FOOD OF WALLEYE POLLOCK, 



THERAGRA CHALCOGRAMMA, IN AN 



EMBAYMENT OF SOUTHEASTERN 



ALASKA 



The walleye pollock, Theragra chalcogramma Pallas, 

 is commercially and ecologically one of the most im- 

 portant fishes in Alaskan waters. In recent years, it has 

 predominated by weight in the catches of commercial 

 groundfish and in demersal trawling surveys of the 

 Bering Sea and the Gulf of Alaska (North Pacific 

 Fishery Management Council 1979a, b; Pereyra et al. 

 1976 1 ; Ronholt et al. 1978 : ). It is similarly abundant 

 in the inside waters of southeastern Alaska (Carlson 

 et al. 1977 3 ), where recent attempts have been made 

 to establish a commercial fishery for walleye pollock. 

 Walleye pollock are also an important component of 

 the food web, primarily as forage for seabirds, marine 

 mammals, and fish. In the eastern Bering Sea, 

 walleye pollock is the most important species in the 

 diet of many seabirds (Hunt et al. 1981) and is a ma- 

 jor food of seals, Phoca spp. (Lowry and Frost 1981); 

 whales (Frost and Lowry 1981); and northern fur 

 seals, Callorhinus ursinus (Harry and Hartley 1981). 

 In southeastern Alaska, juvenile walleye pollock are 

 one of the most common foods of troll-caught Pacific 

 salmon, Oncorhynchus spp. (Wing 1977). 



Despite the importance of walleye pollock, their 

 diet has been little studied. The food of adults and 

 juveniles from the eastern Bering Sea has been inves- 

 tigated during the spring, summer, or fall (Takahashi 

 and Yamaguchi 1972; Mito 1974; Bailey and Dunn 

 1979), but no single study covered more than one 

 season in a given area. There are no published data 

 on the food of walleye pollock in the eastern North 

 Pacific Ocean south of the Bering Sea except for one 

 report of their feeding upon salmon fry in southeast- 

 ern Alaska (Armstrong and Winslow 1968). In this 

 report, I document the foods and seasonal changes in 



the diet of walleye pollock for 1 yr in an area of the in- 

 side waters of southeastern Alaska. 



Methods 



During each of the four seasons, stomachs were 

 collected from walleye pollock in two adjacent bays, 

 Auke Bay and Fritz Cove, near Juneau, Alaska (Ta- 

 ble 1; Fig. 1). The two bays make up a larger embay- 

 ment of Stephens Passage, a prominent fjord in the 

 inside waters of southeastern Alaska. Depending up- 

 on the availability of research vessels, fish were 

 caught with three types of bottom trawls: A 400- 

 mesh (27 m) Eastern otter trawl, a standard 40-ft (12 

 m) Gulf shrimp trawl, and a 12-ft (3.7 m) balloon-type 

 otter trawl. All trawling was done during daylight. 

 The trawls were dragged from deeper to shallower 

 water during each haul, so the exact depth at which 

 fish were captured could not be determined. Trawl- 

 ing depths averaged 46 m (range 16-60 m) in Auke 

 Bay and 90 m (range 55-110 m) in Fritz Cove. It was 

 necessary to collect walleye pollock from both bays to 

 obtain a broad size range offish in each season. When 

 a single tow produced large numbers of walleye 

 pollock, the catch was arbitrarily sampled to obtain 

 about 40 fish in each 100 mm size category; other- 

 wise, all walleye pollock were retained. Standard 

 length (SL) of each retained fish was measured, sex 

 determined when possible, and stomachs removed 

 and preserved in 5% buffered Formalin 4 . Walleye 

 pollock used for the stomach samples were mostly 

 between 150 and 450 mm SL, and ranged from 106 to 

 585 mm SL. Most of these fish were probably 

 juveniles or young adults because walleye pollock 

 mature at lengths between 290 and 350 mm FL (fork 

 length) (Hughes and Hirschhorn 1979). 



A number of authors have discussed various tech- 

 niques for analyzing stomach contents of fish (see 

 Windell 1971), and it appears that any single method 



'Pereyra, W. T., J. E. Reeves, and R. G. Bakkala. 1976. Demersal 

 fish and shellfish resources of the eastern Bering Sea in the baseline 

 year 1975. Processed rep., 619 p. Northwest and Alaska Fisheries 

 Center, National Marine Fisheries Service, NOAA, 2725 Montlake 

 Boulevard E., Seattle, WA 98112. 



2 Ronholt, L. L., H. H. Shippen, and E. S. Brown. 1978. Demersal 

 fish and shellfish resources of the Gulf of Alaska from Cape Spencer 

 to Unimak Pass 1948-1976, a historical review. Processed rep., 4 

 vols., 955 p. Northwest and Alaska Fisheries Center, National 

 Marine Fisheries Service, NOAA, 2725 Montlake Boulevard E., 

 Seattle, WA 981 12. 



'Carlson, H. R., R. E. Haight, and K. J. Krieger. 1977. Species com- 

 position and relative abundance of demersal marine life in waters of 

 southeastern Alaska, 1969-77. Processed rep., 69 p. Northwest and 

 Alaska Fisheries Center, National Marine Fisheries Service, 2725 

 Montlake Boulevard E., Seattle, WA 98112. 



4 Reference to trade names does not imply endorsement by the 

 National Marine Fisheries Service, NOAA. 



TABLE 1. — Summary of walleye pollock stomach samples collected 

 each season, by date and location of collection, Auke Bay and Fritz 

 Cove, southeastern Alaska, 1979-80. 



Season 



Dates of 

 collection 



Number 

 of trawl 

 hauls Auke Bay 



Number of stomachs collected 

 Fritz Cove Total 



FISHERY BULLETIN: VOL. 81, NO. 3, 1983 



637 



