Distribution patterns of sea urchins and barrens in shallow Mediterranean rocky reefs impacted by the illegal fishery of the rock-boring mollusc Lithophaga lithophaga

Shallow rocky habitats in SW Apulia (SE Italy, Mediterranean Sea) were surveyed in late spring 2002 to assess distribution patterns of sea urchins (Paracentrotus lividus and Arbacia lixula) and barren habitats (coralline barrens and bare substrates) in rocky reefs impacted by the destructive fishery of the rock-boring date-mussel Lithophaga lithophaga. Sea urchin density, test size-structure and biomass, and the percent cover of barrens were evaluated at four locations (5-6 km apart from each other), two heavily impacted by the date-mussel fishery and two controls. Sea urchin density and barren habitat cover were assessed at two and three sites (100-300 m apart), respectively, within each location. Sea urchin biomass was evaluated only at the scale of locations. Average density of P. lividus did not significantly change between impacted locations and controls, whereas A. lixula showed a greater density at the impacted locations. Distribution patterns of A. lixula, in addition, differed at the spatial scale of a few metres between impacted locations and controls, being generally more aggregated at the controls. The size-frequency distribution (test diameter) of P. lividus showed a mode at 3-4 cm at the impacted locations compared to a mode at 2-3 cm in the controls. The size-frequency of A. lixula was bimodal at the damaged locations (with modes at 1-2 and 4-5 cm, respectively) and unimodal (with the mode at 4-5 cm) at the controls. Average biomass of both sea urchins (P. lividus and A. lixula) was two- to fourfold greater at the impacted locations (similar to600 g wet wt m(-2)) than at the controls (150-250 g wet wt m(-2)). Barren habitats had a far greater average cover (mainly of macroalgae) at the impacted locations (from 79% to 96%) than at control locations (from 7% to 21%). These results show that the date-mussel fishery may have the potential to affect distribution patterns of sea urchins and to greatly enhance the percent cover of barren grounds in shallow Mediterranean rocky reefs.