2002) survey for large cetaceans off the coast of British Columbia, which included the shelf break, did not result in a single confirmed sei whale sighting (DFO, unpublished data). Extensive aerial and ship-based surveys of the United States Pacific coast conducted by NMFS (cited in Carrettaet al. 2001), extending as far as 131ºW, have produced only one confirmed sei whale sighting in six years of surveys (1989 – 1993, and 1996). This lack of sightings may well be due to the difficulty in distinguishing the species from fin whales through cursory surface observations.

On the Atlantic coast, sei whales are identified more frequently. Aerial surveys (1999 and 2000) found spring concentrations along the southern edge of Georges Bank (Waring et al. 2001). Also, an influx of sei whales into the waters off Maine and Massachusetts was observed between 1986 and 1989 (Waring et al. 2001). These animals very likely moved through Canadian waters later in the season.

Perry et al. (1999) report a 1991 estimate of 4,000 animals for the entire North Atlantic, but this is believed to be a low-precision estimate. The Iceland/Demark Strait stock has been estimated from ship-based surveys as 1,290 (1987) and 1,590 (1989). These estimates were reported without confidence intervals (Perry et al. 1999).

The limited survey data suggests that the British Columbian population is extremely small, and has shown no signs of recovery since the species was protected from commercial whaling after 1976. The Nova Scotia stock may contain around 2,000 individuals, but there is no information available to assess population trends for either coast.

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