Aggregations of Arctic deep-sea scavenging amphipods at large food falls

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There is much speculation about chemoreception being involved in food finding strategies of deep-sea scavengers in the literature (Dahl, 1979; Meador, 1981; Busdosh et al.; 1982, Sainte-Marie, 1992). Most of these ideas have emerged from analysing time-lapse photographs and video recordings of bait deployments in the deep sea (Thurston, 1979; Lampitt et al., 1983; Hargrave, 1985; Priede et al., 1990). However, optical instruments have considerable restrictions in spatial coverage, thus all past efforts in determining any directionality in the appearance of scavengers have been limited from 0.9 to 4 m2 only (Smith, 1985; Wilson and Smith 1984). Here we present data obtained by using a scanning sonar system (SSS) which allows detection of single objects larger than 2 cm at a maximum distance of 50 m in a horizontal plane. Together with the SSS a baited time-lapse camera attached to a free falling lander was used in the Arctic deep sea at two locations in the Fram Strait at about 2500 m water depths. We would like to point out that this combination of optical and acoustical measurements allowed for the first time the long range detection of approaching scavenging amphipods in the deep sea. Eurythenes gryllus (Lichtenstein, 1822), a cosmopolitan deep-sea scavenging amphipod, was recorded to attend our bait experiments with a maximum of 618 individuals with first arrival 12 min. after deployment which is one of the fastest arrivals ever observed. We found a significant temporal correlation between integrated backscatter energy (IBE) based on measurements of the SSS and amphipod individuals counted on photographs.