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 abundance, have been linked to topographic, bathymetric, and biotic factors. The two-stage generalized additive modelling (GAM) approach undertaken in the present study offered biological interpretations of presence/absence of a pelagic 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 depth of the water column, the seabed substrate, and zooplankton biomass appeared to be significant factors modulating presence and relative abundance of a pelagic species within the northern North Sea. Results indicated that habitats with the higher probability of finding herring present were those located in shallower waters having a gravel/sand type of seabed that also carried the highest zooplankton abundances. Species clustering was found to be preferentially higher in these habitats. Notwithstanding their pelagic nature, herring exhibit preferences for areas with specific bathymetric, substratum, and zooplanktonic conditions. These preferences are believed to be linked to the oceanography of the North Sea ecosystem.