Effects of the edible sea urchin, Paracentrotus lividus, fishery along the Apulian rocky coast (SE Italy, Mediterranean Sea)

The effects of the sea urchin Paracentrotus lividus (Lamarck) fishery, and possible indirect influences on the co-occurring (but not harvested) sea urchin Arbacia lixula (L.), were investigated in shallow rocky reefs in southern Apulia (SE Italy, Mediterranean Sea). Density, size (test diameter), biomass and size-frequencies of both species were assessed in summer 2001 at one location heavily impacted by P. lividus fishery (recreational and commercial), and at three controls. Average sea urchin density (pooling P. lividus and A. lixula), and density of P. lividus did not significantly differ between the impacted location and the controls, while average density of A. lixula was significantly greater at the location where P. lividus was fished. The average size of P lividus was significantly lower at the exploited location than at the controls, whereas that of A. lixula did not differ. Total sea urchin biomass (pooling P. lividus and A. lixula) did not differ between the exploited location and the controls. The average biomasses of P. lividus and of A. lixula, conversely, were lower and greater, respectively, at the fished location than at the control sites. In some cases, distribution patterns of the investigated variables (i.e., density, size and biomass) were affected by fishery at the spatial scale of a few metres (i.e., between replicates). At the controls, P lividus larger than 4 cut (the threshold size for commercialisation) accounted for about one third of the populations surveyed, whereas at the fished location specimens of this size were rare. Size-frequency distributions of A. lixula were fairly similar at the fished and control locations. Although appropriate experimentation would be needed to confirm these patterns, the present study provides suggestive evidence that intense R lividus harvesting may cause reduction in average size and biomass of this echinoid because of the selective harvesting of largest specimens. Densities of P lividus at fished and control areas, instead, did not change, which suggests that P lividus populations subject to fishing could have the potential to recover by appropriate management policies (e.g., catch quotas). Indirect effects of the P lividus harvesting on the co-occurring A. lixula suggest a possible competitive dominance by P. lividus in the absence of selective fishing on this latter species, and a sort of compensation when it is fished. Implications for management of P. lividus exploitation, and for possible ecological consequences of the sea urchin fishery, are also discussed.