Ac. Farina et al., DEMERSAL FISH ASSEMBLAGES IN THE GALICIAN CONTINENTAL-SHELF AND UPPERSLOPE (NW SPAIN) - SPATIAL STRUCTURE AND LONG-TERM CHANGES, Estuarine, coastal and shelf science, 44(4), 1997, pp. 435-454
The spatial structure and long-term changes of the demersal fish assem
blage on the continental shelf (100-200 m depth) and upper slope (200-
500 m) in Galicia (NW Spain) were analysed. Annual trawl research surv
eys were carried out (in autumn from 1980 to 1991 and in spring in 198
6 and 1987) during which a total of 105 fish species were caught. The
assemblage was dominated by a small number of species, the most promin
ent 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 s
ole Microchirus variegatus and silver pout Gadiculus argenteus (these
species comprised a total of 91% of the specimens and 75% of the bioma
ss). 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 diversit
y (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 paramete
rs showed significant differences between geographical areas and, part
icularly, between years. Density and biomass increased in the southern
area of the coast (between Mino and Estaca de Bares), while species r
ichness and diversity were greater in the northern one (from Estaca to
Ribadeo). The ordinations of the assemblage obtained through correspo
ndence 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 spec
ies presented a wide distribution range within the study area, and fau
nal zonation with assemblages differentiated in the space were not det
ected. Important differences in assemblage structure were found betwee
n the period 1980-85 and 1988-91. In this sense, several non-commercia
l species (M. variegatus, Argentina sphyraena, Capros aper, Arnoglossu
s laterna) and L. boscii and Trisopterus luscus presented a positive t
emporal trend, whereas the monkfishes Lophius Piscatorius and L. budeg
assa decreased significantly throughout the study period. Moreover, th
e density of L. boscii, A. sphyraena, A. laterna and G. argenteus had
a significant positive correlation with the upwelling index correspond
ing to the period immediately preceeding the cruise. The results are d
iscussed in terms of the environmental factors that may determine the
patterns observed, especially the great fishing effort that takes plac
e in the area, as well as the inter-annual and spatial variability in
upwelling processes. (C) 1997 Academic Press Limited.