Diurnal variations in the fish assemblage at an artificial reef

An important factor contributing to the variation in reef fish assemblages is the diel cycle of the constituting species. To study diurnal variations, visual censuses were carried out during day and night on a 5-year-old artificial reef in the coastal waters of the Algarve (southern Portugal). The reef consists of 735 small cubic modules (2.7 m(3) each) distributed in 21 groups. Counts were made according to a fixed time schedule (7:00, 8:30, 10:00, 11:30, 14:30, 19:00, 20:30, and 23:00 h) by scuba divers in August 1995. A total of 18 species were recorded, most of which belonged to the Sparidae. The number of species present on the reefs varied throughout the day, with minima during night and maxima around midday. The variation in fish density between day and night showed a similar trend. We conclude that most species present have a diurnal activity pattern. About 61% of the species were considered resident, while 33% used the reefs mainly for foraging and/or shelter. (C) 2002 International Council for the Exploration of the Sea. Published by Elsevier Science Ltd. All rights reserved.