THE TARPON 71 



the same lot, living under approximately the same condi- 

 tions, the size of fishes is very markedly influenced by a 

 number of factors, the most important of which are tem- 

 perature and food supply. 



The effect of temperature on the growth of fishes is 

 very marked. In the winter fishes usually grow very slow- 

 ly. As the temperature rises their digestive capacity is 

 increased, they eat more and grow faster. This difference 

 is not only seasonal but is also influenced by latitude. The 

 black bass, for instance, grows more rapidly in southern 

 waters, because of the higher temperature and the longer 

 growing season. It is, therefore, possible for individual 

 fishes of the same age and coming from different bodies of 

 water of varying temperature to differ greatly in size. In 

 considering the effect of an increase of temperature on 

 the growth of fishes, it is to be remembered that there is 

 an optimum temperature, that is, a temperature at which 

 they grow best. If the temperature is raised beyond the 

 optimum point growth will, apparently, be retarded. 

 Moreover the optimum temperature varies with the 

 species. For example, trout thrive best in water of a 

 moderate temperature, and are not able to exist perma- 

 nently in too w^arm water. It may confidently be expected 

 that such species which thrive best in the cooler waters 

 will be retarded in their growth when living in a body of 

 water having a temperature somewhat higher than the 

 optimum. 



The abundance or scarcity of food supply in a body of 

 water is perhaps even of greater effect than temperature 

 in determining the growth of fishes. The young of fishes 

 usually feed on the minute free-swimming organisms, 

 known as plankton, which are present in the water. The 

 larger fishes live on the smaller species and other aquatic 

 organisms, which, in their turn, are dependent on the 

 plankton. Since the supply of plankton varies greatly in 



