Use of Population Parameters in Examining Changes in the Status of the Short-Necked Clam <i>Paphia undulata</i> Born, 1778 (Mollusca, Pelecypoda: Veneridae) in Coastal Waters of Southern Negros Occidental

Abstract

Growth, recruitment, mortality and exploitation rate of the short-necked clam Paphia undulata (Born, 1778) were studied in southern Negros Occidental waters between August 2007 and July 2008 from length-frequency data derived from catches of divers, to be able to compare with earlier data derived and analyzed 13 years ago by Agasen et al. (1998).  Both sets of data were analyzed using the FiSAT software (Gayanilo & Pauly, 1997).  The asymptotic shell length (S¥ = 79 mm) derived from the present data proved to be smaller compared to the earlier data (SL¥ = 81.5 mm) due to the lack of bigger sizes in the present samples. The growth constant (K = 1.0 yr-1) was however comparable indicating it to be a more species-characteristic parameter.  Two recruitment pulses for each study were derived and were found to be correlated with the spawning pattern in the species.  The value of natural mortality (M = 1.57 yr-1) derived from bivalve literature, is deemed more appropriate compared to the earlier estimate (M = 2.89 yr-1) based on Pauly’s (1980) empirical equation developed for fish.   Total mortality (Z) values for both studies were comparable, but a higher level of fishing mortality (F = 4.61 yr-1) was estimated for the present data set, thus resulting likewise in a higher exploitation rate (E = 0.75).  These, together with  fishery information from an accompanying paper (Villarta & del Norte-Campos, 2010), not only validate the earlier findings of overexploitation due to lack of management,  but reveal a worsening condition of the stock, most likely as a result of growth overfishing. Overexploitation can only be mitigated by imposing stringent restrictions in terms of the minimum size for exploitation (45 mm shell length) and closed seasons during the spawning peak (August-November).