Phenotypic plasticity in wild marine fishes associated with fish-cage aquaculture

Two opportunistic fish species associated with fish farms, bogue (Boops boops) and salema (Sarpa salpa), were studied to infer whether changes in their feeding habits may cause a phenotypic response in body and otolith shape. Specimens were collected close to aquaculture cage sites, called 'wild-farmed fish', and from control sites far away from these areas, called 'wild fish'. The fish body shapes were examined with geometric morphometric analysis using 21 landmarks. The otolith contours were analysed using wavelet function, whereas otolith weights were used as indicators of fish growth rates. Statistically significant differences were observed in body shape between wild-farmed fish and wild fish of both species. The wild-farmed fish had a smaller head and eye, and in bogue also a slighter snout curvature and head depth. Otolith shape and weight did not differ between groups in bogue. By contrast, there were significant differences in otolith shape and weight between groups in salema. A complementary study in salema indicated that the wild-farmed fishes grew more in weight and accumulated higher concentrations of 'terrestrial fatty acids' (linoleic and alpha-linolenic acids), which are used in fish feeds. Our findings clearly demonstrated an influence of coastal sea cages on metabolism of some wild fishes.