Spatial distribution of deep-sea decapods and euphausiids near the bottom in the northwestern Mediterranean

This study presents data on the occurrence and relative abundance of deep-sea decapod crustaceans and euphausiids, collected between 389 and 1859 m depth, in the water layer immediately adjacent to the bottom (10–140 cm above the bottom). Species were collected using an automatic opening and closing suprabenthic Macer-Giroq sledge in the Catalan Sea slope (western Mediterranean) during 1991 and 1992. The gear consisted of three rectangular mouths (40 × 80 cm) arranged in three tiers, one above the other, sampling simultaneously three separate levels (subsamples) between 10 and 50, 55 and 95, and 100 and 140 cm above the bottom. Results revealed how mesopelagic species [euphausiids, Sergestes arcticus Kroyer, Pasiphaea sivado (Risso), and Gennadas elegans (S.I. Smith)] and some nektobenthic species were concentrated in the uppermost levels, according to high values in indexes of swimming activity (K2, Kl). In contrast, endobenthic species (e.g. Calocaris macandreae Bell, Alpheus glaber (Olivi), and Crangonidae) were closely associated with the lowermost level. The partitioning of vertical space was analogous to that deduced from stomach contents analysis in deep-sea decapods in the same area. Almost all the specimens of Sergestes arcticus and Gennadas elegans captured were adults. The presence of large individuals of mesopelagic species near the sea floor is ascribed to ontogenetic migrations, which indicate differences between population structure of mesopelagic species near the bottom and in the midwater.