INTERNATIONAL FISHERY INVESTIGATIONS 601 



known migratory fishes which appear to come to the British 

 fishing grounds with a considerable degree of regularity are 

 also under the influence of the same current of warm and 

 dense water. The establishment of such connections between 

 the migrations of the fishes of the type of the hake, cod, and 

 herring with the hydrographical changes in the sea remains as 

 the most difficult of fishery problems, and among the things 

 which still await thorough investigation. 



It is now quite certain that the metabolic processes of marine 

 animals must be influenced by the changes in temperature and 

 other physical properties of the water in which they live. The 

 sea is to an animal inhabiting it exactly what the atmosphere 

 is to a terrestrial animal such as a bird or a mammal. Indeed, 

 a curiously close parallel can be drawn between a migratory 

 bird living on the land and a migratory fish in the sea. In 

 each case we have well-defined periods of reproduction, and 

 periodic migrations, which are repeated from year to year with 

 a certain regularity. Atmospheric changes, temperature, etc., 

 affect the breeding seasons of the terrestrial animal as well as 

 its migratory cycle. It is a priori probable that the changes 

 in the sea which we have already considered also affect the 

 reproductions and migrations of the fish. We have, indeed, 

 direct evidence of this in the influence that temperature exerts 

 on the incubation period of the eggs of a marine fish. The 

 period of development is so dependent on the temperature 

 that it can almost be expressed mathematically. We have seen 

 that in the case of the herring a body of sea water of a certain 

 salinity has been associated with the migrations of this fish, 

 and there is evidence that such a connection also exists in the 

 case of the migrations of other fishes. We have here direct 

 relationships, but it is quite certain that the influence of physical 

 changes in the sea is usually an indirect one, and that fishes 

 and other marine animals are affected by changing temperature 

 and salinity, because these factors exert a more profound 

 reaction on the animals and plants which form their food. On 

 the land the great outburst of vegetable life in the spring 

 has the most intimate effect on terrestrial animals. In the 

 sea there is a similar development of vegetable life in the early 

 months of the year. Among the commonest organisms in the 

 sea are the diatoms or unicellular plants, and these are influenced 

 in the closest manner by physical and chemical environment. 



