Stoner, A.W.

Abstract

The queen conch (Strombus gigas) is a large economically important gastropod that has been severely depleted throughout much of the Caribbean region. The species has determinate growth and reaches maximum shell length before sexual maturation; thereafter the shell grows only in thickness. In this study, queen conch were collected in the Exuma Cays, Bahamas, to evaluate maturity with respect to shell length (SL) (170–255 mm) and shell lip thickness (LT) (2–42 mm). Soft tissue weight and gonad weight increased with SL, but these same variables, along with the gonadosomatic index (gonad weight/soft tissue weight), all had dome-shaped distributions with LT and decreased slightly with LT > 22 mm. This indicates some loss of fecundity with age; however, no loss of reproductive capability was evident in histological data. Gonad maturity lagged substantially behind first formation of the shell lip. Minimum LT for reproductive maturity was 12 mm for females and 9 mm for males, and 50% maturity for the population was achieved at 26 mm LT for females and 24 mm LT for males, higher than previous estimates. A review of fishing regulations indicates that immature queen conch are being harvested legally in most Caribbean nations, providing at least a partial explanation for widespread depletion. While relationships between shell lip thickness, age, and maturity vary geographically, sustainable management of queen conch will require a minimum shell lip thickness for harvest no less than 15 mm, along with other urgently needed management measures. 

Large-scale distribution of a large, commercially significant gastropod, Strombus gigas (queen conch), was investigated in a II ,OOO-haregion of the Great Bahama Bank near Lee Stocking Island, Exuma Cays. Maps of depth and seagrass biomass, generated with Landsat thematic mapper data, and a 4-year survey of juvenile conch distribution showed that most of the juveniles were in aggregations located in 1.5-4.0-m water depth. Although general locations of juvenile conch aggregations remained the same between 1989 and 1992, their total surface area occupied only about 1.5% of the 8,300 ha of seagrass habitat available. Locations of only the most persistent long-term aggregations could be predicted on the basis of preferred seagrass biomass (30-80 g dry weight m-2); however, important conch nurseries were always located in tidal channels which brought clear, oligotrophic water from the Exuma Sound. Harmonic analysis of water temperature data from sites with and without juvenile aggregations showed that conch nurseries were subject to flushing with oceanic water on every tide, whereas non-conch sites reflected only diurnal heating and cooling of bank water. Relationships between circulation and juvenile conch distribution on the Great Bahama Bank may be related directly to larval recruitment, or indirectly to aspects of nutrient cycling and food production; evidence for both mechanisms exists. Although exact locations of conch aggregations shift from year to year, these shifts appear to occur within larger nursery habitats, the boundaries of which are set by a precise combination of physical and biological factors. Because most meadows are probably unsuitable for this severely ovcrfished species, critical nursery habitats should be identified and protected. 

ABSTRACT: Effectiveness of a marine protected area (MPA) in supporting fisheries productivity depends upon replenishment patterns, both in supplying recruits to surrounding fished areas and having a sustainable spawning stock in the MPA. Surveys for queen conch Strombus gigas were made in 2011 at 2 locations in the Exuma Cays, The Bahamas, for direct comparison with surveys conducted during the early 1990s at Warderick Wells (WW) near the center of the Exuma Cays Land and Sea Park (ECLSP) and at a fished site near Lee Stocking Island (LSI). There was no change in adult conch density and abundance in the shallow bank environment at LSI where numbers were already low in 1991, but numbers declined 91% in the deeper shelf waters. At WW, the adult population declined 69% on the bank and 6% on the island shelf. Unlike observations made in the 1990s, queen conch reproductive behavior near LSI is now rare. Average age of adult conch (indicated by shell thickness) at LSI decreased significantly during the 20 yr period between surveys, while average age increased at WW and juvenile abundance decreased. These results show that the LSI population is being overfished and the WW population is senescing because of low recruitment. In 2011, the ECLSP continued to be an important source of larvae for down- stream populations because of abundant spawners in the shelf environment. However, it is clear that the reserve is not self-sustaining for queen conch, and sustainable fishing in the Exuma Cays will depend upon a network of MPAs along with other management measures to reduce fishing mortality. 

ABSTRACT: There is increasing recognition that habitats should be managed as part of fisheries management. It is generally assumed that amount of suitable habitat is linked to production of de- mersal species and that maps of bottom type will provide the information needed to conserve essen- tial habitats. In this review, a synthesis of nursery habitat is made for Strombus gigas (queen conch), a large, economically important gastropod in the Caribbean region. Juveniles occur on a variety of bottom types over their geographic range. In the Bahamas, nurseries occur in specific locations within large, beds of seagrass, while obvious characteristics of the benthic environment such as seagrass density, depth and sediment type are not good predictors of suitable habitat. Rather, nurseries persist where competent larvae are concentrated by tidal circulation and where settlement occurs selec- tively. Nursery locations provide for high juvenile growth resulting from macroalgal production not evident in maps of algal biomass, and they provide for low mortality compared with seemingly simi- lar surroundings. Therefore, critical habitats for queen conch juveniles are determined by the inter- section of habitat features and ecological processes that combine to yield high rates of recruitment and survivorship. While maps of bottom type are a good beginning for habitat management, they can be traps without good knowledge of ecological processes. A demersal species can occupy different substrata over its geographic range, different life stages often depend upon different bottom types, and specific locations can be more important than particular habitat forms. Habitat management must be designed to conserve habitat function and not just form. Implicit in the concept of ‘essential habitat’ is the fact that expendable habitat exists, and we need to prevent losses of working habitat because of inadequate protection, restoration or mitigation. Key nurseries may represent distinctive or even anomalous conditions. 

An important scientific workshop on queen conch was held in Caracas, Ven­ ezuela, in July 1991. This workshop and the proceedings that emerged from it (Appeldoom and Rodriguez, 1994) pro­ vided a good background on the status of research on biology, fisheries, and mariculture of the queen conch. Be­ cause the general biology of the queen conch is already relatively well known, the purpose of this paper is to summa­ rize some of the important advances made in the study of queen conch since the 1991 workshop. Emphasis has been placed on topics related to the ecology of queen conch that are most relevant to fisheries management and stock re­ habilitation. In the following sections an attempt has been made to draw con­ clusions about habitat requirements for the species, mortality of juveniles as it relates to stock rehabilitation and en­ hancement, larval ecology and fisher­ ies oceanography of the species, and the conservation of reproductive stocks. 

Queen conch (Lobatus gigas), is an economically and culturally important marine gastropod. The species is subject to extensive exploitation throughout large parts of the Caribbean which has led to a decrease in population densities across much of the species’ distribution range. Hence, there is a need for protective measures to safeguard the reproductive stock. This requires a better estimation of its size at maturity, which is best quantified as the thickness of the lip that the shell develops after reaching its maximum length. The lip thickness at 50% maturity (LT50) was determined using a logistic and an accumulation model, from seven representative location of distribution of this species in the Wider Caribbean Region. LT50 of both females (7–14 mm) and males (4–11.5 mm) 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. LT50 values estimated with the logistic model were smaller (7–14mm for females, 4–11.5mm for males) than values estimated with the accumulation model (13–26mm for females, 16–24mm for males), showing an overestimation of LT50 in queen conch in previous studies which used the accumulation model to estimate LT50. 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