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Comparing Euphausiid Size-Selectivity by Pollock and Nets

figure 1, see caption
Figure 1.  Length frequency of euphausiids caught by nets (blue) and consumed by walleye pollock (burgundy) from the 2009 EBS hydroacoustic survey.

During an AFSC summer internship project conducted by Kelsey Kappler, euphausiids from the stomachs of walleye pollock were measured for comparison to the euphausiids caught in Methot plankton nets. The walleye pollock stomach samples and Methot euphausiid samples were collected during the 2009 EBS hydroacoustic survey.

figure 2, see caption
Figure 2.  Consumed euphausiid lengths (mm) plotted against walleye pollock lengths (cm).

In general, Kelsey found the length distribution of euphausiids in the diet of walleye pollock was similar to that caught in the nets, with possibly a slight shift to the smaller side of the distribution for euphausiids consumed (Fig. 1). We believe this was not caused by physical or chemical effects of being consumed, nor by differences in measurement methods between the samples, because only intact euphausiids were measured and because the opposite shift in length distribution appears to occur for two of the less abundant species, Thysanoessa longipes and T. spinifera.

Kelsey also found that the body length of consumed euphausiids was found to be similar for all sizes of walleye pollock examined (Fig. 2).

By Troy Buckley, Kelsey Kappler, Patrick Ressler, and Kerim Aydin

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