Fish density, diversity, and size-structure within multiple back reef habitats of Key West National Wildlife Refuge
Abstract:Tropical and subtropical back reef habitats such as seagrass meadows, mangrove prop-roots, and channels bisecting mangrove islands presumably serve as important nursery areas for numerous fishes. This study provides an initial step towards identification of the nursery role of specific habitats within multiple back reef habitats by quantifying fish density, diversity, and size-structure, and was part of a larger study that used aerial photographs, ground-truthing, and GIS software to map putative nursery habitats in the Key West National Wildlife Refuge (KWNWR). Visual surveys assessed fish density, diversity, and size-structure in the Lakes and Marquesas regions of the KWNWR over a 3-mo period and across the marine habitats of concern (seagrass, channels, mangroves, hardbottoms, patch reefs, offshore reefs). A combination of band transects and 10-min surveys provided a more complete overall species assessment than either method in isolation. Mangrove prop-root habitats contained the highest relative mean density and diversity of fish, with abundant forage fish such as silverside minnows (Atherinidae) and herrings (Clupeidae), as well as a high number of piscivores such as gray snapper Lutjanus griseus (Linnaeus, 1758) and barracuda Sphyraena barracuda (Walbaum in Artedi, 1792). Channel habitats contained the greatest diversity of microhabitats, and contained a relatively high diversity of fish compared to seagrass. Channel habitats typically harbored juvenile snappers (Lutjanidae), grunts (Haemulidae), and forage fish (Atherinidae). Qualitatively, we observed greater numbers of relatively large gamefish, as well as rare and threatened species in channel and mangrove habitats than any other habitat. Conversely, seagrass contained higher fish densities than channels. Increases in the size-frequency of certain species, such as S. barracuda, Pomacanthus arcuatus (Linnaeus, 1758), and Gerres cinereus (Walbaum in Artedi, 1792), from backreef habitats such as seagrass and mangroves, to channels and eventually patch and offshore reefs were suggestive of ontogenetic patterns of habitat use. In contrast, the smallest stages of L. griseus were found exclusively in seagrass, but remaining size classes, including adults, were found at all of the habitats surveyed. In contrast, the smallest size classes of Halichoeres bivitattus (Bloch, 1791), Lutjanis synagris (Linnaeus, 1758) and Haemulon sciurus (Shaw, 1803) were found in nearly all of the habitats examined. We found no relationship between fish density and diversity, or seagrass shoot density and blade height. Inclusion of seagrass, mangrove, and channel habitats in future studies of reef fish growth, survival, and emigration should produce a more complete picture of their nursery role in tropical back reef environments.
Document Type: Research Article
Publication date: 2004-09-01
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