FISHERY BULLETIN: VOL. 85, NO. 2 



Gulf of Alaska, so a relationship between the de- 

 clines in northern sea lion numbers and changes in 

 the abundance of their prey cannot be rejected or 

 confirmed. 



The declines of northern sea lion may not have 

 a single cause, but may be due to the effects of a 

 combination of these and other factors. Sources of 

 mortality which alone seem insufficient to account 

 for the declines but which could be important in a 

 combined effect include the pre- 1973 commercial sea 

 lion harvests, entanglement in marine debris, inci- 

 dental taking in fisheries, and the killing of sea lions 

 for bait and predator control. 



Northern sea lions were commercially harvested 

 in the eastern Aleutian Islands and Gulf of Alaska 

 from 1959 to 1972. Six hundred and sixteen adult 

 males were taken in an experimental harvest in 1959 

 (Thorsteinson et al. 1961). A total of 45,178 north- 

 ern sea lion pups of both sexes were harvested in 

 the eastern Aleutian Islands and Gulf of Alaska be- 

 tween 1963 and 1972 (FEIS^i). The largest harvests 

 were conducted between 1963 and 1972 at Sugar- 

 loaf and Marmot Islands where 16,763 and 14,180 

 pups, respectively, were killed, and between 1970 

 and 1972 at Ugamak and Akutan Islands where 

 3,773 and 6,036 pups, respectively, were killed. The 

 pup harvests, which sometimes reached 50% of the 

 total pup production from a rookery (e.g., at Sugar- 

 loaf Island in 1965 and 1968), could have depressed 

 recruitment in the short term. This may partially 

 explain the declines experienced at some sites 

 through the mid-1970s. However, it is unclear why 

 numbers declined in areas where no harvest oc- 

 curred (e.g., the north side of Ugamak Island), while 

 no declines were observed at some harvest sites 

 (e.g.. Marmot Island). In any event, those harvests 

 should not currently be affecting the decline, be- 

 cause populations should have stabilized 3-5 years 

 after the cessation of harvesting as unharvested 

 year classes reached breeding age. Furthermore, 

 these harvests probably cannot explain the declines 

 in numbers counted in the western and central Aleu- 

 tian Island populations. 



Little information exists on the effect of entangle- 

 ment in marine debris on northern sea lions. Despite 

 debris commonly being found in areas northern sea 

 lions frequent (Calkins 1985; Merrell 1985), data 

 from NMML surveys suggest that this is not a prob- 

 lem, at least for adult sea lions. Observed entangle- 



ment rates were 0.07% in the 1985 ship survey 

 (Loughhn et al. fn. 7), 0.09-0.17% in the 1985-86 

 Ugamak Island surveys, and 0.12% at Marmot 

 Island in 1983 (Merrick 1984). Numerous northern 

 sea lion pups were seen in the November 1985 east- 

 ern Aleutian Island entanglement survey (Lough- 

 lin et al. fn. 7), but none were entangled. Never- 

 theless, it is possible that entangled northern sea 

 lion pups drown and are not observed. 



Numerous northern sea lions have been taken in- 

 cidental to fisheries in the Bering Sea and North- 

 east Pacific Ocean since the late 1960s and early 

 1970s (FEIS fn. 21). In 1978-81 the estimated aver- 

 age annual mortality for all foreign vessels was 724 

 animals (Loughlin et al. 1983). This does not, how- 

 ever, include animals taken by U.S. fishermen fish- 

 ing in joint ventures or independently. Loughlin and 

 Nelson (1986) documented the take in the Shelikof 

 Strait joint venture walleye pollock fishery where 

 an estimated 958 to 1,436 northern sea lions were 

 caught by U.S. trawlers in 1982. This take declined 

 to less than 400 animals per season in 1983 and 

 1984, probably due to changes in fishing technique 

 and the area and times fished. The cumulative im- 

 pact of foreign independent and joint venture fish- 

 eries in the Bering Sea and North Pacific Ocean 

 probably now accounts for less than 500 deaths per 

 year (NMML fn. 11). Domestic fishermen now work- 

 ing independently probably take less since they 

 generally are involved in fisheries that catch few sea 

 lions. However, as foreign fishing is phased out of 

 U.S. waters, the domestic take will probably in- 

 crease. The foreign and domestic incidental take 

 contributes to but cannot totally account for the 

 decline. 



We are uncertain how the killing of northern sea 

 lions by fishermen has affected the population. Fish- 

 ermen have been observed to kill adult animals at 

 rookeries, haul-outs, and in the water near boats, 

 but the magnitude of this take is generally unknown. 

 Trawl fisheries attract many northern sea lions dur- 

 ing haulback operations and shooting at these 

 animals is a common occurrence. One of the few 

 estimates of shooting mortality comes from Matkin 

 and Fay22 who calculated that 305 northern sea lions 

 were killed directly (shot) while interferring with 

 fishing operations in the spring 1978 Copper River 

 Delta salmon gill net fishery. Northern sea lions at 



2>Final environmental impact statement (FEIS). 1977. Con- 

 sideration of a waiver of the moratorium and return of manage- 

 ment of certain marine mammals to the State of Alaska. Vol. II. 

 U.S. Dep. Commer. and U.S. Dep. Inter., Interagency Task Group, 

 Wash., D.C., 251 p. 



22Matkin, C. 0., and F. H. Fay. 1980. Marine mammal-fishery 

 interactions on the Copper River and in Prince William Sound, 

 Alaska, 1978. Final rep. for contract MMC-78/07 to Mar. Mam- 

 mal Comm., 71 p. Available from National Technical Information 

 Service, U.S. Department of Commerce, Springfield, VA 22161, 

 as PB80-159536. 



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