xi NUMBER OF FISHES 283 



life of the vast populations of the sea and a glimpse of the factors that 

 control the numbers of each animal species. 



It used to be held that one of the chief dangers of fishing was that 

 it would destroy the breeding-stock, on the grounds that it is axio- 

 matic that something is wrong if fish are destroyed before they have 

 an opportunity to breed. We may detect something of anthropocentric 

 sentimentality here, for reflection shows at once that it is impossible 

 for every fish that is hatched to survive and, further, that the total 

 annual supply of fish can be provided by relatively few adults. The 

 number of fish in the sea, indeed, bears no obvious relation to the 

 number of eggs. A cod may shed as many as 8 million eggs (usually 

 fewer), which float at the sea surface and hatch into planktonic young. 

 Yet herrings, which are much more numerous as adults, lay fewer 

 eggs, usually less than 30,000 each. Presumably the risks of life are 

 less for the herring, but we cannot see clearly why; the herring's eggs 

 are sticky and become attached to gravel or seaweeds, which may give 

 them some protection; they are known to be eaten by other fishes, 

 perhaps on a smaller scale than planktonic eggs. Both herring and 

 cod fry are planktonic and feed on the nauplius and other larval 

 stages of Calamis and other copepods. The cod, like other bottom- 

 fishes, has to undergo the risks of metamorphosis, but this hardly 

 seems sufficient to explain the much greater abundance of herring, 

 especially since many fishes feed on the latter, including the cod them- 

 selves! As Graham puts it, 'no one really knows why the herring, 

 which nearly everything eats, should be able to manage with a less 

 rate of reproduction than the cod, which eats nearly everything'. 

 The greater part of the mortality of pelagic fish larvae is due to pre- 

 dation, including that by members of the same species. This probably 

 provides a method of regulation, the number of each species eaten 

 being dependent on their frequency. 



This brings us to face the difficult question of the pressure, force, 

 or potential that ensures and controls the number of animals. Re- 

 production is only a part of the source of this pressure, the other 

 element being the feeding and growth of the animals as a result of the 

 skill and persistence with which they seek and consume their food. 

 It is somewhat easier to study these questions in a marine than in a 

 land community, the whole population being enclosed in one vast 

 bath, the additions and subtractions to which can be known. 



We must not forget, however, that conditions are far from con- 

 stant, even in the 'unchanging sea'. For instance, the extent to which 

 fish-fry hatch and successfully overcome the hazards of the early 



