GULLAND and BOEREMA: SCIENTIFIC ADVICE ON CATCH LEVELS 



If from 1965 onward, catch quotas had been 

 introduced equal to the replacement yield, the 

 stock would have been maintained at the level 

 of 70,000 whales, and the quota would have 

 been 6,800 whales in 1965, declining by 800 

 per year to 2,800 whales in 1970, after which 

 the number would have remained at this 

 level. 



If, on the other hand, the sustainable yield 

 had been taken every year the catch quota 

 would have been 2,800 in 1965, slowly rising to 

 3,190 in 1970, then declining to 3,130 in 1973 

 and stabilizing at about 3,140 thereafter, at 

 which time the stock size would practically 

 have stabilized at 78,560 whales. 



The highest constant annual yield which 

 could have been taken from 1965 onward — the 

 maintainable yield — would have been about 

 3,125 whales, which would have led to a stock 



size of about 78,200 whales by the mid-1970's 

 (Figure 2a). 



For the purpose of demonstrating the effect of 

 increasing stock size, the effects of different 

 catch quotas have been calculated for the 

 situation in which the stock has risen from 

 70,000 to 80,000 whales over a 5-year period 

 (say, from 1965 to 1970) by 2,000 whales per 

 year. Catch quota equal to the replacement 

 yield, which would maintain the stocks at 

 80,000 whales, would have been 2,400 whales 

 in 1970, increasing by 160 whales per year to 

 3,200 whales in 1975, after which the figure 

 would remain constant. 



If the sustainable yield had been taken, the 

 catch quota in 1970 would have been 3,200 

 whales, decreasing to 3,120 in 1974 and there- 

 after stabilizing at around 3,130, and a stock 

 size of around 78,310 whales. 



1960 



cat(:h 



(000) 



8 



— 3 



1965 



1970 



1975 



1980 



1985 1965 



1970 



1975 



1980 



1985 



1990 



Figure 2. — Examples of catch level and stock size resulting from regulating catches of fin whales at the level of 

 equivalent sustainable yield, maintainable yield, and replacement yield respectively, in the case that the 

 regulations were introduced a) after a constant decline in the stock (in the example assumed to have taken 

 place from 1960 till 1965) and b) after a constant increase in the stock size (in the example assumed to have 

 taken place from 1965 till 1970). Parameters used in these models are specified in the text. 



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