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Fishery Bulletin 100(2) 



Discussion 



A major result of our study was that for all age gi-oups of 

 pollock, the number of stations where pollock occurred at 

 low densities (<1000 fish/km-) increased during 1984-96 

 while the mean density decreased. This pattern suggests 

 that the pollock population was increasing its range, a 

 characteristic often seen in growing populations that is 

 consistent with the "basin hypothesis" iMacCall, 1990). 

 MacCall hypothesized that at low-densities, marine fish 

 will occupy habitats that are optimal for sui-vival. As pop- 

 ulations grow, however, some portions of the populations 

 will expand into locations of less suitable habitat quality. 

 Our analyses of the data indicated that the demersal 

 portion of the pollock population was stable but that the 

 overall (pelagic and demersal) pollock population declined 



during the study (1984-96, Fig. 1. B and C). Year-class 

 strength was only sporadically strong and therefore would 

 not account for the sustained increased positive occurrence 

 by station during the early- to mid-1990s. Under MacCall's 

 hypothesis, the range of the population should have been 

 stable or contracting. Thus, patterns emerging from our da- 

 ta are not entirely consistent with MacCall's model. 



Analysis of CPUE of all positive tows revealed that 

 there was a decrease in mean CPUE over the years from 

 1984 to 1996. The data showed an increase in low-density 

 concentrations of adult pollock stations across all catego- 

 ries but in particular at bottom depths 200-300 m and 

 in the Chirikof and Shumagin regions. We conclude that 

 adult pollock had expanded into deeper water in the 1990s 

 but that the expansion had resulted in a decrease of adult 

 pollock in high-density stations, combined with decreased 

 mean density of adult pollock throughout the region. We 

 hypothesize that the expansion of pollock was due to an 

 increase in the suitability of habitats for adult pollock, 

 possibly caused by a spread in the distribution of pollock 

 forage, during a period of stable or decreasing pollock pop- 

 ulation trends. If predators of pollock rely on high-den- 

 sity patches, the decreased mean density of pollock may 

 have negative ramifications for the successful foraging of 

 top predators. We refer to this as the "forage density hy- 

 pothesis"; i.e. habitat suitability has changed, perhaps on 

 a local scale, such that there has been an expansion of 

 the overall population distribution. However, the expan- 

 sion, combined with decreasing population numbers, had 

 caused the density of pollock patches to decrease below 

 a threshold at which top predators can successfully for- 

 age. The underlying assumption here is that the predators 

 need patches of high prey density rather than a uniform 

 distribution of average or low prey density. 



Analysis of trends in the distribution and abundance 

 of juvenile age groups is more difficult because data for 



