The effect of prey density on the growth of P. hypophthalmus larvae can be accounted as follows. Young larvae of this species swim throughout the water column with their mouth wide open (Hardjamulia et al., 1981; Subagja et al., 1999) and possess little ability for manoeuvring, since their pectoral fins are not developed at the start of exogenous feeding (E. Baras, J. Slembrouck and M. Legendre, unpublished data). With such behavioural and morphological traits, the probability of encountering and capturing food items is directly proportional to prey density, to a greater extent than in larvae of other fish species with greater manoeuvring abilities. Whatever the intrinsic foraging efficiency of a larva of P. hypophthalmus, it is presumably reduced in presence of conspecifics, which either remove prey from the trajectory of a particular forager or interact with its movements. Hence, it presumably takes longer for a particular fish to replenish in the presence of numerous conspecifics, This interpretation is supported by the observation that the food intake of 48-h old larvae at a particular prey density was negatively correlated with fish density, even when food was in excess (Fig. 3). The longer the foraging time, the higher the swimming costs (Ruzicka and Gallager, 2006), and the slower the growth.