574 



Abstract — Aspects of the feeding 

 migration of walleye pollock iTher- 

 agra ehalcogramma) in the eastern 

 Bering Sea (EBS) were investigated 

 by examining the relationship be- 

 tween temperatures and densities 

 of fish encountered during acoustic 

 and bottom trawl surveys conducted 

 in spring and summer between 1982 

 and 2001. Bottom temperature was 

 used as an indicator of spring and 

 summer warming of the EBS. Clus- 

 ters of survey stations were identified 

 where the density of walleye pollock 

 generally increased or decreased with 

 increasing water temperature. Infer- 

 ences about the direction and magni- 

 tude of the spring and summer feeding 

 migration were made for five length 

 categories of walleye pollock. Gener- 

 ally, feeding migrations appeared to 

 be northward and shoreward, and the 

 magnitude of this migration appeared 

 to increase with walleye pollock size 

 up to 50 cm. Pollock larger then 50 cm 

 showed limited migratory behavior. 

 Pollock may benefit from northward 

 feeding migrations because of the 

 changes in temperature, zooplank- 

 ton production, and light conditions. 

 Ongoing climate changes may affect 

 pollock distribution and create new 

 challenges for pollock management 

 in the EBS. 



Variation in the distribution of walleye pollock 

 (Theragra ehalcogramma) with temperature 

 and implications for seasonal migration 



Stan Kotwicki 



Troy W. Buckley 



Taina Honkalehto 



Gary Walters 



Resource Assessment and Conservation Engineering Division 



Alaska Fisheries Science Center 



National Marine Fisheries Service. NOAA 



7600 Sand Point Way NE 



Seattle, Washington 981 1 5-6349 



E-mail (for 5 Kotwicki) Stan kotwicki'5'noaa gov 



Manuscript submitted 20 November 2004 

 to the Scientific Editor's Office. 



Manuscript approved for publication 

 30 March 2005 by the Scientific Editor. 



Fish. Bull 103:574-587 (20051. 



Walleye pollock (Theragra ehalco- 

 gramma; referred to as "pollock" in 

 this article) migrate seasonally. Such 

 migrations have been described for the 

 northern Sea of Japan (Maeda, 1986; 

 Maeda et al., 1988, 1989; Kooka et 

 al., 1998), Korean waters (Shuntov et 

 al., 1993), the Okhotsk Sea (Shuntov 

 et al., 1987), and the western and 

 central Bering Sea (Fadeyev, 1989; 

 Bulatov and Sobolevskiy, 1990; Efim- 

 kin, 1991; Radchenko and Sobolevskiy, 

 1993; Shuntov et al., 1993; Balykin, 

 1996). Generally, these authors have 

 described a spring and summer 

 migration from spawning grounds to 

 forage areas (referred to as a "feed- 

 ing migrations" by many authors) 

 and a winter migration of pollock 

 returning to spawning grounds (e.g., 

 Maeda et al., 1988; Radchenko and 

 Sobolevskiy, 1993). This pattern of 

 migration is believed to occur in the 

 eastern Bering Sea (EBS) where it 

 has received considerable attention 

 (Takahashi and Yamaguchi, 1972 

 Francis and Bailey, 1983; Pola, 1985 

 Shuntov, 1992; Shuntov et al., 1993 

 Stepanenko, 2001), but the evidence 

 for this pattern of migration is sparse. 

 In addition, there is a lack of infor- 

 mation on the magnitude of, routes 

 of, and size-dependent differences in 

 seasonal migrations. 



Temperature (and other factors 

 closely related to temperature) af- 

 fects the distribution and movements 

 of pollock. Pola (1985) simulated 



temperature-induced migrations of 

 pollock in the EBS occurring dur- 

 ing May and June. Pollock appear to 

 avoid some temperatures (Swartzman 

 et al., 1994) and prefer environmen- 

 tal conditions that are linked to food 

 availability associated with tempera- 

 ture gradients and fronts along the 

 EBS slope (Swartzman et al., 1995). 

 Water temperature is an especially 

 important indicator of the transi- 

 tion from winter conditions to those 

 supporting a spring bloom of phyto- 

 plankton and then zooplankton. In 

 the EBS. the simulated onset of the 

 feeding migration of pollock was de- 

 layed in colder years (Pola, 1985). 



Annual surveys documenting the 

 spatial distribution of fishes in re- 

 lation to water temperatures can be 

 used to infer details about their mi- 

 gratory behavior. Using annual sur- 

 vey data, Mountain and Murawski 

 (1992) found that the relationship 

 between the distribution of season- 

 ally migrating species and water 

 temperature could indicate a change 

 in the overwintering location of the 

 fish, or a change in the timing of the 

 spring migration, or both. In the east- 

 ern Bering Sea, bottom trawl (BT) 

 surveys and echo-integration-trawl 

 (EIT) surveys are conducted in late 

 spring and summer (Honkalehto et 

 al 1 ; Acuna et al. 2 ), when water tem- 



• 2 See next page for footnote texts. 



