Examining the ecological function of structure: species assemblages at casitas and coral heads in the Lower Florida Keys

Casitas are artificial structures used widely in the Caribbean to facilitate the harvest of spiny lobsters by divers. Casitas are often used by many non-target species, and therefore have the potential to increase local species diversity and mitigate the loss of natural habitats. Their ecological impact, however, is largely unknown. We examined the distribution of benthic habitat and the abundance and diversity of fish and motile invertebrates at casitas (artificial structures), coral heads (natural habitat with vertical structure >15 cm high), and low-relief hardbottom (natural habitat with vertical structure <15 cm high). Casitas and coral heads exhibited similar habitat features and fish and invertebrate species assemblages. Low-relief hardbottom sites had significantly less abundant and diverse fish and invertebrate species assemblages than at either structure. The only significant difference observed between casitas and coral heads was the abundance of lobsters. Lobsters were found in significantly greater numbers at casitas than at either coral heads or on low-relief hardbottom. Determining how an increased concentration of lobsters at casitas affects the local ecology is complex, but our data suggest that the presence of lobsters in such large aggregations at casitas did not alter the surrounding flora and fauna. In regions with limited shelter, species composition and ecological function at casitas are similar to those at coral heads. This study shows that casitas may be efficient dual-purpose devices; that is, they function not only as lobster fishing gear, but also as the ecological equivalent of coral heads in shelter-limited habitat.