A POPULATION SURVEY OF THE WEST INDIAN TOPSHELL OR WHELK (CITTARIUM PICA) IN THE U.S. VIRGIN ISLANDS

Populations of the West Indian topshell, Cittarium pica (Gastropoda: Trochidae), were surveyed at 41 areas in the U.S. Virgin Islands between June 2003 and May 2004. The size and density of C. pica, known locally as whelk, varied substantially among sites, but was not significantly different between northern islands (St. Thomas, St. John, associated cays) and the southern island (St. Croix). Average size was significantly different between island groups and showed a non-significant negative relation to density. Observed spatial variability was partly explained by habitat exposure: whelk at windward sites (offshore cays, exposed points) had larger average size and occurred at lower density, whelk at leeward sites (bays, semi-protected coasts) had smaller average size but occurred at higher density. Larger adult C. pica were rare or absent at most sites although their densities were comparatively high within two marine protected areas, a restrictedentry area, and at a number of sites characterized by exposure to high wave energy. Comparison to limited historic USVI data for C. pica suggests a decline in the relative abundance of large individuals. Together, these observations suggest that harvesting is a major influence upon, if not the primary determinant of, C. pica population structure in the USVI. Data from paired-sampling (leeward and windward sides) indicate that whelk young-of-the-year (YOY) are more abundant on leeward coasts, which suggests that local patterns of whelk recruitment may be influenced by nearshore oceanographic processes. A peak in YOY density was observed in spring and the significance of this observation is discussed in relation to periods of annual spawning. Recommendations for additional studies are presented. Introduction The West Indian topshell, Cittarium pica, is a trochid gastropod that inhabits rocky shorelines which are exposed to wave action (Randall 1964). It is commonly found in the intertidal and shallow subtidal areas the yellow and pink zones (Lewis 1960, Kaplan 1988) where it grazes on a variety of predominantly filamentous forms of algae (Randall 1964). C. pica is distributed throughout the Caribbean and Bahamas, with comparatively recent extinctions reported from Bermuda and Florida (Abbott 1976). It attains a relatively large size of about 10 to 12 cm and Fisher (1978) reports a maximum length of 13.6 cm. Owing to its edible qualities (Clench and Abbott 1943), C. pica is commonly harvested for food throughout much of its range (Fisher 1978). Near human population centers, larger individuals are rare (Clench and Abbott 1943) and overexploitation has become a concern throughout much of its range (Carter 2002). In the United States Virgin Islands (USVI), West Indian topshells are also a popular food item, known locally as whelks (Randall 1964), and humans have harvested whelks from local shorelines since pre-Columbian times (R. Boulon pers. comm.). Presently, whelks are also harvested commercially in the USVI and whole animals (in the shell) are sold for about US$10 per pound (Holt and Uwate 2004, R. Gomez pers. comm.). Although commercial, recreational, and subsistence harvesting occurs in the USVI (Clavijo et al. 1984), little is known of the fishery and less still is known about the impact of harvesting upon C. pica populations. In the late 1980’s, concerns over declining whelk stocks (deGraaf and Moore 1987) prompted Territorial