Mangrove removal in the belize cays: effects on mangrove-associated fish assemblages in the intertidal and subtidal

We investigated the effects of mangrove cutting on fish assemblages in Twin Cays, Belize, in two habitat types. We conducted visual censuses at two sites in adjoining undisturbed/disturbed (30%–70% of shoreline fringe removed) sub-tidal fringing Rhizophora mangle Linnaeus, 1753. Observers recorded significantly more species and individuals in undisturbed sites, especially among smaller, schooling species (e.g., atherinids, clupeids), where densities were up to 200 times greater in undisturbed habitat. Multivariate analyses showed distinct species assemblages between habitats at both sites. In addition, extensive trapping with wire minnow traps within the intertidal zone in both undisturbed and disturbed fringing and transition (landward) mangrove forests was conducted. Catch rates were low: 638 individuals from 24 species over 563 trap-nights. Trap data, however, indicated that mangrove disturbance had minimal effect on species composition in either forest type (fringe/ transition). different results from the two methods (and habitat types) may be explained by two factors: (1) a larger and more detectable species pool in the subtidal habitat, with visual “access” to all species, and (2) the selective nature of trapping. Our data indicate that even partial clearing of shoreline and more landward mangroves can have a significant impact on local fish assemblages. Among subtropical and tropical coastal marine habitats, mangroves are regarded as unusually biologically diverse and quite productive. In addition, the importance of mangroves in shoreline stabilization and protection of the terrestrial interface, and the linkages to adjacent marine systems (e.g., seagrasses/coral reefs) was recognized long ago (Lugo and Snedaker, 1974). Within the perceived resource values of the mangal is that of a nursery area for juvenile coral reef fishes. The literature is replete with discussions of this nursery concept, and the debate is ongoing. Beck et al. (2001) defined general guidelines for describing marine nursery areas, which included (1) greater densities of juveniles, (2) faster growth rates, (3) enhanced juvenile survival, and (4) successful movement to adult habitats. Nagelkerken et al. (2000) provided a summary of the various aspects of the nursery hypotheses specific to the mangal, the primary bases of which are predator avoidance, food abundance, and larval settlement. Contributing to these aspects are (1) the high structural complexity of the mangal, which both protects against predators and intercepts entrained fish larvae, (2) relatively remote locations from reef or offshore waters, reducing proximity to predators, (3) enhanced turbidity, reducing predation efficiency, (4) high autochthonous food production, and (5) areal extent that could intercept planktonic fish larvae at greater frequency than the reef itself. Many early assumptions of nursery value for both mangroves and seagrasses are now being challenged, since they were based on qualitative observations, with no separation of information on fish size or juvenile/adult ratios (Nagelkerken et al., 2000). Faunce and Serafy (2006) indicated that there is evidence that many fish species use the mangal as a diurnal refuge, and thus some of the fisheries production assumed to have originated there may have other sources. BULLETIN OF MARINE SCIENCE, vOL. 80, NO. 3, 2007 880 The nursery role of mangroves may be strongest at broad scales. For example, the areal extent of mangroves is assumed to correlate with secondary production in the commercial fish catch (Manson et al., 2005). Mumby et al. (2004) estimated fish biomass at three Caribbean atolls with no or very limited mangrove cover and compared them with “mangrove-rich” systems (defined as having ≥ 70 km2 of fringing Rhizophora mangle Linnaeus, 1753 within an area of 200 km2). Fish biomass was much greater (up to 25 times for some species) on the reef systems with abundant, adjacent mangroves. The study concluded that mangroves clearly offer an intermediate nursery area that provides a transition between seagrass beds and patch reefs. The data of Mumby et al. (2004) (comparing intact vs naturally “barren” areas) provide, at best, an indication of the consequences on fisheries of human-caused mangrove destruction. The rate of worldwide mangrove deforestation (now at 35% globally) is highest in the Americas, where it is now estimated at 2251 km2 yr–1 (valiela et al., 2001). With destruction of the mangal so common, it is not surprising that studies assessing the effects of human changes are abundant, but most of this work has focused on the Pacific, and much of the emphasis has been on invertebrates and the physical changes following mangrove alteration (water chemistry/sediment characteristics; Manson et al., 2005). Surprisingly, studies examining the effects of alteration on fishes directly associated with the mangal are sparse. We note only one study (Ellis, 2003) that has examined mangrove canopy trimming (Rookery Bay, FL, USA) and the effect on fish assemblages. No effect on overall assemblage structure was found with moderate trimming and the author concluded that alterations to the habitat may have to be significant before changes occur, as variability between samples was very great in the sites studied. In addition, there are few data on how fishes respond to natural environmental variability within the mangal (Faunce and Serafy, 2006). We believe that to slow the rate of mangrove deforestation, empirical data are needed on the effects of mangrove shoreline alteration on associated fish assemblages. here, we examine the effects of mangrove removal on fishes inhabiting the intertidal and subtidal mangroves on one of the Belize cays.