not enclosed by rectangles, refer to material to the right of eacho 

 To preserve the closest possible connection between text and diagram, 

 the exact phraseology used in the diagram is repeated as headings in 

 the textj, even thoughj, in some instances, it results in rather awkward 

 context o 



Objective 



Underlying the investigations of the Uo So Fish and Wildlife Service 

 on the Pacific sardine or pilchard are certain ideas or theories, the 

 discussion of which will clarify that of objectives » In the first place, 

 it is recognized that before fishing took place the sardine population 

 had filled its ecological niche and that, apart from fluctuations such 

 as occur among all organisms, the deaths, on the average, equaled the 

 births and the population was in equilibrium, ioeo neither increasing 

 nor decreasing in total numbers. Moreover, the deaths under these con- 

 ditions were all due to "natural" causes. Predators were, of course, 

 responsible for a substantial portion of the natural mortalityi but the 

 basic influence tending to keep the population in check must have been 

 competition within the populationo This competition may have been of 

 many different forms, all, however, connected with the density of the 

 population itself « It might be failure in reproduction which could arise 

 from the overabundance of spawners, the debilitating effects of which 

 might affect the viability of their eggs j or if the viability of eggs 

 were unaffected, simple sprea^ into marginal spawning grounds with con- 

 sequent failure of eggs to hatch would keep the population in checko 

 Or the competition might simply increase mortality through division of 

 a limited food supply among more mouths o Or the pilchard population, 

 grown to maximal size, might so well supply predators with food that 

 they would multiply to an extent which would check further increase of 

 the pilchards o Whatever the mechanism, the existence of a depressing 

 effect of population size on population increase is implicit in the idea 

 of natural population equilibrium. It also involves negation of the 

 idea that a fishery can ultimately destroy a populationo 



What, then, happens when an important amoimt of fishing takes place? ' 

 There is an added cause for deaths* These deaths are largely among adults, 

 and to the extent that catch mortality goes beyond replacing natural mor= 

 tality, that is, to the extent that fishermen take fish that would not 

 be eliminated by sharks and other predators, the number of adults in the 

 population is reduced and the size composition is changed in the direction 

 of fewer large individ'oels » But the fact that the population grows lesser 

 in numbers and its individuals smaller in size, is no proof that the fishery 

 is taking more than it ought to take, for another thing is happening at 

 the same time o The depressing effect of population-size on population- 

 increase is diminishingo If internal competition had been strongest among 

 the commercial sizes, catch mortality would largely replace natural mor- 

 tality and the population decrease would be slighto If it had been strong- 

 est among pre -commercial sizes or between commercial sizes on the one hand 

 and pre-commercial sizes on the other hand, then replenishment of the com= 

 mercial population would increase somewhat in proportion to the amount of 

 "thinning oute" 



