Effects of density and habitat structure on growth and survival of harvested coral reef fishes

This study examines four species of ecologically similar, harvested coral-reef fishes to evaluate the influences of habitat on density, growth, and survival and tests for density dependence in growth and survival. I studied populations of Lutfanus apodus, L. synagris, Haemulon flavolineatum and H. plumierii on 8 coral reefs in the Bahamas using underwater visual surveys and mark-recapture studies. For L. apodus I used a large-scale density manipulation experiment to uncouple the relationship between density and habitat. I conducted field and laboratory tests of some of the critical assumptions of mark-recapture methods to ensure that my estimates of growth and survival were unbiased.; A tag retention study revealed that PIT tags were retained well and did not affect rates of growth or survival of individuals. Analysis of mark-recapture data indicated that the assumption of equal probability of survival of fish between sampling periods was met. The fish trapping method was found to provide a representative sample of the population, however, a trap response was detected. This violation of mark-recapture assumptions was accommodated by modifying the mark-recapture model structure to produce unbiased estimates of survival.; The degree of habitat association differed among the four species. Densities of L. apodus and H. flavolineatum were strongly tied to habitat structure (coral boulders); whereas H. plumerii density was unrelated to boulders, and L. synagris density was negatively associated with boulders. Despite the varied relationships between density and habitat features, correlative tests did not reveal any influence of habitat on growth and survival of the four species. Nor did correlative tests reveal any effects of density on growth and survival. The density manipulation experiment with L. apodus, however, revealed that survival was positively affected by density. Growth rates were unaffected by density. The positive effect of density on survival of L. apodus contrasts with the results of most studies on non-exploited coral-reef fishes. The inverse density dependence detected in L. apodus implies that regulatory mechanisms that would help populations recover from overfishing may be absent in this species, but that marine protected areas may be particularly effective for management and conservation of this and similar species.