Fisheries-Induced Changes in the Structure and Function of Shallow Water "Nursery" Habitats:  An Experimental Assessment

Kenneth L. Heck, Jr.1, John F. Valentine 1, James H. Cowan, Jr.2 and D. DeVries3 1 Dauphin Island Sea Lab, Dauphin Island AL 36528 2 Department of Marine Sciences, University of South Alabama, Mobile AL    36528 3 Department of Fisheries and Allied Aquacultures, Auburn University, Auburn, Alabama 36849

Despite the increasing management of nutrient inputs to coastal systems, SAV continues to decline throughout the United States. This suggests that other, as yet unrecognized, factors may be involved in causing or contributing to SAV disappearance. One such factor derives from predictions of the trophic cascade hypothesis developed in freshwater systems - the reductions in top predators that could occur as a result of overfishing. The work we propose will be one of the first field tests of the consequences of large predator removals at biologically relevant scales. We expect to gain new insight into the degree to which "top down" factors influence the structure and function of SAV habitats, and the factors that might promote a shift from a SAV-dominated system to a less plankton-dominated system. Because perturbations of coastal ecosystems are occurring at an advancing rate throughout the world, with concomitant losses of highly productive SAV "nursery" habitat, it is important for managers to understand some of the indirect, but potentially very important, consequences that might result from reducing large predator influence. As the loss of large predators is increasingly recognized as a major change in aquatic ecosystems (Dayton et al 1995; Roberts 1997; Botsford et al. 1997) it is important to begin assessing how fishery-induced changes in species composition have affected aquatic ecosystems. This is particularly true, for example, in areas where very expensive nutrient reduction plans are being implemented or contemplated in hopes of permitting the reestablishment of seagrass beds. If food web effects due to fishing activities are significantly involved in seagrass losses in such areas, costly reductions in nutrient supply may not ensure the return of seagrass habitat. In addition, this work asks whether similar food web linkages exist in both freshwater and marine SAV-dominated habitats. If food web linkages are similar, then we can use the predictive tools developed from work in freshwater systems to enhance our understanding of how food webs in marine systems are structured. If linkages are dissimilar, then we seek to determine how the differences we see might influence our ability to predict the effects of food web perturbations in each, and finally to what extent such differences are due to ecosystem level effects or to idiosyncrasies between the particular suite of species found in the freshwater and marine habitats. Ultimately, we expect this work to contribute materially to our understanding of how ecosystems can be affected by the removal of top consumers.