Structure and Seasonal Fluctuations of Rocky Littoral Fish Assemblages in South-Western Portugal: Implications for Otter Prey Availability

Abstract

The fish assemblages of rocky intertidal and subtidal (<3 m depth) habitats of the south-western Portuguese coast were sampled between May 1992 and November 1993, using traps, angling and hand-netting, to investigate their structure and seasonal dynamics. The main purpose of the study was to describe the patterns of prey availability for marine-feeding otters (Lutra lutra L., Mammalia) in south-western Portugal, and to compare these with other areas in Europe. A total of 475 fish of 14 species was captured over 150 d of trapping (788 trap-days), and 1575 fish of 15 species were captured over 24 d of angling (76.3h). Hand-netting was carried out occasionally, to supplement the observations from the other methods, and only 96 fish of seven species were captured. The three methods yielded different patterns of the fish assemblage: giant gobies (Gobius cobitis Pallas, 33.7%) and rock gobies (G. paganellus L., 22.9%) were the most frequently trapped fish; corkwing wrasse (Symphodus melons (L.), 81.2%) were the usual species caught by angling; shannies (Lipophrys pholis (L.), 42.7%) and Montagu's blennies (Coryphoblennius galerita (L.), 28.1%) were the most frequently netted species. Conger eels (Conger conger (L.)), giant gobies and shannies showed an uneven distribution along the coast, occurring most frequently in schist shelves, rather than in areas of large sandstone boulders. There was a large seasonal variation, with the catches of most species peaking in winter and early spring, including the majority of those most important in the diet of otters: conger eels, shore rocklings (Gaidropsarus mediterraneus L.), rock gobies and corkwing wrasse. This was also the period when the individuals of several species were heaviest.