Spatial and temporal variations of fish assemblages in a shallow Mediterranean soft-bottom area (Gulf of Fos, France)

The fish assemblages of two shallow soft-bottom areas located in the Gulf o f Fos (NW Mediterranean) were studied by means of 57 trawl surveys carried out on both seasonal and diel scales between 1983 and 1985. A total of 47 f ish species was recorded, with a mean density and biomass of 650 individual s ha(-1) and 2.67 kg ha(-1), respectively. Most of the fluctuations observe d were on the spatial scale. Strong differences in fish assemblages were ob served between the two areas, with both higher species richness and biomass in the outer part of the gulf (La Gracieuse), while a higher density of in dividuals characterised its inner part (Bay of Carteau). This spatial patte rn was due to a small number of species, such as the gobiids Gobius niger a nd Pomatoschitus minutus in Carteau, and the flatfish Arnoglossus laterna, Buglossidium luteum and Salen solea in La Gracieuse. These differences were probably induced by differences in the hydrological and benthic characteri stics of the two study areas. Decreases in species richness, density and/or biomass were observed in both areas over the study period, although this t rend was not significant in all cases. These decreases may have been linked to interannual variations in the mean flow rate of the Rhone River (-28.1% between 1983 and 1985), which might have induced a decrease in the density of benthic invertebrate populations. We suggest that a similar reaction oc curred in fish populations, as most of them are known to feed on these inve rtebrates. Diel patterns did not reveal any significant trends. However, an increase in density and biomass at sunset, and a decrease before sunrise w ere observed in summer in both areas, and the presence of certain specific species was recorded in catches at determined hours of the diel cycle. Thes e observations were likely due to the activity cycles (diurnal vs. nocturna l) of the fish species. (C) 2001 Ifremer/CNRS/IRD/Editions scientifiques et medicales Elsevier SAS.