Martin de Graaf

Abstract

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 locationspecific 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.

ABSTRACT: Juvenile queen conch are primarily associated with native seagrass such as Thalassia
testudinum in large parts of their range in the Caribbean and the southern Gulf of Mexico. Here,
a number of non-native seagrass species have been introduced including Halophila stipulacea,
which is natural to the Red Sea and the Indo-Pacific. In the Caribbean, H. stipulacea often creates
dense continuous mats with little or no sediment exposed, compared to native seagrass, which
grows much less dense. We examined the diet and growth of juvenile conch in both native, mixed,
and invasive seagrass beds using stable isotope analysis and an in situ growth enclosure experiment.
Organic material in the sediment (i.e. benthic diatoms and particulate organic matter) was
found to be the most important source of carbon and nitrogen for juvenile queen conch in all 3
habitats investigated, and there was a significantly higher probability of positive growth in the
native seagrass compared to the invasive seagrass. Due to the importance of the organic material
in the sediment as a source of nutrition for juvenile conch, limited access to the sediment in the
invasive seagrass can potentially cause inadequate nutritional conditions to sustain high growth
rates. Thus, it is likely that there is a negative effect on juvenile queen conch growth currently
inhabiting invasive seagrass beds, compared to native seagrass beds, when other potential
sources of nutrition are not available.