Fritz and Brown: Interactions between the Pacific cod fishery and Steller sea lions 



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Figure 1 



The four areas (high and low sampling-effort survey areas, the area east of the survey area, and 

 the area south of the survey area) in the southeastern Bering Sea that were surveyed in February- 

 March 2001 for groundfish with a bottom trawl and used for analysis of Pacific cod iGadus mac- 

 rocephalus) fishery data. Steller sea lion {Eumetopias jubatus) critical habitat is also shown. 



In this article, estimates of Pacific cod biomass from 

 Leslie depletion analyses of fishery data are compared 

 with those derived from a bottom trawl survey con- 

 ducted in the same area at the same time. These two 

 methods are independent because they use completely 

 different data to estimate the same parameter, Pacific 

 cod biomass. If they yield similar results, they would 

 support each other in the estimate of local area cod bio- 

 mass and support the use of Leslie depletion analyses 

 of data from relatively short and spatially well-defined 

 fisheries operations for making such estimates. Fur- 

 thermore, these comparisons increase our understand- 

 ing of the potential local effects of a fishery in areas 

 important for sea lion foraging and permit compari- 

 son with the results of assessments of the Pacific cod 

 stock in the entire eastern Bering Sea (Thompson and 

 Dorn, 2002). In this instance, if the change in Pacific 

 cod abundance attributable to the fisheries north of 

 Unimak Island is not greater than what would have 

 occurred if catch were evenly distributed throughout 

 the year and across the range of the stock, then it 

 could be argued that no localized depletion occurred. 

 However, if the local change in abundance is greater 

 than expected, does this constitute a localized deple- 

 tion of the species? The answer ultimately depends on 

 the extent to which the fishery negatively affects the 

 target species (e.g., by reducing recruitment) or, as 



in our case, by reducing the foraging success of sea 

 lions, which, in turn, could lead to reduced survival or 

 reproductive rates. Although we do not know what the 

 threshold levels of change in local prey densities are 

 for foraging Steller sea lions, it is first necessary to 

 determine the level of change in local abundance that 

 may be attributable to fisheries. 



There are several aspects of Pacific cod life history 

 in the eastern Bering Sea that make it difficult to use 

 fishery data and the Leslie depletion method to estimate 

 local area biomass. The most important may be that the 

 population in the area fished may not be closed over 

 the time period analyzed. Pacific cod spawn north of 

 Unimak Island in late winter but apparently arrive in 

 groups and, after spawning, leave the area and spread 

 out on the eastern Bering Sea shelf to feed during the 

 remainder of the year (Shimada and Kimura, 1994; 

 Thompson and Dorn, 2002). Seasonal emigration from 

 and immigration into spawning areas in critical habi- 

 tat, modeled with a combination of fishery and survey 

 data by NMFS scientists 10 (Fig. 2), provide a baseline 



111 NMFS. 2000. Estimation of monthly Pacific cod biomass 

 inside Steller sea lion critical habitat. In Biological opinion 

 questions, NMFS-AKC analytical team. Unpubl. manuscript, 

 112 p. Alaska Fisheries Science Center, 7600 Sand Point 

 Way NE, Seattle WA 98115. 



