The fishery of Balistes capriscus (Balistidae) in Ghana and possible reasons for its collapse

Abstract

The fish catch statistics of grey triggerfish (Balistes capriscus) in Ghana from 1972 to 2003 have suggested a possible 'regime shift' of triggerfish in coastal waters of Ghana. This suggests possible influence of local environmental parameters (sea surface temperature, salinity and coastal wind speed) and/or exploitation on grey triggerfish resource. The observed variability in environmental conditions and triggerfish landings off Ghana occurred seasonally. Time series analyses of sea surface temperature, salinity and coastal wind speed from alongshore recording stations of Marine Fisheries Research Division in Tema, Ghana and Ghana Meteorological Agency over the period 1974-2004 suggest possible link between local environmental parameters and triggerfish catch in Ghana. Again, the identification of maximum mean temperature partitioning (between the periods 1972-76 and 1985-89) and maximum mean critical temperature (in 1987) support the notion of the contribution of sea warming in the disappearance of the triggerfish resource. The insights from the seasonal time series analysis of salinity and wind speed suggest their seasonal control of sea temperature and hence the major coastal cooling along the coast of Ghana. Size-weight relationships of grey and blue-spotted triggerfish indicate that for a given size grey triggerfish tends to weigh less than blue-spotted triggerfish. However, in both equations the exponent for length is sufficiently close to 3.0, a situation which indicates that B. capriscus and B. punctatus grow isometrically. The maximum age of triggerfish in this study was age 11 which is lower than previously reported age of grey triggerfish in northeastern Gulf of Mexico. The asymptotic length, L∞ was greater and rate of growth, K slower than previously reported for the grey triggerfish in Ghana. The estimated natural mortality, M for grey triggerfish (0.40) in this study (derived from Rikhter and Efanov's method) was found to be lower than Ofori-Danson's estimates (0.81) on Ghana grey triggerfish stocks in 1980. Grey triggerfish exploitation ratio (0.2727) obtained in this study shows that the triggerfish resource is underexploited as compared to the exploitation ratio of 0.67 derived from 1980 growth results of grey triggerfish. There is indication that Balistes capriscus and Balistes punctatus have habitat overlap in the western Gulf of Guinea. The gut analysis of B. capriscus and B. punctatus indicate that both triggerfish species are more planktivorous at juvenile stage (12.0-21.9 cm size class) and more benthivorous at later stage in life. The gut analysis of B. capriscus in this study is much comparable to the previous stomach content analysis in 1980 and hence, the possibility of change in diet might not have been the cause of triggerfish disappearance in Ghanaian coastal waters.