The fish biomass estimates made in this report are in the order of 100 000 - 210 000 tonnes. The potential yield (Y) could be estimated according to the formula given in FAO (1979):
Y = 0.5 (C + MB1)where C is catch, M is natural mortality and B1 the biomass at the time of the survey. In the same paper 0.5 is suggested as an overall value for natural mortality for similar species compositions. The average catch in Tanzania during the period 1975-1980 was about 40 000 tonnes. The potential yield was then estimated at about 45 000 - 70 000 tonnes.
However, the size of the estimated potential yield depends on the chosen value for the natural mortality. If the natural mortality is closer to 1 than 0.5 the potential yield will increase by 50-70 %. Big fish as snappers and rock cods have low natural mortality and thereby these stocks could only support a low fishery. It is possible that larger species in general will decline faster than smaller species because the different species will not react the same way to fishing pressure.
The rather rough estimates of potential yield in Tanzanian waters indicate that the catches may be slightly increased.
