LIVINGSTON: PACIFIC COD PREDATION ON THREE CRAB SPECIES 



J I , 1 ^1 1 I I . 1 1^ 



-60° N 



I I 1 1 I 1 1 ^ 



180° W 



175° 



170° 



165° 



160° 



Figure 1. — Haul locations where Pacific cod, Gadus macrocephalus, stomach samples were taken during 1981, 1984, and 1985 in 



the eastern Bering Sea. 



significance tests of the relationship between 

 levels of each factor, so that a description of 

 which predator sizes and sampling years had 

 higher occurrences of crab predation could be 

 presented. 



Estimates of the total amount of each crab 

 species consumed by the Pacific cod population 

 during the sampling period for each year was 

 calculated according to Mehl and Westgard 

 (1983): 



Ci = DR, -D- Br Pi 



(1) 



where C , is the consumption (by weight) of crab 

 by cod belonging to size gi-oup (', DRf is the daily 

 ration (as a proportion of body weight daily, 

 BWD) of cod size gi'oup (', D is the number of 

 days in the sampling period of May through Sep- 



tember when crab were vulnerable to predation, 

 B, is the biomass of cod size gi'oup i, and P, is the 

 proportion (by weight) of the crab species in the 

 diet of cod size gi'oup ;'. 



The areas of crab consumption by cod were 

 derived by plotting the areal distribution of the 

 percentage by weight of each species of crab in 

 the diet for each year. A polygon encompassing 

 the area where each species of crab was con- 

 sumed was obtained for each year. The percent- 

 age by weight of a crab species in the diet of each 

 size group of cod (P) was calculated solely from 

 stomachs taken inside the crab consumption 

 area. To eliminate spatial sampling bias (i.e., 

 samples unevenly distributed within a crab con- 

 sumption area), the percentage by weight of 

 crab in the diet was calculated by taking the 

 average of the percentages for each 20 nmi wide 



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