Biology, monitoring, and management of a tropical marine gastropod: the Queen conch (Lobatus gigas) in the Caribbean

The marine gastropod queen conch (Lobatus gigas), found throughout the Wider Caribbean Region, supports one of the most important fisheries in the region. However, several biological characteristics (e.g. density-dependent mating and survival, slow-moving, a preference for shallow depths, and aggregating behaviour during the reproductive season) make the species particularly vulnerable to overfishing. The heavy exploitation of queen conch throughout large parts of its natural range, as well as degradation of juvenile and adult habitats, also has in many areas led to a reduction in population densities to unsustainable levels to a point where mating success can be negatively affected. In addition, a new invasive seagrass species (H. stipulacea) has already caused significant alterations to the structure of native seagrass habitats which are in many parts of the Caribbean important to juvenile conch, providing both nutrition and protection from predators. However, the impacts of this invasive seagrass on life-history parameters such as growth and survival remain unknown.

Although many Caribbean nations have implemented policies with regard to queen conch protection and exploitation, recovery of overfished populations has been slow. These measures, however, are still mostly not harmonized among nations and often based on outdated and limited biological information. Management of queen conch is also often complicated by difficulties in data acquisition, partly caused by the logistically demanding and relatively expensive surveying of conch. Conventional survey methods using scuba are also limited to ca. 25 m depth due to safety limitations, making them unsuitable for collecting data across the entire depth range of conch, which extends to 60 m.

The main objectives of this study were to address knowledge gaps of the biology and ecology of the queen conch so that the distribution and dynamics of this species in relation to its environment are better understood. Such information will further improve our knowledge of marine gastropod biology in general, as well as our understanding of the effects of direct and indirect human-induced pressures on queen conch in the Caribbean. In addition, this study aimed to tackle some methodological shortcomings in the surveying and assessment of queen conch. Ultimately, these findings could be instrumental in the management and conservation of the species.

Reproductive characteristics are important biological reference points for the management of species. To improve the knowledge of queen conch reproductive biology (i.e. size-at-maturity and reproductive season), evaluation and comparison of the relationship between shell lip thickness and maturity in queen conch throughout the Wider Caribbean Region, using histological analysis of queen conch gonads was carried out. Furthermore, the influence of seawater temperature on the length of the reproductive season was investigated (chapter 2). We demonstrate a clear positive relationship between the thickness of the shell lip and the onset of maturity in queen conch, and that maturity occurs following the development of the lip. Lip thickness at 50% maturity (LT50) of both females and males varied between different locations in the Caribbean, although it did not correspond with variation in water temperature. In most cases, females had a larger LT50 than males indicating sexual dimorphism. Locations with a relatively high variation in water temperature had a significantly shorter reproductive season. The implementation of adequate minimum size regulation based on lip thickness (ca. 15 mm) and a Caribbean wide seasonal closure (May–September) using the most recent biological information from this study, taking into consideration the local differences in LT50 and reproductive season, will assist in developing a long-term sustainable queen conch fishery in the Caribbean.

To address the methodological shortcomings in the surveying of queen conch, a novel towed video method (TVM) was developed and compared with a conventional survey method (i.e. belt transect [BT] using scuba divers) in a series of calibration transects in two different habitats (i.e. high complexity (HC) and low complexity (LC)) (chapter 3). In both habitats, adult live queen conch had similar counts with both methods. Adult dead conch were not mistaken for live conch and the results validate the use of TVM as a reliable sampling tool to estimate densities of live adult conch in both HC and LC habitats throughout the species’ depth range.

In chapter 4, the spatial distribution of adult queen conch and how it varies in response to a number of known abiotic and biotic variables between sites which vary in environmental conditions was examined. By combining TVM with conventional belt-transects, a more comprehensive survey of conch abundance was performed at three sites in the Eastern Caribbean (Anguilla, St Eustatius, Saba Bank). Adult conch appeared in patchy distributions, mostly caused by spatial dependency, which was likely related to aggregating behaviour during spawning events. Environmental variables, such as algae cover, distance to the open ocean, and depth showed important non-linear effects on conch abundance, although these differed among sites. The proportion of reef and sand cover had important negative effects on conch abundance at all sites. High densities (>100 /ha) of adult conch were found only at depths >17 m at all three sites. The lack of strong generic location over-crossing relationships between abiotic and biotic factors and adult conch abundance and distribution is likely partly due to this spatial dependency, as well as different location-specific factors that affect different stages of the conch’s life-history. Furthermore, the results indicate that intermediate and deep areas (ca. 17 – 45 m) contain most of the reproductive output of conch in the survey sites and are therefore highly important for reproductive capacity. Thus, surveying areas at depths beyond the practical limitation of divers (<25 m) are of great importance to obtain more reliable population estimates.  

To provide a first insight into the possible impact of an invasive seagrass species (H. stipulacea) on queen conch, the diet and growth of juvenile conch in both native, mixed, and invasive seagrass beds was examined using stable isotope analysis and an in situ growth enclosure experiment (chapter 5). Organic material in the sediment (i.e. benthic diatoms and particulate organic matter [POM]) was found to be the most important source of carbon and nitrogen for juvenile queen conch in all three 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. Although much uncertainty still exists regarding the effects of H. stipulacea on the population dynamics on queen conch, if lower growth rates in invasive seagrass beds is a general pattern, it would have ramifications for both births and deaths of conch and the overall carrying capacity of conch populations in the Caribbean.

A better understanding of the spatial genetic structure (SGS) and the factors driving contemporary patterns of gene flow and genetic diversity of queen conch are fundamental for developing conservation and management plans for marine fisheries. A detailed study of SGS and genetic diversity was therefore performed using population genetic and multivariate analyses (chapter 6). Our study found that queen conch does not form a single panmictic population in the greater Caribbean. Significant levels of genetic differentiation were identified between Caribbean countries, within Caribbean countries, and among sites. Gene flow over the spatial scale of the entire Caribbean basin is constrained by oceanic distance, which may impede the natural recovery of overfished queen conch populations. Our results suggest a careful blend of local and international management will be required to ensure long-term sustainability for the species.

This study has provided new insights into queen conch biology and population dynamics as well as methodological shortcomings so that the distribution and dynamics of this species in relation to its environment are better understood. Ultimately, the findings from this study can contribute to improving the management and conservation of the species. However, the species will in the future face new challenges, due to expected changes in abiotic and biotic factors, such as temperature, ocean currents, and seagrass species composition. As body temperature and thus their physiological functions (e.g. growth) are directly dependent on environmental condition in this ectotherm species, it is particularly vulnerable to climate change (Dillon et al. 2010). Consequently, life-history parameters (e.g. size-at-maturity, reproductive season, growth rate, spatial genetic structure) of queen conch should not be considered rigid as these can be expected to change in the short and long-term, putting an unknown time limit to the relevance of the current biological knowledge of these parameters. However, there is still much uncertainty regarding what degree queen conch and other species can adapt to environmental changes induced by climate change and invasive species. Therefore, commitment to long-term research and updates in current biological knowledge, life-history parameters and population dynamics of queen conch throughout its range will be required to adjust subsequent management and conservation strategies to ensure the long-term sustainability of the species.