Shima et a\ Spatial distribution of Theiagra chakogramma in the Gulf of Alaska 



319 



these age groups are strongly influenced by interannual 

 variations in year-class strength. However, spatial trends 

 in distribution were consistent with those seen for adult 

 pollock. For all ajje groups of juvenile pollock there was an 

 uicrease in the proportion of stations where low densities 

 occurred, and a decrease in the mean CPUE during the 

 time period examined. 



The spatial expansion by a species should be reconciled 

 with more detailed analyses of the characteristics of the 

 area it occupied to determine whether changes in habitat 

 suitability have occurred. Adult pollock are not expanding 

 into areas with physical characteristics previously not as- 

 sociated with pollock. Cluster analysis revealed that adult 

 pollock are associated with average bottom temperatures 

 between 5.2°C and 6.4°C. The bottom temperature data 

 from field surveys revealed a consistent range over time of 

 5-6"C (Dorn et al.''). It was difficult to discern a trend from 

 either our bottom temperature or the OBT data. Tempera- 

 tures may have been slightly warmer in the 1980s than in 



6 Dorn. M. W., A. B. Hollowed. E. Brown, B. Megrey, C. Wilson, 

 and J. Blackburn. 1999. Walleye pollock. In Stock assess- 

 ment and fishery evaluation report for the 2000 Gulf of Alaska 

 groundfish fishery, 65 p. North Pacific Fishery Management 

 Council, P.O. Box 103136, Anchorage, Alaska 99510. 



the 1990s. Temperature changes, together with the shoal- 

 ing of the mixed layer depth (Polovina et al., 1995; Shima, 

 1996), may have caused a redistribution of prey, possibly 

 contributing to changes in pollock distribution. 



Evidence of the importance of external forcing on the 

 spatial range of fish species has been noted in other eco- 

 systems. Movement of fish in response to environmental 

 change was recorded in the Barents Sea when Atlantic 

 cod (Gadus morhua Linnaeus, 1758) shifted westward as 

 a consequence of cooler waters (entering from the east) 

 across the region from 1977-81 (Loeng, 1989). Primary 

 shifts in distribution, in response to temperature changes, 

 were seen in younger age classes of fish. Because there 

 were some species (e.g. haddock, Melanogrammus aeglefi- 

 nus (Linnaeus, 1758)) that did not respond to changes in 

 temperature, it may be that the movement of the fish may 

 be dependent on the sensitivity offish prey to the temper- 

 ature shifts (Shevelev et al., 1987). Cod (like pollock) feed 

 mostly on planktonic organisms that may respond rapidly 

 to temperature changes whereas haddock feed mostly on 

 benthic species (Shevelev et al, 1987). 



External forcing may also include anthropogenic fac- 

 tors. Domestication (U.S.) of the pollock fishery occurred 

 during the period investigated (Megrey, 1989) with the re- 

 sult that fishing operations shifted from at-sea processors 



