Resource Ecology & Fisheries Management  (cont.)

Spatial Distribution and Ontogenetic Movement of Walleye Pollock in the Eastern Bering Sea

Walleye pollock is a key species in the Bering Sea and North Pacific ecosystems as well as the target species for one of the world’s largest fisheries. Because of its semipelagic habit and interannual variability in distribution, the ontogenetic movement pattern and factors influencing the spatial distribution of this species are not well understood. Examination of the age-specific spatial distribution of walleye pollock in the eastern Bering Sea (EBS) relative to physical factors and population density may yield insights into their ontogenetic movements, as well as their population structure. 

Bottom trawl surveys of the EBS continental shelf have been conducted every summer since 1982. Centroids (the mean center of abundance) were calculated for ages 1 through 8 in every year based on the catch-per-unit-effort (CPUE) at each station in the standard survey area. The age-specific centroids occur in a dispersed scatter indicating a high degree of interannual variability in the center of abundance across ages and years.

Figure 6.. The average centroids of pollock abundance, ages 1 through 8, for cold and warm survey years.  Triangles are centroids of survey station distribution for each survey year.

Previous studies have shown that walleye pollock avoid water below 0°C. The southern extent of cold water over the EBS middle shelf domain is the major influence on the mean bottom temperature encountered during each survey. We categorized the surveys into cold (< 2°C ), intermediate (2° - < 3°C ) and warm (>= 3°C ) years based on the mean bottom temperature.  The average centroids for warm years are further on-shelf than the average centroids for cold years (Figure 6) and intermediate years, indicating that the broader dispersal onto the shelf in warmer years was detected by this method.

A general ontogenetic pattern of movement can be seen in both the warm and cold years with the average center of abundance shifting southeastward with increasing age. However, the centroids for a few year classes (‘75, ‘76, ‘77, ‘78, ‘89, ‘90) were shifted further southeast than the other year classes (Figure 7 below). There may be a relationship between low adult biomass when these year classes were 2 year olds and the increased southeastern shift in the distribution of walleye pollock assessed by the bottom trawl survey. Thus, density-dependent factors may play an important role in the spatial distribution of a year class, in particular, the degree to which a year class occupies the southeastern portion of the EBS shelf.

Figure 7.  The average centroids of walleye pollock abundance, ages 1 through 8, for year classes that remain concentrated in the northwest area of the EBS shelf and those that shift southeastward.

Walleye pollock biomass is mostly supported by occasional large year classes. The spatial distribution of three of these large year classes (‘82, ‘89, ‘92) differed considerably (see Figure 8). The ‘82 year class was most dense over the northwest area of the EBS shelf. In contrast, the ‘89 year class occupied the southeast area of the shelf in high densities. The ‘92 year class appeared to be confined to the northwest area of the EBS shelf with some very high densities occurring in the northwest end of the standard survey area. Stock assessments following this year class over time indicate that it is either experiencing an unusually low rate of mortality or that fish from this year class are moving into the sampling area. Previous simulation studies by AFSC scientists and the shift in distribution shown in Figure 8 seem to support the latter possibility. The simulation study showed that the ‘92 year class of pollock larvae in the EBS may have been transported farther west than all other year classes.  This may be an indication that the initial juvenile distribution of a year class can persist for years and subsequently affect the observed spatial distribution of the year class as adults. 

Several considerations must be kept in mind when viewing the results of this approach. These are bottom trawl data and walleye pollock is a semipelagic species. Bottom trawl selectivity increases with age as walleye pollock become increasingly demersal. Converting lengths to ages can smear the true distribution of a year class, especially for older fish where there is more overlap in the length of different ages. We attempted to minimize this by using separate age-length conversions for the northwest and southeast areas of the EBS shelf and confining our analysis to younger ages. Border effects also need to be considered where high densities of walleye pollock may move out of, or into, the survey area (as possibly has happened for the 1992 year class).

As we continue this investigation, we hope to refine our understanding of the apparent patterns and relationships, described above, that contribute to the variable spatial distribution of walleye pollock. Seasonal movements related to environmental conditions, spawning, and feeding will also be examined. Currently, exploratory analysis has begun on spatial patterns of the stomach contents of walleye pollock in the EBS.

This study was presented at the PICES X annual meeting in Victoria, British Columbia in October 2001. The extended abstract and associated color figures that contain more of the details of this work can be viewed on the AFSC web site at
http://www.afsc.noaa.gov/refm/reem/doc/Pices2001Poster.pdf

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AFSC Quarterly
Research Reports
Oct-Dec 2001