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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 is a 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 it 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, i.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 



