Catch composition by depth strata from the Iceland Basin trawl samples.

The major groups of species represented in the samples organised according to their acoustic properties. Fish with gas-filled swimbladders (Fish-GSB) give echoes of an order of magnitude higher than fish with regressed swimbladders (Fish-RSB, swimbladders regress with age and becomes lipid filled), which give a much stronger signal than fish without swimbladders (Fish-NSB) of same size. Squid, jellyfish and crustaceans are relatively weak acoustic reflectors compared to fish. Catch data (kg) are from pelagic fish trawls (FT) and krill trawls (KT, g km−1), except copepods caught by a multinet (no. m−3) taken at the end of the vessel track in Figure 1. Copepods contribute little to the recorded acoustic signal. Lengths are standard length for fish and mantle length for squid. Actual fish trawl depth ranges were: FT = 150–200 m, 370–750 m; KT = 10–175 m, 180–845 m, 880–1545 m.