Ecological Applications, 18(6), 2008, pp. 1501–1515 Ó 2008 by the Ecological Society of America EXPLOITED SPECIES IMPACTS ON TROPHIC LINKAGES ALONG REEF–SEAGRASS INTERFACES IN THE FLORIDA KEYS JOHN F. VALENTINE, 1,2,10 KENNETH L. HECK,JR., 1,2 DERRICK BLACKMON, 1,2 MARGENE E. GOECKER, 3 JULIET CHRISTIAN, 4 RYAN M. KROUTIL, 5 BRADLEY J. PETERSON, 6 MATHEW A. VANDERKLIFT, 7 KEVIN D. KIRSCH, 8 AND MIKE BECK 9 1 Dauphin Island Sea Lab, University of South Alabama, 101 Bienville Boulevard, Dauphin Island, Alabama 36528 USA 2 Department of Marine Science, University of South Alabama, Mobile, Alabama 36688 USA 3 Department for Environment and Heritage, Coast and Marine Branch, 75 Liverpool Street, Port Lincoln, South Australia 5606 Australia 4 Ecological Associates, Inc., P.O. Box 405, Jensen Beach, Florida 34958 USA 5 Department of Fisheries and Aquatic Sciences, University of Florida, Gainesville, Florida 32611 USA 6 Marine Sciences Research Center, Stony Brook University, Stony Brook, New York 11794 USA 7 CSIRO Marine Research, Underwood Avenue, Floreat, Western Australia 6014 Australia, and Centre for Ecosystem Management, Edith Cowan University, 100 Joondalup Drive, Joondalup, Western Australia 6027 Australia 8 NOAA Office of Response and Restoration, Assessment and Restoration Division, 263 13th Avenue South, Saint Petersburg, Florida 33701 USA 9 The Nature Conservancy, LML, 100 Shaffer Road, University of California, Santa Cruz, California 95060 USA Abstract. The removal of fish biomass by extensive commercial and recreational fishing has been hypothesized to drastically alter the strength of trophic linkages among adjacent habitats. We evaluated the effects of removing predatory fishes on trophic transfers between coral reefs and adjacent seagrass meadows by comparing fish community structure, grazing intensity, and invertebrate predation potential in predator-rich no-take sites and nearby predator-poor fished sites in the Florida Keys (USA). Exploited fishes were more abundant at the no-take sites than at the fished sites. Most of the exploited fishes were either omnivores or invertivores. More piscivores were recorded at no-take sites, but most (;95%) were moderately fished and unexploited species (barracuda and bar jacks, respectively). Impacts of these consumers on lower trophic levels were modest. Herbivorous and smaller prey fish (,10 cm total length) densities and seagrass grazing diminished with distance from reefs and were not negatively impacted by the elevated densities of exploited fishes at no-take sites. Predation by reef fishes on most tethered invertebrates was high, but exploited species impacts varied with prey type. The results of the study show that, even though abundances of reef- associated fishes have been reduced at fished sites, there is little evidence that this has produced cascading trophic effects or interrupted cross-habitat energy exchanges between coral reefs and seagrasses. Key words: coral reefs; exploited species; food web interactions; seagrass herbivory; trophic cascade; trophic transfer. INTRODUCTION A great deal of evidence suggests that extensive commercial and recreational fishing has drastically altered the structure of marine ecosystems (e.g., Dayton et al. 1995, Botsford et al. 1997, Coleman et al. 2004). Specifically, the historically intense harvesting of higher order consumers has led to substantial reductions of piscivorous fish biomass worldwide (Hutchings 2000, Christensen et al. 2003, Myers and Worm 2003). As higher order consumers became increasingly rare, fishers shifted their harvesting to lower order consumers, including omnivorous and herbivorous fishes and invertebrates (Pauly et al. 1998, 2000, Steneck et al. 2004). The impacts of these food web changes for the transfer of primary production to the survivors in higher trophic levels are uncertain. However, current thinking strongly suggests that the intensity of density-dependent interactions and the transmission of primary production to higher trophic levels are much lower in most modern day oceans than they once were (cf. Steneck et al. 2004, Mumby et al. 2006). Among the other probable impacts of consumer removals are significant reductions in the numbers and strengths of trophic linkages among habitats (Mumby et al. 2004, Valentine and Heck 2005). Although the historical importance of cross habitat transfers of energy in marine food webs is unclear, we do know that once abundant large predators (e.g., groupers, snappers, sharks, and bluefin tuna) fed in multiple habitats to meet their nutritional needs (cf. Dayton et al. 1995, Block et al. 2001, Heithaus et al. 2002, Bonfil et al. 2005). Early studies of coral reefs found that carnivo- rous fish biomass was three to four times greater than Manuscript received 17 October 2007; accepted 5 November 2007; final version received 14 February 2008. Corresponding Editor: P. K. Dayton. 10 E-mail: jvalentine@disl.org 1501