Habitat selection and clustering of a pelagic fish: effects of topography and bathymetry on species dynamics

Two dynamic aspects of the ecology of pelagic fish, distribution and abunda nce, have been linked to topographic, bathymetric, and biotic factors. The two-stage generalized additive modelling (GAM) approach undertaken in the p resent study offered biological interpretations of presence/absence of a pe lagic fish species as distinguished from abundance (if present) in relation to various abiotic and biotic factors. The data were collected during the ICES coordinated Atlantic herring (Clupea harengus) acoustic surveys (ICES Division Na) of July 1992, 1994, and 1995 in the northern North Sea. The de pth of the water column, the seabed substrate, and zooplankton biomass appe ared to be significant factors modulating presence and relative abundance o f a pelagic species within the northern North Sea. Results indicated that h abitats with the higher probability of finding herring present were those l ocated in shallower waters having a gravel/sand type of seabed that also ca rried the highest zooplankton abundances. Species clustering was found to b e preferentially higher in these habitats. Notwithstanding their pelagic na ture, herring exhibit preferences for areas with specific bathymetric, subs tratum, and zooplanktonic conditions. These preferences are believed to be linked to the oceanography of the North Sea ecosystem.