Ontogenetic shifts in habitat use and aggregation in juvenile sparid fishes

Ontogenetic shifts in habitat use, aggregation and dispersion were studied from the onset of settlement to recruitment to the adult population 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 Mediterranean 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 habitat: D. puntazzo and D. sargus showed a clear preference for crannies in the rocks between 0 and 2 m, whereas settlers of D. vulgaris preferred more open areas, generally somewhat deeper. For all species, preference for a given habitat type decreased as the juveniles grew, with distributions more consistent with habitat availability. These ontogenetic changes in habitat use were associated with differences in shoaling behaviour. During settlement, juveniles of the three species formed small monospecific shoals and exhibited a markedly clumped distribution. As the individuals grew, the shoals gradually fragmented. This was reflected in an increase in the number of shoals, lower densities within shoals and a more contagious distribution of shoals. The habitat use and shoaling behaviour was the same during the day and at night and in clear or partially turbid water conditions. However, under conditions of heavy turbulence, the level of aggregation of individuals clearly increased, independently of fish size. Juveniles of the three species 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 place concurrently with the integration of individuals into shoals of adult conspecifics. (C) 1998 Elsevier Science B.V.