502 



Fishery Bulletin 103(3) 



(Kruse et al. 5 ; DeMaster and Atkinson 4 ; NRC, 2003) 

 concluded that "top-down" forces, such as predation or 

 illegal shooting, are greater threats to recovery of the 

 Steller sea lion population, they could not eliminate 

 "bottom-up" factors from consideration. NRC (2003) 

 suggested that NMFS conduct an adaptive manage- 

 ment experiment to determine the magnitude of one 

 such "bottom-up" force, nutritional stress resulting from 

 competition with fisheries for prey (NMFS 67 - 89 ; NRC. 

 2003). The North Pacific is home to some of the largest 

 fisheries in the world, particularly those for groundfish 

 such as Pacific cod (Gadus macrocephalus) and walleye 

 pollock (Theragra chalcogramma). Steller sea lions eat 

 a wide variety offish and cephalopods, including Pacific 

 cod, walleye pollock, Atka mackerel (Pleurogrammus 

 monopterygius), arrowtooth flounder (Atherestes sto- 

 rnias), salmon (Oncorhynehus spp.), herring (Clupea pal- 

 lasi), capelin (Mallotus villosus), eulachon (Thaleichthys 

 pacificus), sand lance {Ammodytes hexapterus), squid, 

 and octopus (Sinclair and Zeppelin, 2002). A large pro- 

 portion of their diet, however, is composed of semide- 

 mersal or pelagic schooling fish, particularly fish in 

 spawning migrations or aggregations nearshore. These 

 same species are often targeted at the same time and 

 in the same areas by groundfish fisheries, particularly 

 those fisheries that use trawl gear. Concerns about the 

 potential of fisheries to create localized depletions of 

 prey in important sea lion foraging habitats have led to 

 controversial groundfish fishery restrictions throughout 

 most of Alaska (NMFS 8 - 9 ). 



5 Kruse, G. H., M. Crow, E. E. Krygier, D. S. Lloyd, K. W. 

 Pitcher, L. D. Rea, M. Ridgway, R. J. Small, J. Stinson and 

 K.M.Wynne. 2001. A review of proposed fishery manage- 

 ment actions and the decline of Steller sea lions lEumetopias 

 jubatus) in Alaska: a report by the Alaska Steller sea lion 

 restoration team. Regional information report 5J01-04, 

 106 p. Alaska Dep. Fish and Game, P.O. Box 25526. Juneau 

 AK 99802. 



H NMFS (National Marine Fisheries Service). 1998. En- 

 dangered Species Act Section 7 Consultation on an Atka 

 mackerel fishery under the BSAI groundfish FMP between 

 1999 and 2002; authorization of a walleye pollock fishery 

 under the BSAI FMP between 1999 and 2002; and under the 

 GOA FMP between 1999 and 2002, 189 p. NMFS Protected 

 Resources Division, Alaska Region, P.O. Box 21668, Juneau, 

 AK 99802. 



7 NMFS. 2000. Endangered Species Act. Section 7: Con- 

 sultation, biological opinion and incidental take statement 

 on the authorization of the Bering Sea-Aleutian Islands and 

 Gulf of Alaska groundfish fisheries based on the Fishery 

 Management Plans, 352 p. NMFS Protected Resources Divi- 

 sion, Alaska Region, P.O. Box 21668, Juneau. AK 99802. 



8 NMFS. 2001. Endangered Species Act. Section 7: Con- 

 sultation, biological opinion and incidental take statement 

 on the authorization of the Bering Sea-Aleutian Islands and 

 Gulf of Alaska groundfish fisheries based on the Fishery 

 Management Plans as modified by Amendments 61 and 70, 

 206 p. NMFS Protected Resources Division, Alaska Region, 

 P.O. Box 21668, Juneau, AK 99802. 



9 NMFS. 2003. Supplement to the Endangered Species Act. 

 Section 7: Consultation, biological opinion and incidental 

 take statement of October 2001, 179 p. NMFS Protected 

 Resources Division, Alaska Region, P.O. Box 21668, Juneau, 

 AK 99802. 



Assessment models and fisheries harvest strategies 

 have determined the overall fishing mortality rate that 

 can be allowed for the stock and the amount of biomass 

 that can be removed. In practice, however, catches are 

 not uniformly distributed across the range of the as- 

 sessed stock nor are they distributed equally through- 

 out the year. Although there is evidence that the Atka 

 mackerel trawl fishery has created localized depletions 

 of its target species (NMFS 6 Lowe and Fritz, 1997; 

 NRC, 2003), this finding has not been generally applied 

 to fisheries for other sea lion prey. Trawl fisheries in 

 the Aleutian Islands may have, in certain instances, 

 reduced local abundances of Atka mackerel by as much 

 as 90% (Lowe and Fritz, 1997). Atka mackerel and its 

 fishery have characteristics that permitted analysis of 

 fishery data in this way. The species does not possess 

 a swim bladder and thus makes a poor acoustic target. 

 As a consequence, the Atka mackerel fishery does not 

 target on an acoustic signal, but instead trawls in ar- 

 eas where the species is known to congregate. Through 

 the analysis of time series of catch and effort statis- 

 tics from local fisheries with Leslie's equation (Ricker, 

 1975; Hilborn and Walters, 1992; Gunderson, 1993), 

 estimates of the initial abundance of Atka mackerel 

 (prefishery) and its catchability (proportion of the stock 

 caught with one unit of effort) were made within the 

 context of certain assumptions, which included the 

 following: 1) the population being fished is closed, or 

 alternatively that immigration and growth are equal 

 to emigration plus natural mortality, 2) catchability 

 over the course of the fishery remains constant, and 

 3) changes in catch per unit of effort (CPUE) are di- 

 rectly related to changes in fish density. These assump- 

 tions may be met for marine species if the area fished 

 is well defined (e.g., is surrounded by habitat that is 

 unsuitable for the species), the duration of the fishing 

 season is relatively short, or the species is relatively 

 sedentary (Polovina, 1986; Ralston, 1986; Joll and 

 Penn, 1990; Miller and Mohn, 1993). Although they 

 indicate that fisheries have created local depletions 

 of Atka mackerel, these models are difficult to apply 

 to other North Pacific fisheries because of a lack of 

 fishery-independent estimates of biomass and by cir- 

 cumstances unique to the Atka mackerel fishery (e.g., 

 the fishery trawls in areas where the species is known 

 to congregate rather than uses acoustic signal, Atka 

 mackerel are patchily distributed, and patches are 

 separated by areas with low fish density). 



To obtain information on the winter distribution of 

 groundfish in areas used by foraging Steller sea lions 

 and groundfish fisheries, the Alaska Fisheries Science 

 Center of the National Marine Fisheries Service con- 

 ducted a bottom trawl survey for groundfish in the 

 southeastern Bering Sea north of Unimak Island in 

 February-March 2001 (Fig. 1). This area is important 

 to the Pacific cod fishery in winter because cod ag- 

 gregate in this area to spawn (Shimada and Kimura, 

 1994). It is also recognized as an important foraging 

 area for Steller sea lions because it is designated as 

 critical habitat under the ESA (NMFS 7 - 8 ). 



