160 



Fishery Bulletin 101(1) 



Table 3 



Biomass (t), natural mortality, fishery catches (t), and predicted Steller sea lion consumption of gadids (walleye pollock and Pacific 

 cod) and hexagrammids (Atka mackerel) in Alaska in 1998. 



Sum of estimated exploitable biomass from Gulf of Alaska (pollock and cod only) (Plan Team for the Groundfish Fisheries of the Gulf of Alaska. 

 1999. Summary. In Stock assessment and fishery evaluation report for the groundfish resources of the Gulf of Alaska, p. 1-31. North Pacific Fishery 

 Management Council, P.O. Box 103136, Anchorage, AK 99510), Aleutian Islands, Bogoslof (pollock only), and eastern Bering Sea (pollock and cod 

 only) regions (Plan Team for the Groundfish Fisheries of the Bering Sea and Aleutian Islands. 1999. Summary. In Stock assessment and fishery 

 evaluation report for the groundfish resources of the Bering Sea/Aleutian Islands regions, p. 1-36. North Pacific Fishery Management Council, P.O. 

 Box 103136. Anchorage, AK 99510). Estimate of exploitable biomass of Atka mackerel in the Gulf of Alaska not available, but the population was 

 much smaller than the Aleutian Islands population. 



Dorn, M. W., A. B. Hollowed, E. Brown, B. Megrey, C. Wilson, and J. Blackburn. 1999. Walleye pollock. In Stock assessment and fishery evaluation 

 report for the groundfish resources of the Gulf of Alaska, p. 33-104. North Pacific Fishery Management Council, P.O. Box 103136, Anchorage. AK 

 99510 



Thompson, G. G., H. H. Zenger and M. W. Dorn. 1999. Assessment of the Pacific cod stock in the Gulf of Alaska. In Stock 'kssessment and fishery 



evaluation report for the groundfish resources of the Gulf of Alaska, p. 105-184. North Pacific Fisherv Management Council, P.O. Box 103136, 



Anchorage, AK 99510. 



Lowe. S. A., and L. W. Fritz. 1999. Assessment of Bering Sea/Aleutian Islands Atka mackerel. In Stock assessment and fishery evaluation report 



for the groundfish re.sources of the Bering Sea/Aleutian Islands regions, p. 569-638. North Pacific Fishery Management Council. P.O. Box 103136. 



Anchorage. AK 99510. 



Annual mortality rate = 1 -e"■^^ 



Assumed to be \&'7c of total biomass based on value used by Trites et al. (1999) for pollock. 



Assumed to be the median M reported by Wespestad and Terry (1984) for 1- and 2-year-old pollock (range=0.45-0.85). 



Sum of catches from Gulf of Alaska (Plan Team for the Groundfish Fisheries of the Gulf of Alaska, see Footnote 1 above), Aleutian Islands, Bogoslof 



(pollock only), and eastern Bering Sea (pollock and cod only) regions (Plan Team for the Groundfish Fisheries of the Bering Sea and Aleutian Islands, 



see Footnote 1 above). 



there may only be minor overlap between the fish taken 

 by humans and the fish taken by Steller sea lions even 

 though our estimate of the gadid biomass consumed by 

 Steller sea lions in Alaska in 1998 was 12'7( of the com- 

 bined pollock and cod catch (Table 3). 



Caution should also be used when making inferences 

 about competition and prey availability even when esti- 

 mates of prey biomass and catch are size-specific. Spatial 

 and temporal distributions of prey (and fishing) at the 

 local scale determine the availability of food resources 

 for Steller sea lions. Estimates of total prey abundance 

 are not enough to make inferences about the food that 

 is available to Steller sea lions. For example, if estimates 



of the amount of food that Steller sea lions require were 

 less than the estimated available prey biomass (minus the 

 prey taken by fisheries), it would not necessarily mean 

 that Steller sea lions had enough to eat. Sea lions may 

 not have access to all of the prey due to local differences 

 between their foraging space and time and the spatial 

 and temporal distribution of the fish. Local prey densities 

 encountered by Steller sea lions are more relevant than 

 absolute abundance when assessing prey availability. 



Our study provides the first estimates of the biomass of 

 prey consumed by Steller sea lions in different regions of 

 Alaska. However, our estimates of prey consumption are 

 neither species-specific nor size-specific and have con- 



