1916 20 2 2 24 26 2B 30 32 34 36 



4^ 42 S 4^ 4^ 50 52 54 56 58 



Fig. 33. Fluctuations in year class strength of haddock in the northern North Sea. Haddock 

 spawn in February-March each year, and by the autumn of the following year the new recruits to 

 the stocks are large enough to be retained in the commercial fishing nets. 



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consideration and it is convenient to divide this chapter into three major 

 sections; one deahng with the larval fish which are themselves planktonic, 

 and the others with the fish as adults, the plankton feeders and those which 

 live on or near the sea floor. 



Starting with the egg stage, we have already seen (p. 97) that the eggs 

 of almost all the commercial fish, except herring, float in the plankton, and 

 that the female fish produce vast numbers of eggs. It was mentioned that 

 a haddock, for example, spawns over | million eggs, and she may spawn 

 for several successive years before being caught. Supposing she lays 4 million 

 eggs, only two of these, a male and a female, need reach maturity to main- 

 tain the population at the present level. The rest, one way or another, form 

 the food of other creatures, including man. This is a mortality of 99-99995 

 per cent ! Should the mortahty, for some reason or other, be changed to 

 99-9995 per cent the difference would seem so slight as to be almost ineffect- 

 ual, but in fact the survival rate is ten times as great. If this were kept up for 

 several years haddock would be tremendously abundant. Natural fluctuations 

 greater than this do occur, as is shown in Fig. 33, but big increases can never 

 be maintained because there would be insufficient food for all the extra 

 haddock, and in turn there would be an increase in the number of predators 

 to feed on them, which could result in very poor broods indeed. 



The numbers of young recruited each year to the fisheries is thus not so 



127 



