What constitutes essential nursery habitat for a marine species? A case study of habitat form and function for queen conch
ABSTRACT: There is increasing recognition that habitats should be managed as part of fisheries management. It is generally assumed that amount of suitable habitat is linked to production of de- mersal species and that maps of bottom type will provide the information needed to conserve essen- tial habitats. In this review, a synthesis of nursery habitat is made for Strombus gigas (queen conch), a large, economically important gastropod in the Caribbean region. Juveniles occur on a variety of bottom types over their geographic range. In the Bahamas, nurseries occur in specific locations within large, beds of seagrass, while obvious characteristics of the benthic environment such as seagrass density, depth and sediment type are not good predictors of suitable habitat. Rather, nurseries persist where competent larvae are concentrated by tidal circulation and where settlement occurs selec- tively. Nursery locations provide for high juvenile growth resulting from macroalgal production not evident in maps of algal biomass, and they provide for low mortality compared with seemingly simi- lar surroundings. Therefore, critical habitats for queen conch juveniles are determined by the inter- section of habitat features and ecological processes that combine to yield high rates of recruitment and survivorship. While maps of bottom type are a good beginning for habitat management, they can be traps without good knowledge of ecological processes. A demersal species can occupy different substrata over its geographic range, different life stages often depend upon different bottom types, and specific locations can be more important than particular habitat forms. Habitat management must be designed to conserve habitat function and not just form. Implicit in the concept of ‘essential habitat’ is the fact that expendable habitat exists, and we need to prevent losses of working habitat because of inadequate protection, restoration or mitigation. Key nurseries may represent distinctive or even anomalous conditions.