Demersal fish assemblages in the Galician continental shelf and upper slope (NW Spain): Spatial structure and long-term changes

The spatial structure and long-term changes of the demersal fish assemblage on the continental shelf (100-200 m depth) and upper slope (200-500 m) in Galicia (NW Spain) were analysed. Annual trawl research surveys were carried out (in autumn from 1980 to 1991 and in spring in 1986 and 1987) during which a total of 105 fish species were caught. The assemblage was dominated by a small number of species, the most prominent of which were the blue whiting Micromesistius poutassou (making up 57% of the total number of individuals caught and 67% of the biomass), hake Merluccius merluccius, megrim Lepidorhombus boscii, thickback sole Microchirus variegatus and silver pout Gadiculus argenteus (these species comprised a total of 91% of the specimens and 75% of the biomass). Average density and biomass were 3396 ind. and 91.0 kg per 30 min tow. An average of 16.8 species were caught per tow and mean diversity (index H' of Shannon-Wiener) was 1.54. Density, biomass and species richness decreased significantly with depth, whereas the index H' did not show a significant bathymetric change. However, all these parameters showed significant differences between geographical areas and, particularly, between years. Density and biomass increased in the southern area of the coast (between Mino and Estaca de Bares), while species richness and diversity were greater in the northern one (from Estaca to Ribadeo). The ordinations of the assemblage obtained through correspondence analysis were similar in autumn and spring. The variability in assemblage structure was determined mainly by temporal changes and, to a lesser extent, by depth and geographical location. Most of the species presented a wide distribution range within the study area, and faunal zonation with assemblages differentiated in the space were not detected. Important differences in assemblage structure were found between the period 1980-85 and 1988-91. In this sense, several non-commercial species (M. variegatus, Argentina sphyraena, Capros aper, Arnoglossus laterna) and L. boscii and Trisopterus luscus presented a positive temporal trend, whereas the monkfishes Lophius Piscatorius and L. budegassa decreased significantly throughout the study period. Moreover, the density of L. boscii, A. sphyraena, A. laterna and G. argenteus had a significant positive correlation with the upwelling index corresponding to the period immediately preceeding the cruise. The results are discussed in terms of the environmental factors that may determine the patterns observed, especially the great fishing effort that takes place in the area, as well as the inter-annual and spatial variability in upwelling processes. (C) 1997 Academic Press Limited.