ONTOGENIC SHIFTS IN HABITAT USE AND AGGREGATION IN JUVENILE SPARID FISHES

Ontogenetic shifts in habitat use, aggregation and dispersion were stu died from the onset of settlement to recruitment to the adult populati on for juvenile littoral fishes of the genus Diplodus. These processes were examined for three species (D. puntazzo (Gmelin), D. sargus (L.) and D. vulgaris (Geoffroy Saint-Hilaire)) in the north-west Mediterra nean Sea between May 1993 and June 1996. On settlement, juveniles (1.0 -1.5 cm in length) of the three species showed non-random use of the h abitat: D. puntazzo and D. sargus showed a clear preference for cranni es in the rocks between 0 and 2 m, whereas settlers of D. vulgaris pre ferred more open areas, generally somewhat deeper. For all species, pr eference for a given habitat type decreased as the juveniles grew, wit h distributions more consistent with habitat availability. These ontog enetic changes in habitat use were associated with differences in shoa ling behaviour. During settlement, juveniles of the three species form ed small monospecific shoals and exhibited a markedly clumped distribu tion. As the individuals grew, the shoals gradually fragmented. This w as reflected in an increase in the number of shoals, lower densities w ithin shoals and a more contagious distribution of shoals. The habitat use and shoaling behaviour was the same during the day and at night a nd in clear or partially turbid water conditions. However, under condi tions of heavy turbulence, the level of aggregation of individuals cle arly increased, independently of fish size. Juveniles of the three spe cies showed a high degree of fidelity to the nursery area, where they remained for a number of months. Dispersal outside this area occurred when the individuals reached 4.5-5.5 cm in length. This process took p lace concurrently with the integration of individuals into shoals of a dult conspecifics. (C) 1998 Elsevier Science B.V.