Cartilaginous fishes along the North-Norwegian coast. Distributions and densities with regard to fishing and sea temperature

Abstract

Scientists are becoming increasing aware that many of the world’s cartilaginous fish stocks are particularly vulnerable to overexploitation by modern fishing activities. Very little is known about the cartilaginous fish stocks found in the North Norwegian coastal area. In other areas some of these species have been severely depleted by fishing. The aim of this study was to broaden the knowledge these species within this area and determine what factors play an important role in controlling their abundances. Demersal trawl catch data, from 1992 to 2005, was applied for analyzing abundances and distributions of the species present along the North Norwegian coastline. Abundances were determined using the swept area method. Distributions were assessed with respect to position along the coastline and depth. Further investigation focused on whether temporal and spatial differences in abundance could be linked to changes in sea temperature or shrimp trawl fishing effort. For most species, clear distribution patterns could not be determined, either because sample variances were too high or observed abundances were too low. Rabbitfish, velvetbelly lantern shark, piked dogfish and blackmouth catshark were all found in higher abundances at lower latitudes. Thorny skate was distributed along the entire coastline, but found in higher densities at northern latitudes. Between 1992 and 2005, these species abundances appeared not to significantly change. Their distributions appear to be related to spatial variations in temperature, but no conclusion could be made as to whether shrimp trawl fishing or temporal differences in temperature did significantly affect distributions or abundances. Further work is necessary in order to improve abundance estimations, clarify species identification. Data relating to more appropriate fisheries is required for determining how fishing effort may influence species abundance.