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Sei whale Listed: 6/2/1970 Status since listing: Unknown The sei whale (Balaenoptera physalus) is one of the least studied of the endangered “great whales” [1]. They occur in all oceans, but generally stay in more temperate waters within a smaller range of latitudes than other baleen whales and tend to be found in deeper waters associated with continental slopes and edges [2]. Little is known about either their current or historic status, and the structure of sei whale stocks and their migrations patterns are still relatively unknown [1]. Whaling in the 1950s through the early 1970s reduced most stocks of sei whales, some of them drastically [1]. The sei whale won international protection in 1970, when catch quotas for the North Pacific began to be set on a species basis. In 1976 the sei whale was given complete protection from commercial whaling in the North Pacific. In 1986, all legal commercial hunting for sei whales ended. Although it is expected that the cessation of whaling would result in increased sei whale numbers, the status and population trends for this species are unknown [3]. Currently, stock assessment reports by NOAA consider three stocks of sei whales in U.S. waters: 1) the Eastern North Pacific stock (California, Oregon and Washington waters) 2) the Hawaiian stock, and 3) the Nova Scotia stock (includes waters off the northeastern U.S. Atlantic coast) [4]. Although there have been no direct estimates (based on sighting surveys) of sei whale abundance for the entire North Pacific, populations are thought to have been reduced to 20% of their pre-whaling abundance [3]. The eastern North Pacific stock of this population also was likely depleted by whaling [3]. Records indicate that 384 sei whales were taken by shore-based whaling stations in central California between 1958 and 1965; 26 were taken off central and northern California between 1919 and 1926 [3]. Aerial surveys conducted off the coasts of California, Oregon and Washington in 1991, 1992, 1993, 1996 and 2001 produced only two confirmed sei whale sightings [3]. The sei whales’ offshore distribution along the continental slope may explain, at least in part, the infrequency of sightings between northern California and Washington [1]. An abundance estimate of 56 (CV = 0.61) sei whales was produced from shipboard surveys conducted in 1996 and 2001 in California, Oregon and Washington waters out to 300 nmi [3]. Population trends for this stock are unknown [3]. Little is known about the status of sei whales in Hawaiian waters [4]. A 2002 shipboard line-transect survey of the entire Hawaiian Islands Exclusive Economic Zone resulted in a summer/fall abundance estimate of 77 (CV=1.06) sei whales [4]. Currently, this is the best (and only) available abundance estimate for this stock [4]. At this time of year, however, the majority of sei whales would probably be at higher-latitude feeding grounds [4]. In the North Atlantic, the range of the Nova Scotia sei whale stock includes the continental shelf waters of the northeastern U.S., and extends northeastward to south of Newfoundland [5]. During the spring and summer, sei whales from this stock are found in waters of the Gulf of Maine to Georges Bank [5]. The total number of sei whales that enter U.S. Atlantic waters is unknown [5], but Sei whales are not thought to be common anywhere in the U.S. Atlantic [1]. Tag-recapture surveys conducted in the 1970s in Nova Scotia estimated the Nova Scotian stock to number between 1,393 and 2,248 [5]. Spring aerial surveys conducted in U.S. waters from 1979-1981 (CeTAP surveys) estimated an abundance of 280 sei whales in the stock [5]. This estimate is not considered statistically reliable, however, because of low survey effort and inaccuracy in accounting for submerged animals [2]. In 1986, an influx of sei whales into the southern Gulf of Maine was reported [5]. The cause of this phenomenon remains unknown, but similar events have been reported in other parts of the world [5]. There are no recent abundance estimates for this stock and population trends are unknown [5]. Today, the possibility of illegal whaling or a resumption of legal whaling represent potential direct threats to sei whale populations [1]. Japan currently conducts yearly harvests of sei whales for “scientific research” [2]. Ship collisions, entanglement in fishing gear, reduced prey abundance due to overfishing and habitat degradation, and disturbance from low-frequency noise may also pose threats to sei whales [1]. Since sei whales tend to inhabit deeper waters, however, they are probably less prone to collisions with fishing gear and ships than are most of the other Northern Hemisphere baleen whales [1]. [1] National Marine Fisheries Service. 1998. Draft Recovery Plan for the Fin Whale (Balaenoptera physalus) and Sei Whale (Balaenoptera Borealis). Silver Spring, MD. |
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