Kotwicki et al Variation in the distribution of Theragra chalcogramma 



575 



peratures are generally rising 

 on the eastern Bering Sea shelf 

 (Overland et al., 1999; Stabeno 

 et al., 2001). Interannual vari- 

 ability in climatic conditions 

 and survey timing create vari- 

 ability in mean water tempera- 

 tures encountered during the 

 surveys (Acuna et al. 2 ). 



We describe the variability in 

 distribution of pollock with tem- 

 perature and propose that this 

 variability may be explained by 

 the fact that pollock migrate to 

 feeding grounds during spring 

 and summer. Temperature is 

 used in our study as an indica- 

 tor of how far into an idealized 

 seasonal warming cycle each 

 survey has occurred. Thus, the 

 distribution of pollock observed 

 in a warm year would be con- 

 sidered to be representative of 

 that seen later in a seasonal 

 warming cycle in a cold year. 

 Generally, 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 mi- 

 gratory behavior. Pollock may 

 benefit from northward feed- 

 ing migrations because of the 

 changes in temperature, zoo- 

 plankton production, and light 

 conditions. 



Materials and methods 



64 N - 



62"N 



60"N 



58' N 



56'N - 



54"N - 



52 N 



BERING SEA 



rf* °Sk 



gg *- ~ssg£ 



66 N 



64 "N 



62"N 



60N 



58"N 



56°N 



54-N 



176°W 



172°W 



168W 



164°W 



1 60"W 



Figure 1 



Locations of AFSC bottom trawl stations (dots) and echo-integration survey 

 transects (lines) in the eastern Bering Sea where walleye pollock {Theragra 

 chalcogramma) were collected during bottom trawl surveys and echo-inte- 

 gration trawl surveys in spring and summer between 1982 and 2001. 



Data used in this investigation were collected by BT 

 and EIT surveys conducted by the Alaska Fisheries 

 Science Center. 



Since 1982, BT surveys have been conducted annu- 

 ally over a standard area of the EBS, at the centers of 

 20x20 nautical-mile grids (Fig. 1). The corners of the 

 grid block were also sampled in areas surrounding St. 

 Matthew Island and the Pribilof Islands. The same 



1 Honkalehto, T., N. Williamson, and S. de Blois. 2002a. Echo 

 integration-trawl survey results for walleye pollock (Theragra 

 chalcogramma) on the Bering Sea shelf and slope during 

 summer 1999. U.S Dep. Commerce, NOAA Tech. Memo. 

 NMFS-AFSC-125,77 p. 



- Acuna, E., P. Goddard, and S. Kotwicki (compilers). 

 2003. 2002 bottom trawl survey of the eastern Bering Sea 

 continental shelf. AFSC Processed Report 2003-01, 169 p. 

 Alaska Fish. Sci. Cent., NOAA Natl. Mar. Fish. Serv., 7600 

 Sand Point Way NE, Seattle, WA 98115. 



standard trawl (83-112 eastern otter trawl) was used 

 every year (Acuna et al. 2 ) and surveys usually began in 

 late May or early June, and ended in August. Surveys 

 always began in the northeastern corner of the Bristol 

 Bay and proceeded westward. Samples were collected 

 by towing for 30 minutes at 1.54 m/s (intended speed). 

 Temperature data were collected during each tow us- 

 ing an expendable bathythermograph (XBT) until 1992 

 and after 1992 with a micro-bathythermograph (MBT) 

 attached to the headrope of the trawl. Catches were 

 sorted by species and weight; number of fish caught and 

 length-frequency data were collected for each tow. 



Echo integration trawl survey transects were de- 

 signed to coincide with north-south lines of BT sta- 

 tions. Similar to the BT survey, the EIT survey began 

 also in the eastern Bristol Bay and proceeded west- 

 ward. The time lag between the survey varied from 

 to 30 days. Acoustic data were collected with a Simrad 

 EK500 quantitative echo sounding system. Biological 



