Faunal assemblages of benthic megainvertebrates inhabiting sea scallop grounds from eastern Georges Bank, in relation to environmental factors.

Abstract : Faunal assemblages of benthic megainvertebrates from Georges Bank (NW Atlantic) commercial scallop beds were defined from video-monitored sled-dredge samples. The distribution, diversity, abundance, and biomass of the megafauna were studied in relation to water depth and sediment types. A total of 140 species of megainvertebrates representing 10 phyla were idenhfied, from which epibenthic taxa accounted for ca 76 %. Molluscs, crustaceans, annelids, and echinoderms were best represented, with bivalves ranking first in abundance (55 %) and biomass (86 %). Significant changes of species diversity, total abundance, and total biomass were found according to sediment type, but not according to depth (except for total biomass). No clear bathymetric pattern was observed, although the mean number of species, mean total density (below 80 m), and mean total biomass (below 60 m) decreased with increasing depth in the deepest zone. Maximum megafaunal richness was found in biogenic bottoms while minimum value occurred in sand dunes. Total biomass and total density were consistently dominated by a small number of taxa. Three bivalve species (Spisula solidissima, Arctica islandica, Placopecten rnagellanicus) made up to 7 l % of total biomass overall, while 14 species (predominantly Anornia spp., Arctica islandica, and Ophjopholis aculeata) accounted for 70 % of total density. Six megafaunal associations related to 2 major assemblages (biogenic sand-gravel and sandshell fauna) were defined from multivariate analyses. Sediment type, tidal current speed, turbulent mixing, and food availability appear to be the major distribution-regulating factors of the megabenthos in the 55 to 105 m depth range. Density-dependent predator-prey relationshps were the main biological associahons shown by megainvertebrates