Comparative analysis of the feeding ecology of two pelagic forage fishes: capelin Mallotus villosus and walleye pollock Theragra chalcogramma

Abstract

Pelagic forage fishes are an important component in the transfer of trophic energy in marine ecosystems. Capelin Mallotus villosus and age-0 walleye pollock Theragra chalcogramma are pelagic forage fishes whose alternate fluctuations in abundance have been associated with system- wide biotic reorganization of the Gulf of Alaska (GOA) ecosystem. To better understand differences between these fishes, we compared their feeding ecology using samples of similar-sized (39 to 126 mm) individuals collected together in midwater trawl hauls during September 2000 to 2001 in the western GOA. Zooplankton samples were used to characterize prey availability. The comparative analysis focused on diet, feeding chronology, daily ration, and prey selectivity. The diet of both spe- cies mostly consisted of small copepods, large copepods, adult and juvenile euphausiids, thecosome pteropods, and larvaceans; consequently, the overall diet overlap was high. The diet overlap of fish from some samples, however, was low. Capelin fed mostly during crepuscular periods while pollock mostly fed at night. The daily ration estimate for capelin was low (1.8% body weight (BW)) relative to pollock (5.2% BW). Chesson's index of selectivity indicated preferential selection of euphausiids (adult and juvenile) by capelin and age-0 pollock. At euphausiid-poor locations, capelin stomachs were mostly empty while pollock stomachs contained alternate prey. Selectivity of amphipods, large copepods, larvaceans, and reptantians was higher for age-0 pollock than for capelin. Consumption of euphausiids increased with their abundance in the plankton, and consumption:biomass ratios indi- cate that localized depletion could occur. We hypothesize that capelin increase foraging mobility while age-0 pollock switch to alternate prey as a competitive response to limited euphausiid avail- ability.