The effect of light intensity on the availability of walleye pollock (Theragra chalcogramma) to bottom trawl and acoustic surveys

Quantitative assessment of semidemersal fish such as walleye pollock (Theragra chalcogramma) is difficult because the proportion of walleye pollock available to standardized surveys varies temporally and spatially. The US National Marine Fisheries Service's Alaska Fisheries Science Center conducts bottom trawl (BT) surveys to estimate the demersal portion of the walleye pollock population and acoustic trawl (AT) surveys to estimate the pelagic portion. Both surveys are conducted during daylight hours to minimize variability due to diel changes in vertical distribution. To test if daytime near-bottom light intensity affects the proportion of walleye pollock available to the BT survey, we concurrently measured light and walleye pollock abundance on the Bering Sea shelf. Logistic regression models demonstrated that both light and depth affected walleye pollock abundance estimates by either BT or AT surveys, with more walleye pollock available to the BT survey under high illumination and at shallow depths and less walleye pollock available to the AT survey under these conditions. This finding suggests that daytime survey catchability for walleye pollock depends on depth and light intensity and that incorporation of light measurements could improve the precision of abundance estimates of semidemersal species such as walleye pollock.