182 Part [I]. —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 knowkedge 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 or GrowrTH. 
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 double their size and increase their weight about eight 
times after they have reached sexual maturity ; or that 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. Noél Paton, 
whose investigation of the changes in the salmon are well-known, as well as one on the 
rate of digestion in fishes. + Twentieth Annual Report, Part IIT., p. 394. 
