Habitat triage for exploited fishes: Can we identify essential “Essential Fish Habitat?”

There is little doubt that estuarine habitat is important for some exploited fish species, at some times, and in some places. However, it is also clear that we do not have enough resources to conserve or restore all estuarine habitat. Consequently, a simple, quantitative and transparent approach to prioritizing estuarine habitat management is required. Here, we present a general framework for identifying critical habitats of exploited fishes. Our approach requires three basic steps: (1) develop stage-structured models and identify sensitive life history stages; (2) determine what habitats, if any, are important to these stages; and (3) identify sites in which high densities of critical life stages occur in important habitat. We will illustrate the utility of this approach using red drum, Sciaenops ocellatus. Results of a simulation-based sensitivity analysis of a stage-structured matrix model show that most of the variability in population growth rate (λ) of red drum is explained by larval and juvenile survival rates. Thus, this approach indicates that larval/juvenile red drum habitat should be given higher priority for conservation and/or restoration than habitats used by other life history stages. To illustrate the potential importance of juvenile habitat to red drum, we modeled the growth of a hypothetical red drum population using different population matrices as manifestations of varying habitat conditions. These numerical experiments revealed that restoration of both marsh and seagrass habitats would yield a ca. 24% increase in post-settlement survival and would result in a ca. 2% increase in λ-an increase sufficient to stem a long-term population decline. Our results illustrate that protection of fish habitat depends not only on protecting sites where fish occur but also on protecting the ecological processes that allow populations to expand. Quantitative and synthetic analyses of ecological data are a first step in this direction.