Use of bottom-mounted echo sounders in exploring behavior of mesopelagic fishes

We deployed an upward-facing echo sounder mounted on the bottom and cabled to shore in a ̃400 m fjord location for long-term studies of small mesopelagic fish and their potential predators. The population of the myctophid Benthosema glaciale displayed diverse diel vertical migration (DVM) behaviors, including normal DVM to surface waters at night, reverse DVM in the lower part of the water column in which fish ascended to ̃200 m at day, as well as nonmigration of some individuals. The relative prevalence of these behavioral modes varied with season. Acoustic target tracking of individuals in deep water showed that B. glaciale was conspicuously inactive and drifted back and forth with weak tidal currents, essentially acting as plankton. Swimming was largely restricted to infrequent short bouts. More active swimming occasionally occurred in the vertical direction, and then in a stepwise pattern. Potential predators in deep water were swimming at speeds of <0.5 body length s^-1, with maximum speeds of ̃1 body length s^-1. These results show that submerged echo sounders provide a means of non-intrusively studying both individual and population behavior of deep-living organisms.