Sei whale (Balaenoptera borealis) COSEWIC assessment and status report: chapter 6

Biology

Reproduction

Sei whales reach sexual maturity between 5 and 15 years of age. In both the hemispheres the apparent age at sexual maturity declined from 10-11 years to 8 years between the 1930s and the 1960s (IWC 1977). Estimates of pregnancy rates towards the end of the fishery ranged from 30 to 69% of mature females (Mizroch 1980). While it was thought for a time that the Antarctic baleen stocks were responding to depletion with an increased pregnancy rate, Mizroch (1980)demonstrated that this assumed density-dependent response was spurious, and likely the result of inappropriate pooling of data.

The gestation period is estimated at 10.5 months (North Pacific, Masaki 1976) to 12 months (Antarctic, Gambell 1968), with conception and calving occurring in winter. Calves are weaned on the feeding grounds after a lactation period of about 6 months, and the calving interval is 2 – 3 years (Gambell 1985a).

Survival

Estimates for adult natural mortality range from 4.7 to 10.3% (IWC 1977) depending on sex and stock. Six percent was believed to be a reasonable approximation (IWC 1977). A mean juvenile mortality of 10% has also been reported. Allen (1980) estimated adult mortality at 7.5% and suggested there was little evidence to assume a higher rate for juveniles.

The species is known to have carried both endo- and ectoparasites (Andrews 1916, Rice 1974, 1977). According to Rice (1974), sei whales are more prone to heavy infestations of parasitic helminthes (i.e. flatworms) than other baleen species. While these are not normally pathogenic, a sufficiently large infestation, particularly of the liver or kidneys, could cause mortality.

Rice (1974) also reported that 7% of sei whales landed in California between 1959 and 1970 were infected with a disease that caused the shedding of baleen plates. However, the stomachs of these whales contained fish, and they appeared to be in good condition. The degree to which this and parasitic infections continue to affect the species is unknown.

Predation by killer whales and sharks (Rice 1968, Perry et al. 1999), and aggressive interactions with swordfish (Brown 1960) are cited as potential sources of mortality for sei whales; however, no definitive reports of such attacks were found in the literature. While killer whales are known to feed on baleen whales, the interaction with swordfish and sharks appears to be coincidental. Townsend (1923) provided a number of explanations for the co-occurrence of these species with large whales, and concluded that any reported attacks on whales by sharks or swordfish were actually attacks by killer whales. Thus, interactions with sharks or swordfish are likely inadvertent and the result of co-occurrence in areas of high prey abundance.

The population level impacts of killer whale predation is not known. However, predation is more likely to affect immature or weakened individuals (Perry et al. 1999).

There are no contemporary estimates available for the growth rate of sei whale populations. The default maximum net productivity rate for cetaceans used by the NMFS is 4% (Waring et al. 2001).

Species characteristics and physiology 

The average size for adult sei whales is 15 m in length and 19 tonnes (Horwood 1987). The maximum reported size for a female in the northern hemisphere was 18.6 m, while in the southern hemisphere a maximum length of 20 m was recorded (Gambell 1985b). As with all balaenopterids, males are somewhat smaller. Sei whales in the northern hemisphere appear to be smaller than those in the southern hemisphere (Tomilin 1967 cited in Horwood 1987)while western Atlantic animals appear to be heavier for a given length than those in the North Pacific (Lockyer and Waters 1986). Sei whales can live up to 60 years (Lockyer 1974).

Sei whales are exceptionally fast swimmers. Andrews (1916) estimated that a sei whale could reach 30 knots (56 kmh) in its first rush after being struck with a harpoon. A marked sei whale in the southern ocean moved 2,200 nmi in a 10-day period implying an average speed of 9 knots (17 kmh). However migration speeds are likely to be somewhat slower (Horwood 1987).

The sei whale is not considered a deep diver. On surfacing, it does not arch as much as the other rorquals. Instead, it approaches the surface at a shallow angle, and often exposes its dorsal fin at the same time as it blows. It then tends to sink below the surface rather than dive (Andrews 1916).

The reason for the restriction to temperate waters may be based on temperature. Antarctic catch records indicate that most sei whales were caught in waters between 8 and 18°C, with the Antarctic Convergence acting as a barrier to all but the larger whales (Kawamura 1974). In the northern hemisphere, the summer whaling grounds were in temperate waters containing major frontal systems. The range appeared to extend into more polar waters only with favourable temperatures (Horwood 1987). 

Migration

As is typical of baleen whales, sei whales in both hemispheres migrate from low-latitude wintering areas to high-latitude summer feeding grounds. There is evidence from catch records that migrations in all basins were segregated according to length (i.e. age), sex, and reproductive status. Pregnant females appear to lead the migration to the feeding grounds, while the youngest animals arrive last and leave first, and do not go as far poleward (Lockyer 1977, Horwood 1987, Gregr et al. 2000).

Off central California, sei whales were most abundant in late summer and early fall, but were widely and sparsely distributed in winter (Rice 1974). Numbers off British Columbia peaked in July, and the distribution moved progressively further offshore as the summer progressed (Gregr et al. 2000). The timing of these observations is difficult to interpret, but if sei whales peaked in abundance off California and British Columbia at the same time, and were from the same stock, then this suggests a widely dispersed migration, with significant segregation either by sex or age class.

In the Northwest Atlantic, sei whales were reported to migrate along the continental slope (northward) in July-August, and return through the same area in September-November (Mitchell and Chapman 1977). Mitchell (1974) assumed that the northward migration took the animals to the south coast of Newfoundland by August and September. However, Andrews (1916) suggested that their occurrence off Newfoundland was sporadic. The Atlantic catch record reflects this episodic availability (Mitchell 1974).

Diet composition

Sei whales use both “skimming” and “engulfing” (or gulping) feeding strategies (Nemoto 1959). The finer baleen possessed by sei whales is more similar to that found in balaenids such as the right (Eubaleana spp.) and bowhead (Balaena mysticetus) whales (Mead 1977). Also like the balaenids, the sei whale feeds primarily on calanoid copepods (Calanus spp.), but also takes euphausiids, amphipods, and a variety of schooling fish and squid, particularly in the North Pacific (Nemoto and Kawamura 1977, Flinn et al. 2002).

Stomach content analysis by Nemoto and Kawamura (1977) revealed that the diet composition of sei whales from the North Pacific and the Antarctic differed substantially. In the Antarctic, euphausiids (54%) were the primary prey item, followed by significant proportions of copepods (30%) and amphipods (14%). In the North Pacific, copepods (Calanus spp.) dominated the diet (83%), while euphausiids (13%) and fishes and squid (5%) provided minor contributions. This diet composition was confirmed by Kawamura (1982) who examined the contents of 1,072 stomachs from animals caught in the decade following Nemoto’s study (1969 to 1979).

Nemoto and Kawamura (1977) suggested that sei whales caught in coastal waters had a more diverse diet. This is corroborated by Flinn et al. (2002), whose analysis of stomach contents from coastal British Columbia stations showed that copepods dominated the diet in 3 of 5 years, while fish and euphausiids each dominated in one of the other years.

This diversity is not apparent in the North Atlantic, where sei whales appear to be much more stenophagous. Of 52 stomachs examined at Norwegian whaling stations between 1952 and 1953, all were either empty or contained only crustaceans (copepods or euphausiids) (Jonsgård and Darling 1977). On the Nova Scotian shelf, Mitchell et al. (1986)reports that none of 134 stomachs examined in 1972, and only 2 of the 68 stomachs examined between 1966 and 1972, contained fish or squid.

Nemoto and Kawamura (1977) attributed the difference in stomach contents between the North Pacific and the Antarctic to the different trophic structures in the two basins: In the Antarctic, the majority of biomass is in the form of plankton. In the North Pacific, on the other hand, there is a greater abundance of plankton consumers, increasing the abundance at higher trophic levels. Thus, the diet preferences observed may be at least partially related to prey availability.

Differences in stomach contents may also be partially explained by seasonal differences in diet. Flinn et al. (2002) found a monthly trend in diet composition, with fish dominating in the early (May) and late (September) part of the season. Rice (1977) reported that sei whales off California fed primarily on anchovies in June through August, and on euphausiids in September and October.

Foraging

Sei whales are unpredictable in their distributions, often appearing on the same feeding ground for a number of years and then disappearing for extended periods. These year to year differences in arrival time and distribution of sei whales on the summer feeding grounds have been documented wherever sei whales were caught. In the Antarctic, whalers spoke of “sei whale years” (Gambell 1985a), while in the eastern Atlantic, years with high numbers of sei whales were called “invasion years” (Jonsgård and Darling 1977). The British Columbia whaling records also provide evidence for erratic annual abundances in the eastern North Pacific (Gregr et al. 2000).

The close association of sei whales with oceanic fronts suggests that sei whales take advantage of the physical oceanographic processes that enhance production and entrainment of plankton. These observations, combined with an apparent temperature limitation (Kawamura 1974, Horwood 1987), suggest that sei whales may use the warmer, poleward moving currents in each of the ocean basins to penetrate into higher latitude feeding grounds.

A close relationship between sei whale feeding grounds and oceanographic conditions explains the occurrence of “invasion years”. The well-documented dispersal of feeding sei whales after storm events (Horwood 1987) fits well with this assumption, if the weather was severe enough to break down the oceanographic feature being exploited by the whales.

Recently, occasional intrusions have been observed into the shallow inshore waters of the Northwest Atlantic (i.e. Stellwagen Bank, Great South Channel) in times of increased copepod abundance (Payne et al. 1990, Waring et al. 2001, Murison, personal communication). This could be related to shifts in preferred habitat, or degradation of more preferred habitats further offshore. 

Interspecific interactions 

Sei whales appear to be opportunistic feeders, taking advantage of localized prey concentrations during their annual migrations. This would allow them to minimize competition with other baleen species. For example, in the southern hemisphere, a clear latitudinal segregation of baleen whales and preferred prey species has been observed (Laws 1977). There is also evidence for a staged arrival, with blue and humpback whales arriving first, and fin and sei whales arriving later in the season. In addition, there appears to be separation by depth, with minke, sei and right whales tending to feed at the surface, while fin and blue whales feed at greater depths (Horwood 1987). The extensive, early depletion of right whales (by the end of the 19th century) in both the North Atlantic and the North Pacific may have allowed sei whale populations to increase as a result of reduced competition for copepods.

On the Scotian shelf, sei whales were observed to have a sympatric distribution with right whales between 1966 and 1972 (Mitchell et al. 1986). Given the overlap in diets, this is not unexpected. However, more recently (1982 – 1988), the observed distributions of sei and right whales on the Atlantic coast have been mostly allopatric, except in years of high copepod abundance (Payne et al. 1990). Payne et al. (1990) present a strong argument for competition between right whales and sandlance (Ammodytes spp.) for copepods, and suggest that planktivorous fishes may play a significant role in the distribution of baleen whale populations.

Adaptability

The flexible feeding strategy of sei whales is at least partially a function of baleen plates that are intermediate between the fine sieves of right whales and the coarser plates that facilitate gulping. With its ability to apply both skimming and gulping feeding strategies, the sei whale is also better able to adapt to fluctuations in prey populations than the more stenophagic right whale, but perhaps not as well as the more generalist fin whale. If competition is primarily with other planktivores (whales or fishes), then the ability to take advantage of a variety of prey items that become abundant under different oceanographic conditions or in different areas will enhance the survival of the species.

The diet information from the North Pacific and the Antarctic suggests that sei whales can adapt their diet to different prey distributions. However, recent observations, exclusively from the Nova Scotian shelf, suggest a continuing preference for copepods.

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