ALBERS and ANDERSON: DIET OF PACIFIC COD 



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SEPTEMBER 198 



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10 



15 20 



Carapace Length(mm) 



25 



30 



Figure 3— Size distribution of pink shrimp from trawl samples and Pacific cod stomachs for August 1980 and May and September 

 1981. 



biomass decline of pink shrimp between May and 

 September 1981 would also be greater. 



Pacific cod are probably feeding on pink shrimp 

 in Pavlof Bay the entire year although the largest 

 concentrations of cod are likely to occur from spring 

 through fall. Trawl survey data from Pavlof Bay in- 

 dicate that cod biomass decreased from 93 t in 

 September 1978 to 20 t in February 1979 and then 

 increased to 371 t in May 1979. Pacific cod are 

 migratory; they move to shallow areas (<90 m) in 

 spring to feed and return to deeper areas (165-247 

 m) offshore in fall or winter to spawn (Moiseev 1953). 

 The majority of Pavlof Bay is <90 m deep which is 

 not preferred winter habitat. 



Although we believe that Pacific cod predation has 

 an effect on the present reduced population of pink 

 shrimp, predation probably was not the primary 

 reason for the initial decline of pink shrimp in Pavlof 

 Bay that began in 1977. At that time Pacific cod 

 abundance was just beginning to increase (Fig. 2). 

 Fishing removed about 3,819 t (calculated from 

 ADF&G commercial catch data and NMFS survey 

 data) of pink shrimp between the 1977 and 1978 

 surveys, which was 30% of the estimated available 



biomass in 1977. This harvest and the dying out of 

 the strong 1971 year class (Anderson 1981) were 

 probably responsible for most of that initial decrease 

 Cod predation did become a factor, however, once 

 the pink shrimp resource was reduced. This impact 

 on pink shrimp appears substantial despite the 

 reduction of cod in Pavlof Bay (Fig. 2). 



ADF&G (footnote 4) has reported diminishing pink 

 shrimp stocks in other areas of the western Gulf of 

 Alaska. Some areas that once contained high con- 

 centrations of pink shrimp experienced reductions 

 in abundance at the same time as Pavlof Bay. In most 

 areas, no increase in pink shrimp abundance was 

 observed through 1982, though many areas were 

 closed to fishing. Like Pavlof Bay, these other areas 

 experienced an increase in Pacific cod abundance 

 about the same time as pink shrimp populations were 

 declining. Cod predation may play a role in keeping 

 these reduced pink shrimp stocks from rebuilding 

 to former levels. 



ACKNOWLEDGMENTS 



Thanks to Pete Jackson and Dave Jackson, Alaska 



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