182 Part III. — Twenty-fourth Annual Report 



material for the growth of the reproductive organs, as is commonly 

 supposed, but is also connected with maintaining their nutrition over the 

 colder portion of the year. Fat is often got in immature fishes in 

 quantities, as in the herring and Norway pout, for example.* 



There are reasons for the belieff that growth is modified with respect 

 to period and amount in the deeper waters of the northern part of the 

 North Sea, compared with the waters near the coast, but to what extent, 

 the growth of fishes in the sea in deep and moderately deep water, as on 

 the fishing banks in the North Sea, is affected by the changes in the 

 temperature of the water is not yet clear, there being a want of sufficient 

 observations as to the changes in the temperature that actually occur 

 there. 



It is clear that a knowledge of the changes in temperature that take 

 place is necessary to understand not only the growth of fishes and its 

 variation, but their biology generally. All the other observations ought 

 to be correlated with the temperature changes, just as the biological 

 changes on land are, and what is wanted is a calendar of physical 

 conditions throughout the year to which the biological observations may 

 be referred, whether they relate to plankton, food of fishes, spawning 

 periods, development, growth, or migrations. 



The salinity of the water is another condition which probably modifies 

 growth to a considerable extent, and it is not unlikely that it is one of 

 the causes which produce a change in the range of size and the average 

 size in species in certain localities. Some fishes, as the plaice, the dab, 

 and the lesser weever, I have found to be of smaller dimensions, and of 

 slower growth, in the Solway Firth, where the salinity is reduced, than 

 on the East Coast, and the same cause probably acts on other forms. 

 The subject is one which has not yet been much investigated. 



A Law of Growth. 



During the researches on the growth of fishes, it has become apparent 

 to me that there exists a relationship between the size at which sexual 

 maturity occurrs in the various species and the general maximum size 

 to which they attain. It may be expressed in one way by saying that 

 fishes approximately douhle their size and increase their weight about eight 

 times after they have reached sexual maturity ; or t\mt fishes attain sexual 

 maturity when they reach about half their maximum length and about 

 one eight of their maximum weight. 



It cannot be said at present that the law is more than approximately 

 correct, for our knowledge of the precise average size at which the males and 

 females of many fishes first spawn is as yet meagre — it is not well determined 

 even for the cod — and the same is true as to the general maximum size 

 to which many fishes attain ; a limit, moreover, which, in some instances 

 at least, may have been modified by the action of man. For example, 

 the maximum size of most fishes at Iceland is larger than in the North 

 Sea at present, though there is no reason to suppose that growth is 

 quicker there; and it is known that when the Dogger Bank was first 

 worked by trawlers the general maximum size for plaice was higher than 

 it is now. 



* A research on this subject is at present being made for the Board by Dr. Noel Paton, 

 whose investigation of the changes in the salmon are well-known, as well as one on the 

 rate of digestion in fishes. f Twentieth Annual Report, Part IIT. , p. 394. 



