Demersal assemblages off Portugal: Mapping, seasonal, and temporal patterns

Analysis of trawl surveys (1989-1999, 22 surveys) conducted off Portugal (36-710 m depth) indicated the existence of five spatially distinct fish assemblages: shallow and intermediate (northern and southern), and deep assemblages. Depth and latitude correlated with major directions of biological turnover on the shelf, and accordingly, determined the geographical location of the assemblage boundaries. These did not change significantly between the summer and fall surveys, but there were seasonal changes in relative species composition within assemblages, which are discussed in light of known patterns of planktonic production associated with the seasonal upwelling. On the shelf plateau (<150 m), horse mackerel (Trachurus trachurus) was more important in autumn assemblages, whereas the pelagic crab (Polybius henslowii), and boarfish (Capros aper) dominated summer assemblages to the north and south, respectively. On the upper slope, the fish community was dominated by blue whiting (Micromesistius poutassou). Most species were confined to certain depth and latitudinal ranges, and in ubiquitous species (European hake, Merluccius merluccius, horse mackerel), mean body size increased from the shallower to the deeper assemblages.