Fishery-induced reductions in density and size truncation of sea urchin Loxechinus albus affects diversity and species composition in benthic communities

Fishery assessments are frequently focused on economically important target species, thus, effects of fisheries on associated habitats and communities are often neglected and/or poorly understood. In Northwest Patagonia, commercial beds of the red sea urchin Loxechinus albus are subjected to intense fishing, which along with large environmental variability are believed to affect the diversity and structure of benthic communities. We evaluated the potential of two types of L. albus demographic indexes: density and size structure, and of three environmental covariates: temperature, salinity and chlorophyll-a concentration to explain the richness, diversity and species composition of L. albus benthic communities. Then, we compared models containing L. albus demographic indexes and environmental covariates, against alternative models based upon simple geographic variables and against null models. Richness and diversity presented positive relationships with both L. albus density and size structure indicators, suggesting that L. albus beds subjected to higher exploitation levels present less rich and diverse communities than less exploited ones. Size structure had a significant effect on richness, diversity and species composition. The environmental variables temperature and salinity had a significant effect on most response variables, while chlorophyll-a concentration had a significant effect only on species composition. Models containing demographic indexes and environmental covariates were more informative than geographic and null models. Our results showed that L. albus fishery affects not only this target population but also the associated benthic communities. These findings highlight the importance of adopting a more integral ecosystem based approach to assess the impact of benthic fisheries.