FISHERY BULLETIN: VOL. 71, NO. 2 



stock is changing and is below the level of 

 MSY, however, the maintainable yield is equal 

 neither to the replacement yield nor to the 

 sustainable yield corresponding to the abundance 

 of either parent or present stock, but will be at 

 a level between the sustainable yields of these 

 two stock sizes. Maintenance of the catch at 

 the level of maintainable yield will ultimately 

 lead to an equilibrium situation with a stock 

 size in between the original "parent" and 

 "present" stock sizes. 



It should be noted that if the stocks are 

 below MSY level and have recently been de- 

 creasing, the maintainable yield is lower than 

 the replacement yield, whereas if the stocks 

 have recently been increasing, the maintainable 

 yield is the higher of the two. Any catch lower 

 than the maintainable yield, if kept at the same 

 level for a sufficient period of time, will ulti- 

 mately lead to a rebuilding of the stocks, even 

 though in the short term it may cause some 

 decrease. 



If the population is below the MSY level, 

 setting the catch equal to the estimated sus- 

 tainable yield will, as has been frequently 

 pointed out, lead to an unstable situation. If the 

 estimate used is only just too high the stock 

 will start to decline, the gap between the catch 

 and the actual sustainable yield will widen at 

 an accelerating rate, and the stock will decline 

 ever more rapidly until the catch is adjusted. 



As pointed out by Y. Fukuda (pers. comm.) 

 this can occur, if there are natural fluctuations, 

 even if the catch taken is exactly equal to the 

 sustainable yield under average conditions. 

 Suppose, for example, there is a poor year for 

 reproduction, leading to reduced recruitment. 

 Then the catch, if unchanged, will reduce the 

 stock, and this reduced stock will have a smaller 

 sustainable yield, even under average condi- 

 tions. A fixed catch will then continue to 

 deplete the stock at an accelerating rate. Simi- 

 larly, a favorable if transient fluctuation and a 

 fixed catch will result in a continuous increase 

 in population to above the MSY level. 



The significance of the differences among the 

 various terms can perhaps best be illustrated 

 with some hypothetical examples of different 

 actions which might have been taken after the 

 serious decline in the fin whale stocks in the 

 late 1950's and early 1960's. For simplicity, it 

 has been assumed that the stocks have been 

 declining by 10,000 whales per year from a 

 level of 120,000 in 1960 to 70,000 in 1965, 

 figures which are roughly similar to the decline 

 as estimated by the Scientific Committee of the 

 IWC. Recruitment rate has been taken for the 

 purpose of this example at 8%, recruitment age 

 at 5 years, and natural mortality rate at 4% , 

 which values were assumed to remain constant 

 within the range of stock sizes of the examples 

 (see Table 1). 



Table 1. — Trends in population and catch of Antarctic fin whales, if, after 1965, 

 catches were set equal to the replacement yield, or to the sustainable yield. 



(Thousands of animals.) 



' N = numbers dying from natural causes = 4% of current stock. 

 2 R = numbers of recruits = 8°o of adult stock 5 years earlier. 



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