Growth, mortality and potential yield of bonga, Ethmalosa fimbriata (Bowdich 1825) of Nigerian inshore waters

The distribution of and fisheries for bonga, Ethmalosa fimbriata (Clupeidae), in Nigerian inshore waters east of the Niger delta are discussed. Data on length frequency collected between 1973 and 1982 are analysed to obtain the von Bertalanffy's growth parameters ( L ∞=30 cm, K=0.43 and T 0=−0.20) which are then used to estimate the instantaneous total ( Z), natural ( M) and fishing ( F) mortality coefficients. Catch-per-effort statistics (1973–1982) allow for a simple graphical treatment of Schaefer's logistic model from which the maximum sustainable yield ( Y max=30 075 t year −1) is derived for the stock of bonga in this part of the Nigerian coast. Considering the level of mean catch (25 030 t year −1), which is very close to the optimum level of yield ( Y ec) and the mean value of the exploitation ratio ( F Z =0.57 ), it is concluded that the stock of E. fimbriata of the inshore waters east of the Niger delta is being fished at about its Y ec. Although this is the desirable level of exploitation if profit is to be maximized, this fishery might be allowed to expand beyond Y ec to contribute to the solution of an important socio-economic problem, namely, reduction of unemployment, but fishing at Y max should be avoided to prevent the collapse of the stock.