POPULATION FACTORS AND SELECTED POPULATION PROBLEMS 



379 



essentials of the problem without confusing 

 complications. But, largely, the various 

 other factors are omitted because not 

 enough is known of their operation in 

 natural populations subjected to such or- 

 ganized human predation. This is well 

 recognized by Russell, of course, who, in 

 discussing the relation of growth rate to the 

 entire problem states: 



This problem of the optimal yield ob- 

 viously could be developed in much more 

 detail for fish populations and for other 

 groups as well. Our responsibility has been 

 to indicate the nature of the problem, to 

 suggest its considerable theoretical signifi- 

 cance and pragmatic importance, and to 

 stress that much in the way of further re- 

 search remains to be done. 



YEAR IS' 2"<i 



3^" 



4th 



YEAR |s» 2"'' Z"' 



4th 



CAPTURE 90% per year 

 ond 

 NATURAL MORTALITY 57operyr 



6 UNITS 



EFFORT treble! 



WEIGHT OF CATCH 26'/2 units(opprox) 

 (90%ofeoch group) 



CAPTURE 30% per year 

 and 

 NATURAL MORTALITY 5%peryr. 



RELATIVE I, 

 WEIGHT /4 



UNITS 



m 



WEfGHT OF CATCH 26/2 units (opprox) 

 (30% of eoch group) 



Fig. 132. Comparison of the effects of high and low intensity of fishing on the exploited popu- 

 lations. (After Graham.) 



"We see then that the reduction in yield due 

 to increased intensity of fishing may be counter- 

 balanced in \arying degree by an increase in 

 growth-rate, and this may serve to remind us 

 again of the complexity of the factors involved 

 in the overfishing problem. Increase in growth- 

 rate is not an unmixed blessing, for it means 

 that the fish are exposed to capture at an earlier 

 stage in their life. To quote Raitt [1939] 

 again: 'Reduction in numbers means less com- 

 petition for food, which means greater growth- 

 rate, which means earlier fishing out, all of 

 which indicates reduction of potential fertility, 

 which in turn would mean still further reduc- 

 tion in numbers and so on. On the other hand, 

 once decrease in rate of depletion were estab- 

 lished, greater survival would mean more com- 

 petition for food, less growth rate, later entry 

 into the trawl, greater survival to spawning age, 

 larger broods and so on.' There are therefore 

 many biological factors involved, and their 

 interrelations are complex" (pp. 95-96). 



We have suggested that the optimal 

 yield problem is an aspect of the larger 

 problem of predation. In the examples dis- 

 cussed here the predator, man, has ex- 

 ploited marine fish populations that are rel- 

 atively simple in the sense that they are 

 not so highly organized as are certain bird 

 and mammal groups. We wish to direct the 

 reader's attention again to the general dis- 

 cussion of predation immediately preced- 

 ing in which it was shown that frequently 

 these more highly organized populations 

 so compensate for predation pressure that 

 predation becomes somewhat incidental 

 rather than causative in terms of its efiFect 

 upon giowth form. 



HOST-PABASITE INTERACTIONS 



"Workers with an appreciation of mod- 

 em developments in biology are finding 



