Species composition and numbers of bony fish were studied in a small artificial reef complex off the northern Mediterranean coast of Israel. On this site visual censuses of fish carried out by SCUBA divers once a month over a twelve month period in 1985-6 and again in 1995 showed that only one species of fish and one species of macro-invertebrate were added in 1995 to a total of 36 species of fish and four macro-invertebrates counted ten years earlier. Of the fish species recorded on the artificial reef 18.7% were Lessepsian migrants from the Red Sea, passing to the eastern Mediterranean through the Suez canal; 75.7% were of Atlanto-Mediterranean origin. The contribution of Red Sea species to the overall fish biomass in the artificial reef complex however, increased from 64% in 1985-6 to 94% in 1995. The change was due mainly to a significant increase (244%) in the population of the Red Squirrelfish Sargocentron rubrum and a decrease in the population of sparids (41-90%) and Dusky Grouper, Epinephelus marginatus (44.8%). Similar increases in Red Sea immigrants were reported by coastal fisheries and observed in other artificial and natural outcrops. One possible explanation for this phenomenon is the competitive exclusion of autochtonous species by a successful Lessepsian migrant, taking over spatial and perhaps food resources. Over-fishing of the commercially important groupers and sparids may provide an alternative explanation for this biomass change in the ichthyofauna. In view of these findings, artificial reefs are suggested as representative sites for observations on long-term ecological changes in bony fish populations.
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