Spatial dependency in abundance of Queen conch, Aliger gigas, in the Caribbean, indicates the importance of surveying deep-water distributions

Aim: To evaluate how the spatial distribution of a heavily exploited marine gastropod (i.e. Queen conch) varies in response to a number of known biotic and abiotic variables within and between study areas that vary in environmental conditions. Location: Three study areas in the north-eastern Caribbean, Anguilla, Saba Bank and St. Eustatius. Methods: A novel towed video system and complemented belt transects to estimate adult queen conch densities throughout its depth range. Bayesian hierarchical spatial models (integrated nested Laplace approximations) modelled distribution patterns of adult conch. Results: Our study revealed patchy distribution patterns of adult conch caused by spatial dependency. This dependency is most likely related to aggregating behaviour during spawning events. Environmental variables, such as algal cover, distance to the open ocean and depth, showed important nonlinear effects on conch abundance, although these differed among study areas. Intermediate and deep areas (ca. 17–45 m) contain most of the reproductive output of conch in the study areas and are highly important for the reproductive capacity. Main conclusions: The general patchy distribution pattern and the lack of strong generic relationships between biotic and abiotic factors and adult conch abundance and distribution are likely to be at least partly due to spatial dependency and location-specific factors, which affect different phases of the conch's life history. The depth distribution of conch also indicates that surveying areas at depths beyond the practical limitation of divers is of great importance to obtain more reliable population estimates.