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Recovery Strategy for Blue, Fin, and Sei Whales in Pacific Canadian Waters [Final]

4 Sei Whale Background

4.1 Current status

Common name: Sei whale

Scientific name: Balaenoptera borealis

Legal listing (SARA): January 2005 (Endangered)

Assessment summary: May 2003

COSEWIC status: Endangered

Reason for designation: This was one of the most abundant species sought by whalers off the British Columbia coast (with over 4000 individuals killed) and was also commonly taken in other areas of the eastern North Pacific. Sei whales have not been reported in British Columbia since whaling ended and may now be gone. There are a few, if any, mature individuals remaining in British Columbia waters, and there is clear evidence of a dramatic decline caused by whaling and no sign of recovery. (www.cosewic.gc.ca)

Occurrence in Canada: North Atlantic and North Pacific

Status history: Designated Endangered in May 2003.

4.2 Species description

The sei whale is the third largest member of the Balaenopteridae, after the blue and fin whales. Sei whales are cosmopolitan in their distribution, though they appear somewhat restricted to temperate waters, occurring within a more restricted range of latitudes than all other rorquals except Bryde’s whales (COSEWIC 2003). There is evidence for three stocks of sei whales (western, central and eastern) in the Pacific (Masaki 1977).

An average adult sei whale is 15 m long and weighs 19 tonnes (Horwood 1987). Females are larger than males. Animals in the northern hemisphere appear to be smaller than those in the southern hemisphere (Tomilin 1967). The maximum reported lengths for a female were 18.6 m in the northern hemisphere and 20 m in the south (Gambell 1985a).

Sei whales are dark to bluish grey dorsally and white to cream coloured ventrally. The ventral grooves commonly have a white or light-coloured area extending from the chin to the umbilicus, although colouration is extremely variable. Oval-shaped scars from cookie-cutter shark bites and lampreys, and infestations of ectoparasitic copepods often occur on the lateral and ventral sides. The curved, slender dorsal fin is prominent measuring 0.25-0.75 m, and is set further forward on the body compared to blue and fin whales. The pectoral flippers are relatively short measuring only 9-10% of the body length, dark grey ventrally, and pointed at the tips. The dark grey flukes are rarely raised to “fluke-up” before dives (COSEWIC 2003).

In the eastern North Pacific, fin and sei whales overlap morphologically in body size, colouration, and dorsal fin shape making the two easily confused. However, sei whales lack the asymmetric white colouration of the right jaw and ventral side that is diagnostic for fin whales. Confusion with fin whales, and to a lesser degree with Bryde’s and minke (B. acutorostrata) whales, implies that sei whale population size and range could easily be underestimated (COSEWIC 2003).

Sei whales migrate from low-latitude wintering areas to high-latitude, summer feeding grounds. Catch records indicate that migrations are segregated according to length (i.e., age), sex, and reproductive status, with pregnant females leading the migration to the feeding grounds. The youngest animals arrive last and leave first, and travel to lower latitudes than adults. The wintering grounds of sei whales are largely unknown, though they are thought to occur far offshore (COSEWIC 2003).

Males and females reach sexual maturity between 5 and 15 years of age and live to approximately 60 years of age. In both hemispheres, the age of sexual maturity declined from 10-11 years to 8 years between the 1930s and the 1960s, likely in response to exploitation. Mating, followed by a gestation period of 10.5 to 12 months, and calving occur in winter. Calves nurse for about 6 months and are weaned on the feeding grounds. The calving interval is 2-3 years (COSEWIC 2003).

4.3 Population size, trends, and distribution

Historically the most abundant of the baleen whales, sei whales are now considered rare in U.S. and Pacific Canadian waters. They were once described as abundant off the west coast of Vancouver Island, British Columbia, in June through August (Pike and MacAskie 1969). The pre-whaling North Pacific population, estimated at between 58,000 and 62,000 individuals, was reduced to between 7260 and 12,620 animals by 1974 (COSEWIC 2003). Although whaling from British Columbia coastal stations stopped after 1967, international whaling in the North Pacific continued to target this species until 1975. Almost 40% of the total (62,550) reported North Pacific sei whale catch was taken after 1967 (IWC database, J. Breiwick, NMML, AFSC, NMFS, 7600 Sand Pt Way NE, Seattle, WA 98115-0070). Since the total catch exceeds the estimated pre-exploitation population size, it is clear this species was severely depleted.

The IWC recognizes a single stock of sei whales in the North Pacific (Donovan 1991). However, evidence does exist for multiple populations, at least historically. A review of marking studies, catch distributions, sighting and baleen morphology revealed three North Pacific stocks separated by 175°W and 155°W longitude (Masaki 1977). Fujino (1964) suggested a difference between sei whales caught in the inner Gulf of Alaska and off Vancouver Island based on examination of blood type. Different forms of parasites observed at opposite sides of the Pacific imply the existence of at least an eastern and western population (Rice 1974).

NMFS recognizes an eastern and a western stock in the North Pacific, divided by 180°W (Carretta et al. 2002). The stock boundary is arbitrary due to a lack of information on population structure. An abundance estimate of 56 animals (CV = 0.61) was recently calculated by Barlow (2003 cited in Carretta et al. 2003) for the Eastern North Pacific stock, to a distance of 300 nm (560 km) from shore. No population trend data are available. It is presumed that sei whales in Canadian Pacific waters are part of the eastern North Pacific population.

4.3.1 Canadian Pacific

Historic records clearly demonstrate that Pacific Canadian waters were once extensively used by sei whales (Figure 1), with a sharp peak in seasonal abundance during July (Figure 2). In recent years, cetacean surveys off the British Columbia coast and shelf-break region have not resulted in a single confirmed sei whale sighting (CRP-DFO, unpublished data). The BCCSN database contains 3 high confidence sei whale sightings.

Two confirmed sei whale sightings and 5 possible sightings (recorded as sei or Bryde’s whales) were made off California, Oregon, and Washington during ship and aerial surveys between 1991 and 2001 (Carretta et al. 2003). These few sightings are the basis for the recent abundance estimate in that region.

Based on sighting data, the number of sei whales currently occurring in Pacific Canadian waters appears quite small and has shown no measurable signs of recovery since the species received protection from commercial whaling in 1976. No reliable information is available to estimate population trends.

4.4 Biological needs, ecological role and limiting factors

Sei whales are low trophic level foragers that feed primarily on calanoid copepods. However, their diet also contains euphausiids, amphipods, and schooling fish and squid, particularly in the North Pacific (Nemoto and Kawamura 1977, Flinn et al. 2002). Stomach content analyses have revealed substantial regional differences in diet. In the Antarctic, euphausiids represented 54% of the sei whale diet, whereas calanoid copepods represented 83% of the diet in the North Pacific (Nemoto and Kawamura 1977, Kawamura 1982).

Sei whales may forage more opportunistically in coastal waters, taking advantage of a more diverse prey base than is available in pelagic waters (Kawamura 1982). Stomach contents from British Columbia whaling stations reveal that copepods were the most common in 3 of 5 years, whereas fish and euphausiids each dominated in one of the other years (Flinn et al. 2002).

Differences in stomach contents between the North Pacific and the Antarctic may be due to the different trophic structures and prey availability in the two regions. In the Antarctic, the majority of biomass is in the form of zooplankton. In the North Pacific on the other hand, there is a greater abundance of zooplankton consumers, increasing the prey abundance at higher trophic levels (Nemoto and Kawamura 1977). Seasonal trends in stomach contents may indicate a seasonal shift from a spring diet dominated by fish to one dominated by euphausiids or copepods later in the summer (Rice 1977, Flinn et al. 2002).

The ecological role of sei whales therefore seems to be that of a generalist, low-trophic level feeder. Whether this ability to generalize diet is a characteristic of all individuals, or if different individuals tend to specialize on different prey types, is unknown.

The likelihood of trophic displacement by finfish stocks, as has been suggested for other large whales (Payne et al. 1990), may be lower for sei whales, whose diet diversity may enable them to adapt to fluctuations in prey quality and abundance. The species does appear to alter its distribution in response to fluctuations in prey availability. However, given the limited sightings in the eastern North Pacific, there is a possibility that the remnant sei whale population may be too small to recover.

Sei whales are reported to carry both endo- and ectoparasites, and appear to be more susceptible to heavy infestations of parasitic helminthes (i.e., flatworms) than other baleen species. Although these parasites are typically not pathogenic, a sufficiently large infestation of the liver or kidneys can be fatal. The degree to which parasitic infections currently affect sei whales is unknown. Seven percent of sei whales killed in California between 1959 and 1970 were infected with a disease that caused the shedding of baleen plates. Apart from the missing baleen, these whales had fish in their stomachs and were in good condition (COSEWIC 2003).

Some predation of sei whales is possible by killer whales and sharks, though the degree of predation is unknown (Reeves et al. in press). Increased abundance could lead to increased predation.

4.5 Habitat needs

Sei whales use both “skimming” and “engulfing” (or gulping) feeding strategies (Nemoto and Kawamura 1977)and feed primarily on calanoid copepods. They are typically found in relatively deep waters, primarily associated with offshore, pelagic habitats. In the northwest Atlantic, sei whales are associated with the continental shelf-break (Hain et al. 1985). In British Columbia, less than 0.5% of the historical coastal whaling catch for which positions were recorded were on the continental shelf (Gregr 2002).

Examinations of baleen whale distributions in relation to oceanographic conditions suggest a close association with oceanic fronts. Sei whales are reportedly observed along major mixing zones and eddies that had broken away from fronts and the animals may follow these fronts seasonally (Nasu 1966). These fronts can be somewhat permanent, near predictable oceanic features such as major upwelling areas, or they can be associated with more dynamic features such as eddies or jets formed near topographical features shearing off major currents (COSEWIC 2003).

Sei whales are often observed on the same foraging ground for many years and then disappear for prolonged periods of time. Whalers spoke of “sei whale years” in the Antarctic (Gambell 1985a). In Norwegian waters, these dramatic influxes of sei whales were called “invasion years” and were correlated with high abundances of pollock (likely Theragra finnmarchica) (Jonsgård and Darling 1977). Unpredictable arrivals of sei whales to the North Pacific feeding grounds are also evident in the British Columbia whaling record (Gregr et al. 2000).