Quantitative Studies of Arctic Cod (<i>Boreogadus saida</i>) Schools: Important Energy Stores in the Arctic Food Web
Aggregations of Arctic cod (Boreogadus saida) in Allen Bay, on the southern coast of Cornwallis Island, Lancaster Sound were studied with fishery acoustics techniques in 1989 and 1990. Nonschooling fish were also examined in several Cornwallis and Devon Island bays. Although acoustic signal shading within the schools minimized abundance estimates (error may exceed 50%), measurements in 1989 indicated that two aggregations of adult Arctic cod together comprised at least 900 million individuals. Combined surface area of these two schools was 59 hectares, and total biomass was 30 715 tonnes (520 t/ha). Mean density in one school was 91 fish/cu m. In the other it was 307 fish/cu m, each fish occupying a space equivalent to about one body length cubed. A smaller school studied in the same location a year later had a mean density of 72 fish/cu m. Mean length of net-captured fish from that school was 18.7 cm (TL), and mean weight was 44.5 g. School surface area was about 4.6 hectares, and it contained an estimated 2835 t of biomass (616 t/ha). Given that significant concentrations of biomass occurred in this small portion of the Lancaster Sound region, we concluded that, in theory, sufficient biomass was sequestered in fish schools within the region to support energy flows through the food web. Acoustic measurements indicated that fish at a school edge were larger (target strength [TS] = -44.7 dB, equivalent size = 19.3 cm) than nonschooling fish elsewhere (TS = -53.4 dB, equivalent size = 8.4 cm). Zooplankton abundance near a school was markedly reduced. Near Cornwallis Island, where water temperatures throughout the water column were >-1.3 C, fish were scattered from surface to bottom. But near Devon Island, fish were common only in warmer surface waters (>2.0 C) above a marked thermocline at about 35 m, but not deeper, where water temperature was -1.5 C.