SCIENTIFIC ADVICE ON CATCH LEVELS

The sustainable yield, or maximum sustainable yield, has been used to provide, on an objective scientific basis, target figures for the catches to be taken from a heavily exploited stock that is under regulation. The simple concept of sustainable yield does not, however, provide a completely adequate guide when the biological system is complex. Certain other quantities-the replacement yield, the sustainable yield from a stock in equilibrium, the maintainable yield, and the catch for desired harvesting rate-are defined which correspond more closely to the biological reality. One or other of these will provide a better guide for management, depending on the nature of the divergence from the simple model. In whale populations the major divergence is the lag between changes in adult stock and changes in recruitment; replacement yield or maintainable yield are the most useful. In many fish stocks, fluctuations in year-class strength are more important: catch for desired harvesting rate may be better. Various fishing research organizations are concerned with the setting of annual catch quotas or other measures for the management of the resources which are their responsibility. Agreement on the level of these quotas is more easily reached if they are determined by objec­ tive scientific criteria. Apart from the continuing difficulty in making precise assessments con­ cerning any wild animals, this guidance is difficult to provide without some agreed basis on how the "correct" catch should be calculated. At present there is not a single theoretical model for determining this catch that combines all the desirable features of (a) being readily understandable (at least in general outline) to the decision makers, (b) describing and predict­ ing in a realistic manner, and to an acceptable degree of precision the events in every fish stock to which it may need to be applied, and (c) capable of being applied to a specific fishery without great demands in data and analysis. The models associated with Schaefer (1954) on the one hand and with Ricker (1958) and Beverton and Holt (1957) (in the simple form when no account is taken of fishery-induced changes in recruitment) on the other fail, for many important fisheries, to satisfy the second