501 



Abstract — Survey- and fishery- 

 derived biomass estimates have 

 indicated that the harvest indices 

 for Pacific cod iGadus macrocepha- 

 lus) within a portion of Steller sea 

 lion (Eumetopias jubatus) critical 

 habitat in February and March 2001 

 were five to 16 times greater than 

 the annual rate for the entire Bering 

 Sea-Aleutian Islands stock. A bottom 

 trawl survey yielded a cod biomass 

 estimate of 49,032 metric tons (t) for 

 the entire area surveyed, of which 

 less than half (23,329 t) was located 

 within the area used primarily by 

 the commercial fishery, which caught 

 11,631 t of Pacific cod. Leslie deple- 

 tion analyses of fishery data yielded 

 biomass estimates of approximately 

 14,500 t (95% confidence intervals of 

 approximately 9,000-25,000 t), which 

 are within the 95 f> r confidence inter- 

 val on the fished area survey estimate 

 (12,846-33,812 t). These data indicate 

 that Leslie analyses may be useful 

 in estimating local fish biomass and 

 harvest indices for certain marine 

 fisheries that are well constrained 

 spatially and relatively short in dura- 

 tion (weeks). In addition, fishery 

 effects on prey availability within 

 the time and space scales relevant 

 to foraging sea lions may be much 

 greater than the effects indicated by 

 annual harvest rates estimated from 

 stock assessments averaged across the 

 range of the target species. 



Survey- and fishery-derived estimates of 

 Pacific cod (Gadus macrocephalus) biomass: 

 implications for strategies to reduce interactions 

 between groundfish fisheries and Steller sea lions 

 (Eumetopias jubatus) 



Lowell W. Fritz 



National Marine Mammal Laboratory 

 Alaska Fisheries Science Center 

 National Marine Fisheries Service 

 7600 Sand Point Way NE 

 Seattle, Washington 98115 

 E-mail address lowell.fntz@noaa gov 



Eric S. Brown 



Resource Assessment and Conservation Engineering 

 Alaska Fisheries Science Center 

 National Marine Fisheries Service 

 7600 Sand Point Way NE 

 Seattle, Washington 98115 



Manuscript submitted 20 May 2004 to 

 the Scientific Editor's Office. 



Manuscript approved for publication 

 23 March 2005 by the Scientific Editor. 



Fish. Bull. 103:501-515 (20051. 



For the past 30 years, the Steller sea 

 lion (Eumetopias jubatus) popula- 

 tion in western Alaska has declined 

 (Braham et al., 1980; Sease and Gud- 

 mundson 1 ). The species was listed as 

 threatened under the U.S. Endangered 

 Species Act (ESA) in 1990 after evi- 

 dence of a major decline in abundance 

 in the core of its range from the Kenai 

 Peninsula in south-central Alaska to 

 Kiska Island in the western Aleutian 

 Islands (Braham et al., 1980; Merrick 

 et al., 1987). After the decline was 

 first observed in the eastern Aleutian 

 Islands in the early 1970s (Braham 

 et al., 1980), it spread eastward to 

 Prince William Sound and west- 

 ward through Russia during the next 

 decade (Merrick et al., 1987; Loughlin 

 et al., 1992). From the early 1970s 

 to 1990, counts of adult and juvenile 

 Steller sea lions declined by over 70%, 

 but annual rates of decline were most 

 severe between 1985 and 1989 (-15%/ 

 yr; Loughlin et al., 1992). During the 

 1990s, the decline slowed to approxi- 

 mately -5%/yr and may have tempo- 

 rarily abated in many areas by 2002 

 (Sease and Gudmundson 1 ). 



Understanding the causes for the 

 decline and lack of recovery in the 

 Steller sea lion population has large- 

 ly eluded scientists and managers. 



despite the millions of dollars spent 

 on scientific research (Ferrero and 

 Fritz 2 ) and numerous reviews by aca- 

 demic (Alaska Sea Grant 3 ; DeMaster 

 and Atkinson 4 ; NRC, 1996; 2003) and 

 governmental panels (Kruse et al. 5 ; 

 NMFS 6 ' 7 ' 8 - 9 ). Although recent reviews 



1 Sease, J. L., and C. J. Gudmundson. 

 2002. Aerial and land-based surveys 

 of Steller sea lions (Eumetopias jubatus) 

 from the western stock in Alaska, June 

 and July 2001 and 2002. NOAA Tech. 

 Memo. NMFS-AFSC-131, 45 p. Alaska 

 Fisheries Science Center, 7600 Sand 

 Point Way NE, Seattle WA 98115. 



2 Ferrero, R. C. and L. W. Fritz. 2002. 

 Steller sea lion research coordination: 

 a brief history and summary of recent 

 progress. NOAA Tech. Memo. NMFS- 

 AFSC-129, 34 p. Alaska Fisheries Sci- 

 ence Center, 7600 Sand Point Way NE, 

 Seattle WA 98115. 



3 Alaska Sea Grant. 1993. Is it food?: 

 Addressing marine mammal and sea- 

 bird declines. Workshop summary 

 rep. AK-SG-93-01, 59 p. Univ. Alaska 

 Fairbanks, Alaska Sea Grant College 

 Program, Fairbanks AK 99775. 



4 DeMaster, D., and S. Atkinson, (eds.l. 

 2002. Steller sea lion decline: Is it food? 

 II. Workshop summary, rep. AK-SG- 

 02-02, 80 p. Univ. Alaska Fairbanks, 

 Alaska Sea Grant College Program, 

 Fairbanks AK 99775. 



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