Biology of the sweeper, Pempheris vanicolensis Cuvier & Valenciennes, a Lessepsian migrant in the eastern Mediterranean, with a comparison with the original Red Sea population

Various aspects of the biology of Pempheris vanicolensis Cuvier & Valenciennes, a recent Lessepsian (Suez Canal) immigrant into the Mediterranean, are given. Red Sea and Mediterranean populations were compared, and the results indicate that the spawning season is shortened in the Mediterranean, continuing from April to September, as opposed to year‐round in the Red Sea. Descriptions of a ripe ovary and post‐larva from the Mediterranean are given. Individuals reach a mean size of 108 mm (t.l.) in their first year, and 144 mm in the second year. Little change has occurred in the diet of the immigrant population, and both populations feed nocturnally, chiefly on larval and adult stages of planktonic crustaceans. Direct underwater observations on the diurnal behaviour of the species show that the fish leave their daytime cave shelter at sunset, congregate at a nearby site and then migrate inshore and disperse into small groups to feed. Before dawn, they reassemble at the cave's entrance, and finally enter it at sunrise, after the school has built up. Observations on Mediterranean sweepers show that the fish are segregated into size groups, ranging from 30 mm juveniles in the high littoral to 100–150 mm adults at depths of 4–5 m.