Implications of overlooked seasonal growth dynamics in tropical fisheries assessment: A test case of an oyster (Crassostrea tulipa) fishery in the Densu Delta, Ghana

Abstract

Tropical fish stocks are perceived not to exhibit seasonality in growth. For this reason the standard von Bertalanffy growth function (VBGF) has been used widely to fit the growth of tropical fish populations compared to the seasonally-oscillating VBGF (soVBGF), which is seldom used in tropical fisheries. To advocate for the use of the soVBGF in assessment of tropical fish stocks, this study compared the outputs of the two methods using ELEFAN_GA_boot on oysters from the Densu Delta, Ghana. Sampling of mangrove oyster, Crassostrea tulipa covered a period of 12 months and the data were analysed for VBGF growth parameters and its prediction models. The intensity of growth oscillation showed that C. tulipa exhibited seasonal growth (C = 0.50), and the soVBGF fitted the growth of the oyster better than the standard VBGF method. Estimates of some of the stock parameters (t50,tmax, Fcur, Ecur and Ycur) were comparatively higher for the standard VBGF than the soVBGF approach. In both approaches, the oysters were underexploited (Ecur < Emsy). However, unlike the soVBGF method where Fcur < F0.5, the standard method indicated that Fcur > F0.5. In view of the disparities, studies which adopt standard VBGF on tropical stocks that exhibit seasonality would likely generate comparatively higher outputs for relevant biological reference points, which may ultimately mislead management decisions. Given the sedentary nature of oysters which could render the organisms more susceptible to seasonal variations in environmental conditions to show seasonality as observed in this study, we recommend further works on tropical shell- and finfishes to corroborate the current findings or otherwise.