GULLAND and BOEREMA: SCIENTIFIC ADVICE ON CATCH LEVELS 



second would give assumptions that could be 

 made about the recruitment; the third and 

 fourth columns would then give the fishing 

 mortality necessary to achieve the objective and 

 the corresponding catch in the foithcoming 

 season. For example, for a purely hypothetical 

 stock the information could be given as follows: 



Objeciixe and 

 recniitiiient 



Maximum physical yield: 

 Constant 



Moderately density-dependent 

 Strongly density-dependent 



Maximum economic yield:' 

 Constant 



Moderately density-dependent 

 Strongly density-dependent 



1 For a certain set of assumptions on costs and prices. 



While this still leaves a wide choice open — 

 a range of over twofold, the more extreme values 

 might be ignored so that the real choice may 

 not be large, and the information provides a 

 useful basis for determining management action. 



REBUILDING DEPLETED STOCKS 



Bank, since the vessels concerned can switch to 

 other fisheries, than to do the same for Antarctic 



whales. 



The general policy to be followed by the 

 management body — to aim for the maximum 

 physical yield from a given stock, or some lesser 

 physical yield but with greater economic bene- 

 fits, to rebuild a depleted stock quickly or 

 merely prevent its further decline — must be 

 determined by that body taking all factors into 

 account and cannot be decided purely on bio- 

 logical grounds. What can be determined by 

 objective scientific calculations is the actual 

 procedure to be followed — specifically the catch 

 to be taken in each season — to implement the 

 chosen policy. This note has been concerned 

 purely with a discussion of this second step. 



ACKNOWLEDGMENTS 



Our thanks are due to D. G. Chapman, Y. 

 Fukuda, R. Hennemuth, T. D. lies, J. Joseph, 

 and B. Rothschild for helpful comments on 

 early drafts of this paper. 



LITERATURE CITED 



A special problem arises when advising on 

 the management of badly depleted stocks, 

 especially one (e.g., of whales) in which recruit- 

 ment is closely dependent on the abundance of 

 the adult stock. 



For such a stock calculations can be made of 

 the replacement yield, or equivalent sustainable 

 yield, but this should not imply that these are 

 the proper objectives. Rather action should be 

 taken to rebuild the stock. This is done most 

 rapidly by cutting the catches to zero for a 

 period. The optimum strategy between this 

 (making the greatest present sacrifice for the 

 maximum long-term benefit) and merely main- 

 taining the situation (making no present sacri- 

 fice) will be determined by a number of factors, 

 mainly economic. This will include the relative 

 values placed on present and future catches, 

 alternative employment for the excess men and 

 equipment in the present fisheries, etc. For 

 example, it would be much easier to stop for a 

 period the offshore herring fishery on Georges 



Beverton, R. J. H., AND S. J. Holt 



1957. On the dynamics of exploited fish populations. 

 Fish. Invest. Minist. Agric. Fish. Food (G. B.) 

 Ser. II, 19,533 p. 



GuLLAND, J. A. 



1968a. The concept of the maximum sustainable 



yield, and fishery management. FAO (Food Agric. 



Organ. U.N.) Fish. Tech. Pap. 70, 30 p. 

 1968b. The concept of the marginal yield from 



exploited fish stocks. J. Cons. 32:256-261. 

 International Commission for the Northwest Atlantic 



Fisheries. 

 1970. Report of Assessment Sub-Committee to 1970 



Annual Meeting. Int. Comm. Northwest Atl. Fish. 



Annu. Proc, p. 33-65. 

 1972. Report of Assessment Sub-Committee to 1972 



Annual Meeting. Int. Comm. Northwest Atl. Fish. 



Annu. Proc, p. 17-42. 

 Ricker, W. E. 



1958. Handbook of computations for biological 

 statistics of fish populations. Fish. Res. Board 

 Can., Bull. 119. 300 p. 



Schaefer, M. B. 



1954. Some aspects of the dynamics of populations 

 important to the management of commercial 

 marine fisheries. Inter-Am. Trop. Tuna Comm., 

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335 



