Comparison of diet and otolith growth of juvenile wild fish communities at fish farms and natural habitats

Juvenile fish of > 20 different species use offshore floating sea cages as settlement habitats in the Mediterranean Sea. In the present work, surveys were conducted to identify differences between species composition and abundance of new settlers among farms and control rocky-shore environments and to test for variation in their diets. Along with this, otolith growth and morphology analyses were also applied to better understand the consequences for the ecology and growth performance of several fish species that use coastal farms as recruitment habitats in the southwestern Mediterranean Sea. Results showed that fish farms are acting as a new habitat for the settlement of juvenile fish and that detected abundance could be compared with that found at natural habitats such as shallow rocky environments. However, a consistent pattern for all the studied species was a significant variation in otolith-shape descriptors, principally identified through the Aspect Ratio index, which was lower for the farm-associated species Atherina boyeri, Oblada melanura, and Sarpa salpa. Otolith modifications were effectively detected by elliptic Fourier descriptors since multivariate analysis correctly identified farm or control origin at 78.8, 85.1, and 86.1% for A. boyeri, O. melanura, and S. salpa individuals, respectively. According to these results, fish farms may offer new settlement habitat for several fish species on open coastal areas and provide resources equivalent to rocky habitats, but at the same time causing some effect on fish growth as a potential consequence of the availability of particulate organic matter derived from feed pellets.