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Resource Ecology and Ecosystem Modeling Program

figure 1, see caption
Figure 1.  The average number of copepods eaten per fish by smaller pollock plotted against the copepod density found at that station.

figure 2, see caption
Figure 2.  The average number of euphausiids eaten per fish by larger pollock plotted against the euphausiid density found at that station.

FEAST Model Parameterization

Relationships between walleye pollock and their zooplankton prey are being investigated to support parameterization of the FEAST model for BSIERP. One source of zooplankton community data is the bongo net tows conducted during the AFSC's bottom trawl survey of the eastern Bering Sea continental shelf. Walleye pollock stomach samples were targeted for collection at nearly the same location and time during 2006, 2007, and 2008. The dedication of the AFSC survey personnel and the captain and crew of the chartered fishing vessel Arcturus made collection of these samples possible.

Generally, the bongo net captured mostly copepods, some chaetognaths, and fewer euphausiids and larvaceans. As is typical in the pollock diet, copepods tended to be a greater percentage of the consumed zooplankton along the outer edge of the continental shelf, and copepods were generally more important in the diet of smaller pollock (fork length (FL) < 45 cm) than larger pollock (> 46 cm FL).

Our preliminary results indicate that for smaller pollock, when copepods are eaten at the sampling station, there may be a weak positive relationship between number of copepods eaten per fish and local copepod density in the water (Fig. 1). At stations where euphausiids occur in the Bongo net catch, the average number of euphausiids eaten by larger pollock may have a weak negative relationship to euphausiid density in the water (Fig. 2). In other words, when more euphausiids are found in pollock stomachs, there appears to be fewer in the water.

This may suggest that pollock can rapidly graze down the local abundance of euphausiids.As more data from sampled stations become available, we will be able to refine our analyses to tighter subsets of stations (geographically), of pollock (narrower size categories), and of consumed zooplankton (digestion state).

By Troy Buckley, Todd TenBrink, Jeffrey Napp, and Kerim Aydin

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