DIET OF PACIFIC COD, GADUS MACROCEPHALUS, AND 

 PREDATION ON THE NORTHERN PINK SHRIMP, 

 PANDALUS BOREALIS, IN PAVLOF BAY, ALASKA 



W. D. Albers and p. J. Anderson' 



ABSTRACT 



Analysis of 455 Pacific cod, Gadus macrocephalus, stomachs collected in 1980 and 1981 from Pavlof Bay, 

 in the western Gulf of Alaska, showed considerable predation on northern pink shimp, Pandalus borealis. 

 The most frequently occurring prey items were pink shrimp, P. borealis, 63%; euphausids, 41%; walleye 

 pollock, Theragra chalcogramma, 27%; and capelin, Mallotus villosus, 26%. Pandalid shrimp and snow 

 (Tknner) crab occurred more frequently with increasing cod size (30-69 cm fork length). Euphausids 

 decreased in frequency of occurrence with increasing cod size Pink shrimp length distributions from cod 

 stomachs and trawl samples were similar. Estimated consumption of pink shrimp by cod in Pavlof Bay 

 ranged from 142 to 857 t over a 112-day period from late May through mid-September 1981. Cod preda- 

 tion may be one reason for the failure of the pink shrimp stock to rebuild in Pavlof Bay following closure 

 of the commercial fishery in 1979. Cod predation may also play a role in keeping other reduced pink shrimp 

 stocks in the western Gulf of Alaska from rebuilding to former levels. 



) 



Pacific cod, Gadus macrocephalus, predation on 

 northern pink shrimp, Pandalus borealis, in Pavlof 

 Bay (Fig. 1) was studied to determine if it is a fac- 

 tor in keeping the pink shrimp stock from rebuilding 

 thera National Marine Fisheries Service (NMFS) 

 and Alaska Department of Fish and Game (ADF&G) 

 survey data from the late 1970's indicate that when 

 pink shrimp populations in regions of western Alaska 

 began to decrease, cod abundance started to in- 

 crease Pink shrimp has been reported to be an im- 

 portant food item in the diet of Pacific cod in the 

 Gulf of Alaska (Jewett 1978; Hunter 1979). Preda- 

 tion of pink shrimp by cod may have substantial in- 

 fluence on shrimp stock abundance 



Pavlof Bay was chosen as the study area because 

 it supported a commercial fishery for pandalid 

 shrimp in the 1970's and is suspected to contain a 

 geographically isolated stock of pink shrimp (Ander- 

 son 1981). From 1972 through 1979, 13,641 1 of pink 

 shrimp were commercially harvested from Pavlof 

 Bay (calculated from ADF&G commercial catch data 

 and NMFS survey data). Survey data from Pavlof 

 Bay indicate that in 1977 and 1978 when pink shrimp 

 abundance began decreasing, cod abundance began 

 increasing (Fig. 2). Following the 1979 season the 

 bay was closed to commercial shrimping due to 

 depressed shrimp abundance levels which remain- 

 ed low through 1983. 



This report presents data which suggest that 

 Pacific cod predation is a factor in keeping shrimp 

 stocks from rebuilding. The summer diet of cod, prey 

 size selectivity, and an estimate of pink shrimp 

 biomass consumed by cod in Pavlof Bay during a 

 112-d period from late May through mid-September 

 1981 are discussed. 



MATERIALS AND METHODS 



Pacific cod were collected from 31 tows during 

 three trawl surveys. The first collection was done by 

 NMFS on 25-26 August 1980, the second by ADF&G 

 on 23-25 May 1981, and the third by NMFS on 10-11 

 September 1981. The collecting was done during 

 daylight hours over a period of about 14 h a day. All 

 three surveys used a high-opening shrimp trawl with 

 an 18.6 m headrope and footrope described by 

 Wathne (1977). Mesh size of the trawl is 32 mm and 

 path-width is about 10 m. Each tow was about 1.8 

 km in length. Randomly selected sampling locations 

 were restricted to depths >55 m since previous 

 surveys showed that neither shrimp nor cod were 

 found in abundance in shallow water. Both shrimp 

 and cod are uniformly distributed at depths >55 m 

 in Pavlof Bay. 



When possible, five stomachs per 5 cm interval of 

 fork length (FL) were removed from every trawl 

 catch and preserved in 10% Formalin^. In the 



'Northwest and Alaska Fisheries Center, National Marine 

 Fisheries Service, NOAA, 2725 Montlake Blvd. E., Seattle, WA 

 98112. 



^Reference to trade names does not imply endorsement by the 

 National Marine Fisheries Service, NOAA. 



Manuscript accepted January 1985. 

 FISHERY BULLETIN: VOL. 83, NO. 4, 1985. 



601 



