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J. A. GULLAND 



recruits. This latter of course depends on the growth pattern and fishing and 

 natural mortality rates already described. If this point is on the ascending 

 part of the stock-recruitment curve, or on a not too steep part of the descend- 

 ing part, the position will be stable; that is, following any minor disturbances, 

 such as a transitory increase in the amount of fishing, the stock will return to 

 the equihbrium state. If however the point lies on a steep part of the descend- 

 ing limb the position will be unstable, and any disturbances, however brief 

 and small, will cause large and permanent oscillations. With the possible 

 exception of the haddock {Melanogrammus aeglejims) on Georges Bank there 

 no clear example of such oscillations occurring in a major natural fish 



are 



O stock 



Pjg. 3. — Possible relations between stock and recruitment, 



population. In many fisheries there are large fluctuations which Hjort (1914) 

 and others have shown to be due to varying numbers of recruits entering the 

 fishery, and though these fluctuations have been beUeved to be caused by 

 environmental factors this has not often been shown clearly and at least some 

 influence of stock size is possible. In laboratory experiments stock-recruit- 

 ment curves with right-hand Umbs steep enough to cause large oscillations 

 have been shown for several invertebrates. Silliman & Outsell (1958) kept 

 populations of guppy {Lebistes reticulatus) at several fishing rates. Though 

 these exploitation rates were kept at any one value for only a comparatively 

 short time, at rates of 25 per cent and 10 per cent per three-week period the 

 population showed definite signs of oscillations. The control, unexploited 

 populations also showed minor oscillations. In Fig. 4 the data from their 



