Vertical distribution and population structure of the necrophagous amphipod Eurythenes gryllus in the West European Basin

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Abstract
During 3 cruises to the West European Basin benthopelagic nekton was captured by use of an acoustically released vertical array of 8 to 10 baited traps. The traps were exposed from 5 to 1000 m above the bottom. Nearly all of the 1962 amphipods trapped in total during 18 depolyments were identified as the lysianassid amphipod Eurythenes gryllus Lichtenstein. Most individuals were recovered from traps depolyed 15 m above the sea floor. Above 30 m, catch rates decreased significantly. However, the vertical distribution differed by age and sex: indeterminate juveniles and adult females were only trapped up to 50 m above the bottom. Catch rates of juvenile males and females decreased with increasing distance from the bottom, whereas adult males showed a bimodal distribution with maxima at 15 and 300 m above the sea floor. Males wee prevalent in all but 2 depolyments. The characteristic vertical distribution and the population structure at the depths sampled imply an ontogenic vertical migration of E. gryllus: females release their brood near the bottom. The youngest stages stay within a few meters above the sea floor, whereas older stages, especially adult males, migrate into higher water layers.