SCIENTIFIC ADVICE ON CATCH LEVELS 



J. A. GULLAND AND L. K. BOEREMA' 



ABSTRACT 



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 



' Department of Fisheries, FAO, Rome, Italy. 



Manuscript accepted October 1972. 



FISHERY BULLETIN: VOL. 71. NO. 2, 1973. 



criterion. The density -dependent form of the 

 Ricker-Beverton Holt approach is more satis- 

 factory in this regard, but makes heavy demands 

 on data and analysis. Despite these difficulties, 

 and in the absence of a single uniformly 

 acceptable model, some objective guidance can 

 be given on the magnitude of the catches that 

 can be taken from heavily exploited stocks, and 

 such guidance is being used by many regional 

 commissions [e.g., IWC (International Whaling 

 Commission), Inter-American Tropical Tuna 

 Commission, ICNAF (International Commission 

 for the Northwest Atlantic Fisheries)] in fram- 

 ing regulations. This paper explores some of the 

 bases of such guidance, in the hope of facilitating 

 the future preparation of advice on desirable 

 catch levels. 



SUSTAINABLE YIELD 



The concept of a sustainable yield as often 

 applied in practice, and more particularly the 

 idea of the maximum sustainable yield, is based 

 on a simplified model of a natural animal 

 population in which the population is treated 

 as a single unit, ignoring the fact that there are 

 individuals of different ages, etc.; it also ignores 

 all disturbing influences on the population, 

 other than removals by man. If such a popula- 

 tion has been reduced below the limiting carrying 



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