Interannual Trends in Pacific Cod, Gadus 

 macrocephalus, Predation on Three Commercially 

 Important Crab Species in the Eastern Bering Sea 



Patricia A. Livingston 



ABSTRACT: Pacific cod. Gadiis macrocephalus, 

 food habits data from 1981. 1981. and 1985 in the 

 eastern Bering Sea were analyzed to determine in- 

 terannual trends in consumption of three commer- 

 cially important species of crabs: the red king crab. 

 Paralithodes camtschatica, and two species of snow 

 crabs. Chionoecetes bairdi and ('. opilio. Soft-shell 

 female red king crab were consumed during spring 

 in Bristol Bay. Estimates of percentages of female 

 red king crab standing stock consumed by the Pa- 

 cific cod population were 3.8%. 2.8% and 1.4% in the 

 respective sampling years of 1981. 1984. and 1985. 

 This implied that Pacific cod were not the major 

 force behind the observed decline in numbers of 

 female red king crab in the population from 1981 to 

 1985. Predation mortality of C. bairdi by cod was 

 estimated to be about 84%, 95%. and 94% of the 

 population of age 1 crab during 1981. 1984. and 

 1985. respectively. Annual predation removal of age 

 1 C. opilio was 28%. 57%, and 27%. respectively, of 

 the reconstructed population numbers of age 1 crab 

 in the .3 years sampled. Although these calculations 

 indicated that cod may consume large proportions 

 of the age 1 snow crab populations, the estimates 

 are tentative partly because of uncertainties in re- 

 constructing the population numbers of age 1 crab. 

 The results also indicate that C. bairdi are more 

 vulnerable to cod predation because of their high 

 spatial overlap with cod populations, whereas an 

 unknown fraction of the C. opilio juvenile popula- 

 tion is north of the main suney area and does not 

 overlap with cod. Results from this study suggest 

 that predation by cod may be an important factor 

 influencing survival of ages 1 and 2 snow crab in the 

 eastern Bering Sea. 



Pacific cod, Gadus macrocephaliis, biomass in 

 the eastern Bering Sea has increased by about 

 800,000 t over the last decade primarily because 

 of two strong year classes spawned in 1978 and 

 1979. Stock biomass levels have been about 1 

 million metric tons since 1982 and a growing 

 domestic fishery for this species is responsible 



Patricia A. Livingston, Alaska Fisheries Science Center, 

 National Marine Fisheries Service, NOAA, 7600 Sand Point 

 Way NE, Seattle, WA 98115. 



for most of the catch (Thompson and Shimada 

 1987). Because Pacific cod are documented 

 predators of soft-shell red king crab, Paralith- 

 odes camtschatica, and juvenile snow crab, 

 Chionoecetes opilio and C. bairdi, (Mito 1974; 

 Feder 1977; Jewett 1978; Blau 1986; Livingston 

 et al. 1986), there has been an increase in spec- 

 ulation linking the decline of crab stocks with the 

 increase in Pacific cod population size. The 

 eastern Bering Sea red king crab population has 

 decreased by an order of magnitude from a maxi- 

 mum of 365 million crab in 1977. Otto (1986) 

 suggested reasons for the decline including weak 

 year class production and large increases in na- 

 tural mortality, which might be attributed to 

 predation by Pacific cod, disease, or incidental 

 catch in trawl fisheries. Pacific cod may also be 

 implicated in the decline of red king crab abun- 

 dance in the Kodiak region of the Gulf of Alaska 

 (Blau 1986) and in the disappearance of certain 

 year classes of C. opilio snow crab in the eastern 

 Bering Sea (Incze and Schumacher 1986). Al- 

 though circumstantial evidence has implicated 

 Pacific cod predation in these crab population 

 declines, direct evidence is so far lacking. 



The regulation of prey population size by a 

 predator requires that prey mortality rate in- 

 crease with prey population size (direct density- 

 dependent mortality. Holling 1959). Thus ex- 

 amination of changes in Pacific cod diet with 

 changes in crab population abundance is neces- 

 sary to determine whether the rate of cod preda- 

 tion on crab changes when crab abundance 

 changes. Total removals of crab by cod need to 

 be estimated and compared with crab population 

 size. If jjercentage of removal of crab by cod 

 alters with changes in crab density, then cod 

 predation is a likely density-dependent factor 

 regulating crab population size. 



I examined the interannual trends in Pacific 

 cod predation on three commercially important 

 crab species in the estern Bering Sea: red king 

 crab, C. opilio, and C. bairdi. Three years of 

 Pacific cod food habits data from 1981. 1984, and 

 1985 were analvzed to determine 1) the areas 



Manuscript accepted March 1989. 

 Fishery Bulletin. U.S. 87: 807-827. 



807 



