52 



ARE THE MIGRATIONS OF FISH INFLUENCED 



BY HYDROGRAPHICAL AND FOOD 



CONDITIONS ? 



By ALEXANDER MEEK. 



Every year the North Atlantic passes through phases of 

 warming and cooling, and the warming is carried to the north- 

 east of the region by the North Atlantic current. Accompanjdng 

 the increase in temperature there is a consx^icuous increase in 

 the quantity of the temporary and complete occupiers of the 

 current, and this increased plankton is carried to and added to 

 that of the north and north-east by the Gulf Stream and the 

 Atlantic current continuous therewith, influencing all the branches. 

 The annual warming is accompanied likewise by a general advance 

 of fish and other forms having the power of movement. Thus 

 it may be said that migrations are related to the general annual 

 change in temperature and the trend of the food supph^ 



This is so far obvious that it need not be supported by an 

 appeal to the actual results relating to plankton, and the general 

 effect so far as fishes are concerned has been given in my work 

 on the " Migrations of Fish." 



On reflexion it is just as obvious that fish as a whole, and 

 onlj' a few of the invertebrates, are capable of making a contra- 

 natation. The main body of the j^elagic life is annually renewed 

 from the more tropical, proximal regions of the circulatory 

 system. The widely spread holoplanktonic forms are carried into 

 currents which lead them north or south, and their degree of 

 resistance to the cooling influences to which they are gradually 

 introduced is indicated by the successive regions where they dis- 

 appear. These organisms have no power of contranatation, and 

 only those which gain a return current of a circulatory system 

 manage to effect a retreat. 



It is manifest therefore that holoplanktonic and mero- 

 planktonic, non-contranatant organisms have a resistance to 



