SEA FISHERIES LABORATORY. 1h 
by adding to the number of young in the district, help the 
species to recover itself while it is still worth helping. 
I think it is worth pointing out also that if the average 
size of a species of fish caught in a particular district is 
decreasing, then that isa sure indication that the area is 
being over-fished, and that it is high time to take steps to 
increase the supply of young fish and so let the species 
in question have a fair chance of keeping pace with the 
destruction going on. The decrease in size of the average 
fish shows that a greater proportion each year of the pop- 
ulation is being prevented from becoming adult. In a 
fishery the aim should be, if 1t were completely under 
control, to adjust the deaths to the births, as you would 
the outflow to the inflow of a tank you wish to keep at a 
constant level. 
The argument, sometimes heard, that since a species of 
fish which is becoming scarce from over-fishing can produce 
a very large number of eggs, it will soon recover in numbers 
if left alone, 7.e., if restrictions are put upon the fishery, is 
not a sound one. It is not by any means the fish that pro- 
duce the largest number of eggs which are the commonest. 
The very abundant herring, which is perhaps the safest to 
last of all our fish, produces only about 30,000 ova as against 
the million of the very much rarer sole and the ten 
million of the comparatively rare turbot; while the dog- 
fishes which are apparently increasing very greatly in 
numbers and are said to be becoming a plague in some 
parts of the Scottish waters produce only a very few young 
at a time. ‘The fact that a fish produces a very large 
number of ova indicates to the biologist that that species 
is exposed to very exceptional risks during its embryonic 
and other young stages, and that there is consequently 
such a great mortality that unless the huge number of ova 
started existence none would arrive at maturity. The 
