292 FISHES AND MAN xi 



of very great importance. If man is the chief predator, then change in 

 his activities will greatly influence the populations. 



Knowing the number of fishes available and the rates at which they 

 are removed by natural causes and fishing in a given case, we still have 

 the even more difficult task of formulating the way in which the popu- 

 lation grows and then deciding whether some other method of fishing 

 would be more profitable. Probably there is a maximum total biomass 

 of fish of any given sort that can be supported in any area, determined 

 ultimately by the supply of inorganic salts. In the absence of fishing 

 this biomass is carried mainly in the form of large fish, whose presence 

 makes the growth of all fish in the population slow. The effect of fish- 

 ing is to remove mainly these larger animals, with a resultant increased 

 growth from all younger groups. The curve showing the relationship 

 between the yield per recruit and the intensity of fishing effort shows 

 a maximum (Fig. 171c). It should be possible to define a fishing mor- 

 tality rate at which the decrease in numbers is balanced by the incre- 

 ments in weight of the survivors. 



There is every reason to hope that with further study of growth, 

 mortality, reproductive potential, and utilization of food by fish the 

 yield could be increased and the effort of getting it reduced, making 

 fishing more profitable to the fisherman and providing the maximum 

 amount of food. At present the economic conditions and psychology 

 of the fishing populations interact with the factors limiting the stocks 

 and the growth of the fish to produce a complicated system of inter- 

 relations that is unstable and unsatisfactory to all parties. 



The increase in weight of the population is perhaps the most diffi- 

 cult feature of the pattern to express mathematically. Weight plotted 

 against age shows an asymmetrical sigmoid shape, with an inflexion 

 (Fig. 171B). The curve fitted is arbitrarily chosen, being that deduced 

 from the hypothesis of Bertalanffy, that the weight is subject to 

 opposing forces of anabolism and catabolism, taken as proportional to 

 the absorbing surfaces, that is, to the squares of the linear dimensions. 

 As before, the fit is poor for the lower points, probably because there 

 are incompletely recruited classes. Elsewhere the fit is good but the 

 important point for us is the inflexion, since it indicates that the 

 growth-rate decreases in the later part of life. The older plaice are 

 from this point of view inefficient in providing more biomass. Fur- 

 ther, the longer a fish has to be kept alive in the sea before it is eaten, 

 the more of the limited raw materials are devoted to this end and the 

 less to providing human food. With present methods it is not possible 

 to give full weight to all of these factors in deciding what is the best 



