Assessing Fishery Potentials of Lakes and Reservoirs

Timeliness in estimating fishery potentials seems more important than precision, at least in the earlier periods of development of specific fisheries. Comparative studies of several sets of lakes, particularly of those supporting developing fisheries in Africa, suggest that potential yield may be related to several simple indices of production. The morphoedaphic index, derived from measures of total dissolved solids and the mean depth, has provided the simplest and most general approach to the problem of initial estimates of potential yield in lakes and reservoirs. Other methods of relating yield to nutrient concentration and primary production reinforce the concept of the dependence of potential yield on lake productivity. During the developing phase of a fishery more direct methods of estimating potential yield, through estimates of the production of the extant stocks, are also being used with some success. These have been adapted from fishery models developed for single stocks. In keeping with the need for rapid assessments during development, the simplest models have been emphasized. An equivalence between production and the product of biomass and mortality rate, along with the assumption that from 30 to 50% of the production of a stock is potentially harvestable, appears particularly useful. The problem of additivity of the potential yields of individual stocks seems relatively unimportant in first-order estimates, though the question of interactions is being examined, particularly in Lake Tanganyika. Programs of monitoring relative abundance of stocks over time have also been useful, both in indicating the approach of actual yield to potential yield and, after "calibration" against actual yields, as indices of stock and total biomasses.