Near-bottom vertical zonation during daytime of deep-living hyperbenthic mysids (Crustacea: Mysidacea)

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Abstract
Animals were collected in a Norwegian fjord from 220 m depth with an epibenthic sledge sampling in three levels above the bottom (0–30, 30–80, and 80–130 cm). A height index, Hy, was calculated for 9 sampling dates between February 1980 and January 1981. Different species differed in height above the bottom. There was a general trend that juveniles remained closer to the bottom than older age stages. Statistically significant difference in Hy between sexes was found only for one species. Five species differed significantly in Hy between dates. Hy, calculated from one 305 m locality and from other data sets, showed that species may have different Hy’s at different localities whereas interspecific rank in height above the bottom is similar. Preferred swimming height, or swimming activity, above the bottom may be important when interpreting population fluctuations on a locality with bottom currents. No single environmental or biological factor seems to regulate the daytime height above the bottom.