IN THE BAYS ON THE SOUTH-EAST COAST OF DEVON. 495 



exceptional, and it is interesting to note that Holt records the same 

 phenomenon, also in Teignmouth Bay. 



" The migratory round-fishes — herring, mackerel, and sprat — spawn in 

 Torbay and Teignmouth Bay during the spring, summer, and autumn, 

 but as these are captured by seines and not by trawls they do not 

 concern us here. 



3. AVEKAGE SIZE AT FIEST MATURITY. 



"The size at which fish become mature for the first time is of the 

 greatest interest from the biological standpoint. It marks the stage 

 where the species begins to reproduce its kind, and if for the moment 

 we regard this as having been determined by natural selection, we may 

 use the convenient terms of that theory in order to grasp the great 

 significance of the line which divides the immature from the mature. 

 It would be more correct to speak of age rather than size, but for 

 present purposes the terms are interchangeable, whilst 'average size' 

 conveys as yet more meaning to naturalists than ' average age.' 



" We might say, then, that natural selection has regulated the size at 

 which each species becomes mature for the first time, as it is of the 

 greatest importance to the species under natural conditions that its 

 individual components should reproduce as early as possible. If there 

 is any risk of the species becoming exterminated, that is to say, if its 

 enemies are many, then the sooner its individuals reproduce the greater 

 will be the chances of the species surviving. Conversely, if its enemies 

 beyond a certain size are comparatively few, so that the inevitable 

 'struggle' is luithin the species, then the later the stage of first 

 maturity the stronger will be the individuals and the chances of 

 extinction from ordinary causes practically nil. The latter seems to 

 have been the condition of the plaice before man ventured on the 

 deep-sea trawling, and even in these days the new fishing-grounds 

 which are being opened up testify to the same thing. When man 

 appears the conditions are slowly reversed, the large fish apparently grow 

 scarcer, the average size of the race or species is consequently lowered, 

 and a premium placed upon early reproduction. Is the species able, how- 

 ever, to adapt itself to the requirements of its new enemy ? Would 

 the average size at which the individuals reproduce for the first time 

 — which size tends to be raised under natural conditions — become 

 lower ? In other words, would the individuals under the strain of the 

 new conditions become mature in their third year, say, instead of their 

 fourth as formerly ? 



" The circumstances which have given rise to the suggestion of this 

 possibility are the following. Man's influence on the deep-sea fishing 



