Spatial and demographic consequences of nursery-dependence in reef fishes: an empirical and simulation study
The rainbow parrotfish Scarus guacamaia has an obligate dependence on man- groves at juvenile stages, and, as the largest herbivorous fish in the Caribbean region, its distribution has important implications for coral reefs. The effect of connectivity with mangroves on relative density, biomass and size of S. guacamaia was assessed from over 65 km of visual sur- veys from Bonaire, Caribbean Netherlands. In addition, an individual-based, age-structured, mechanistic model (IBM) was developed to explain dispersal patterns from nurseries for S. guaca- maia. In the IBM, mortality was constant, growth was determined by a von Bertalanffy growth equation, and movement was modeled through a random walk process. Using the IBM, simula- tions were run to generate patterns of density, biomass, and size with distance from nurseries. Rainbow parrotfish were observed as far as 42 km away from the nearest mangroves on Bonaire. Relative density and biomass showed significant exponential declines with distance from the pri- mary mangrove nursery and were significantly higher in high versus low complexity non-man- grove habitats. Mean size increased linearly with distance (r2 = 0.74), reflecting an absence of smaller individuals with greater distance. These results were closely mirrored by the simulation study: density and biomass declined exponentially with distance from nurseries, and size and age increased following saturating functions. The results suggest that mangroves may have the poten- tial to supply individuals much further than previously thought. Both the empirical and simulation studies reaffirm calls to prioritise protection of reef habitats close to nurseries as well as the nurseries themselves.