THE AGE OF FISHES AND THE RATE AT WHICH THEY GROW. 401 
requires no elaborate argument to prove that the study of this matter 
is of the first importance if we are to give a rational account of the possible 
productiveness of a fishery, of the rate at which the fishery can be re- 
plenished, and of the intensity of fishing which may be prosecuted with- 
out endangering its future prospects as a means of profit to the fishermen 
and a source of food supply for the people. 
To begin at the beginning, I need hardly remind you of the now well- 
known fact that the eggs of the majority of our marketable marine fishes 
are small, transparent, spherical bodies, which are buoyant and float 
freely in the sea. The fact that the eggs of a fish are of this character, 
which we describe as pelagic, was first discovered by G. O. Sars in Nor- 
way, in 1864, whose observations were made on the Cod. It was dis- 
covered independently in Cornwall in 1871, in the case of the Pilchard, 
by that enthusiastic fisherman-naturalist and acute observer, whose 
name will be well known to you, the late Matthias Dunn, of Mevagissey. 
The only important British sea-fish which is an exception to this rule 
of having pelagic eggs, apart from the skates and dogfishes, whose rate of 
growth I propose to leave out of consideration altogether, is the Herring, 
the spawn of which is deposited on the sea floor and attached to shells, 
stones and gravel. 
The time occupied in the development of the eggs of different fishes, 
from the time they are spawned to the time of hatching, was shown by 
Dannevig (5) to be dependent upon the temperature of the water in 
which they float, the increase in the rate of development being in direct 
proportion to any increase of temperature. 
The following table compiled by Dannevig shows the average number 
of days occupied in the development of the eggs of certain species of fish at 
different temperatures > 
Temperature in Centigrade 1 +3 4 5 6 8 10 12 If 
Cod 
(Gadus morrhua) . L 42.23) “202 .- kr lof 122 OF 92) 8t 
Whiting 
(Gadus merlangus) . : ~ = — 153 13} ~10} 8 627 192 
Haddock 
(Gadus wglefinus) . : 42 23, 20 Ais 15k 6S lO? 935 S32 
Plaice 
(Pleuronectes platessa). . 18} 144 12 1b — 
Flounder 
(Pls flesus) . F é = 61 5} 44 330 = 
Time of ticubation ti days (24 hovrs). 
These results have since been confirmed by Johansen and Krogh (16), 
working with more elaborate and accurate apparatus, and they have 
illustrated the relation between temperature and growth rate shown by 
Dannevig’s figures by means of a graph," in which the loci of the different 
* The address was illustrated by lantern slides and the graph was shown. 
NEW SERIES.—VOL. XI. NO. 3. DECEMBER, 1917. 2D 
