WHAT IS OVEK-FISHING ? 591 



of eggs, and that it is only Group III which has been reduced in 

 average size, and is less numerous than before. 



Then there is the possibility that the two other groups, I and II, 

 are growing faster now than before because they have more room, and 

 each of them consequently more food. Group II still consists of sale- 

 able fish, and they grow up a year quicker than Group III ; not so 

 many of them die or are eaten by animals, because they are only two 

 years old. For these reasons it is very likely that it would pay better 

 to base the fishery for the greater part on this group and not on the 

 III Group. 



This problem depends upon the value of the fisli in Group II com- 

 pared with that of the fish in Group III, and upon the mortality in 

 the third year. If we only knew the rate of growth, the mortality 

 during this year, and the price of the fish, the question might be 

 exactly solved by mathematics. In the third year we know that the 

 " P " increases its value four times by growing. If the mortality is 

 as high as one-half during this year, which is not probable, it would 

 pay to prevent all fishing for the II Group. Failure to prevent all 

 fishing for this group would then involve the " destruction of under- 

 sized fish." A size limit for the fish would, under these circumstances, 

 be desirable. Studies on the rate of growth, and of predatory animals 

 feeding on the " P," would greatly add to our knowledge on this point. 



So far I have dealt with the following three cases : — 



I. Over-fishing of an accumulated stock. 



II, Over-fishing of the mature fish to so great an extent that they 

 cannot render a sufficient number of eggs to supply the 

 stock with young fishes. 



III. Pieduction of the average size of the fish to such an extent that 

 they are not sufficiently saleable. This case we may name 

 the " relative destruction of immature fish." 



It is, nevertheless, possible to imagine another kind of over-fishing, 

 viz. : — 



IV. The Group I of the "P" is living in certain very restricted 

 areas close to the shore where shrimp-trawling is going on. It is 

 possible that shrimp-trawling can destroy too many young ones. This 

 I Group has no value in the market at all, and if such individuals are 

 killed by fishing for shrimps or by other methods, it may, in the true 

 sense of the word, be called " destruction of immature fish." 



It is again possible, e.g. in the Kattegat, that the stock of " P " is 

 reduced a good deal by fishing, while other allied species, e.(j. the dab 

 {Pleuronedes limanda), are less affected. If this be the case, the last- 



