SEA-FISHERIES LABORATORY. 295 



cooling down towards the minimum ; and (2) in the 

 months June to August, when the sea is warming up 

 towards the maximum. Although the whole area under 

 consideration is quite small, yet the different parts present 

 differences which seem to be of significance, if it is 

 assumed that physical changes may he factors which 

 influence the migrations of fishes. The whole sea does 

 not, in fact, heat up and cool down at the same rate in 

 its different parts, and it is probable that these 

 differences, with the temperature gradients which they 

 set up, may be the factors of fish migration to be 

 investigated. 



The assumption that temperature changes are factors 

 of this kind is probably justified by all that is known of 

 the life history of fishes. The rate of development of 

 the egg, and the length of the periods between fertilisa- 

 tion and hatching and metamorphosis are certainly 

 affected by the temperature, and it is very probable that 

 an empirical formula might be obtained from sufficient 

 data, that would enable these periods to be calculated, 

 the temperature being known. The rate of growth of 

 adult fishes is also probably affected by temperature, and 

 so also with the relation of length to weight : fishes of 

 the same length are heavier in summer than they are in 

 winter. During the colder winter months plaice either 

 do not feed at all, or, as a rule, contain little in their 

 alimentary canals. The rate of metabolism of fishes is 

 certainly affected by changes of temperature : the 

 frequency of the respiratory movements of the mouth 

 and gills, for instance, which varies with the temperature. 

 Generally speaking, all these things are particular cases 

 of van't Hoff's law, that is, the rate of a chemical 

 reaction is a function of the temperature at which it is 

 carried on : the vital processes which have been mentioned 

 above are all cases of directed chemical reactions. 



