SIZES OF WALLEYE POLLOCK, 

 THERAGRA CHALCOGRAMMA, CONSUMED 

 BY MARINE MAMMALS IN THE BERING SEA 



In the Bering Sea at least 11 species of marine mam- 

 mals, 13 seabirds, and 10 fishes are known to feed 

 on walleye pollock, Theragra chalcogramma (Frost 

 and Lowry 1981a). Walleye pollock are a major food 

 of most pinnipeds, particularly in the southern Ber- 

 ing Sea (Lowry and Frost 1981), and are sometimes 

 eaten by several species of baleen and toothed whales 

 (Frost and Lowry 1981b). 



In recent years, walleye pollock have been the prin- 

 cipal target species in the Bering Sea commercial 

 groundfish fishery. Annual catches have been as high 

 as 1,840,000 t in 1972 (Bakkala et al. 1981). While 

 there can be little doubt that both the fishery and 

 marine mammal predation affect pollock stocks and 

 perhaps also one another, the interactions are poorly 

 understood at present (Lowry et al. 1 ; Swartzman and 

 Harr 1983). 



An important aspect of marine mammal-fishery 

 interactions is the size composition of fishes eaten 

 in relation to that of the commercial catch. For ex- 

 ample, if a marine mammal consumes fishes smaller 

 than those taken by the fishery, the fishery would 

 be unlikely to influence availability of food to the 

 predator unless it affected recruitment. If marine 



mammals and the fishery remove fishes of similar 

 sizes, competition would be expected (IUCN 2 ). 



Stomach contents of marine mammals seldom con- 

 tain intact fishes in a condition suitable for mea- 

 suring. However, the sagittal otoliths of species such 

 as walleye pollock are easily identified (Frost 1981), 

 and equations are available that estimate the length 

 and weight of fishes from otolith lengths (Frost and 

 Lowry 1981a). We present here information on the 

 sizes of walleye pollock consumed by marine mam- 

 mals in the Bering Sea, based on otoliths from 

 gastrointestinal tracts. 



Methods 



Specimens were collected during the months of 

 March to October 1975-81, at the locations shown 

 in Table 1. With the exception of a minke whale, 

 Balaenoptera acutorostrata, which was stranded on 

 shore, all specimens were from animals collected for 

 scientific purposes. Stomachs were removed and 

 opened, and the contents gently washed on a 1 mm 

 mesh sieve. Otoliths were sorted from other ingesta 

 and identified using the descriptions of Morrow 

 (1979) and Frost (1981). Since fresh walleye pollock 

 otoliths have fine lobulations around their perimeter 

 (Frost 1981) which disappear during digestion, 

 degraded otoliths were easily detected by compari- 



'Lowry, L. F., K. J. Frost, D. G. Calkins, G. L. Swartzman, and 

 S. Hills. 1982. Feeding habits, food requirements, and status of 

 Bering Sea marine mammals. North Pac Fish. Manage Counc. 

 Doc. 19 and 19A, Anchorage, Alaska, Contract 81-4, 574 p. 



2 IUCN. 1981. Report of IUCN workshop on marine mammal- 

 fishery interactions, La Jolla, Calif., 30 March-2 April. IUCN, 

 Gland, Switzerland, 68 p. 



Table 1. — Location and dates of capture of marine mammals from which otoliths of walleye pollock 



were obtained. 



192 



FISHERY BULLETIN: VOL. 84, NO. 1, 1986. 



