LIVINGSTON: PACIFIC COD PREDATION ON THREE CRAB SPECIES 



Table 3. — Total number (A/) and proportion of female 

 snow crabs (PF), Chionoecetes bairdi and C. opilio, 

 in Pacific cod stomachs and resource assessment 

 trawl surveys for tfie years 1981, 1984, and 1985 in 

 tfie eastern Bering Sea. 



'Sex ratios and number of crab sexed provided by Re- 

 source Assessment and Conservation Engineenng Division, 

 Kodiak Laboratory for crab <95 mm CW 



^Crab sex was not consistently recorded by stomach 

 analysts dunng this year 



^Observed proportion was not significantly different from 

 0.5 (P > 0.05) using the normal approximation to the bi- 

 nomial test. 



achs, the proportions of females for both species 

 of snow crabs during 1981 were much larger than 

 other years. In 1984 and 1985, the proportions of 

 females in cod stomachs were fairly close to 0.5, 

 although in the normal appro.ximation to the bi- 

 nomial test (Zar 1974) only the proportions in 

 1984 were not significantly different (P > 0.05) 

 from 0.5; a test of an even sex ratio. Trawl survey 

 estimates of the proportion of female snow crab 

 <95 mm CW range from 0.43 to 0.67. In some 

 cases the proportion of females observed in 

 stomach contents differed from the proportion 

 estimated from the trawl survey. No trends seem 

 readily apparent except that trawl survey 

 estimated proportions, and those proportions in 

 Pacific cod stomach contents were more similar in 

 1984 and 1985 than during 1981. 



Pacific Cod Population Consumption of 

 Crab 



Daily ration estimates range from about 0.5 to 

 0.9% BWD with small cod consuming larger ra- 

 tions as a percentage of their body weight than 

 large cod in a given year (Table 4). Rations for 

 both cod groups were smaller in 1985 than in 

 1981 and 1984 because of the lower bottom tem- 

 perature and the large individual prey weight for 

 large cod in that year. 



The parameters necessary for calculating pop- 

 ulation consumption are the cod biomass in the 

 prey area, the percentages by weight of each 

 crab species in the diet, and the number of days 

 the crab species is vulnerable to cod predation 

 (Tables 5-7). The calculations for the total 

 amount of red king crab consumed by cod as- 

 sume that only female crabs are consumed, and 

 the period of predation vulnerability during the 

 sampling period is 30 days in May when female 

 crabs are in the soft-shell condition. Further, 

 because of the small number of crabs measured 

 from stomach contents in each year, the average 

 carapace length of all red king crab measured in 

 a year was used to calculate the total number of 

 crab consumed (1981 = 104 mm CL and 1984-85 

 = 111 mm CL; corresponding to about age 6 for 

 females). 



Both total biomass and numbers of red king 

 crab consumed by cod dechned from 1981 to 1985 

 by a factor of 10 (Table 5). The total amount of C. 

 bairdi consumed by cod during the 153 d sam- 

 pling period in each year, in terms of total 

 weight and numbers, decreased slightly from 

 1981 to 1984 and increased about threefold from 

 1984 to 1985 (Table 6). Although the total bio- 

 mass of C. opilio consumed by cod dropped dur- 



Table 4. — Parameters used to derive daily ration estimates and the 

 estimated daily rations tor two size groups of Pacific cod in 1981, 

 1 984, and 1 985 in the eastern Bering Sea. (% BWD is percentage of 

 body weight daily.) 



817 



