So far the argument has been speculative <> But it is not necessary 

 to rest on thiSo There have been many experiments in which populations 

 have been reared under laboratory'- conditionSo These have involved such 

 diverse forms as yeast,, diatoms^ protozoans, insects, and even fishes o 

 In all of them the rate of increase was rapid while the population was 

 small and slowed down as it grew larger, finally reaching a size beyond 

 which there was no increase o Initiallyp when the population is small, 

 even though the reproductive rate is high, relatively few individuals 

 are added because there are only a few producers. Finally, when the 

 population is at its maximum and there are many reproducers, there are 

 also relatively few individuals added, either because the reproductive 

 rate has become depressed or because mortality has become so high as 

 to offset the additions from reproductiono Usually both a depressed birth 

 rate and incre' sed mortality operate together to keep the net increase 

 in numbers of individuals at a minimurrio In contrast to the small increase 

 in population~size at both low and high population densities, is the large 

 increase that takes place when the population is about midway between 

 these extremes. Then, the population is large enough to have manyre = 

 producers and small enough so that the reproductive rate has not been 

 greatly lowered and mortality has not yet been intensified. In this state, 

 the increase in numbers of individuals per unit of time is large* 



The lesson is obvious o A population has its maximum increase when 

 it is neither at maximum, nor minimum, size, but when middling in sizej 

 and that is the level at which the most individuals can be regularly 

 removed and still be fully replenished by the population's inherent ten= 

 dency to growo 



Of course, with a population of fishes in their natural habitat, 

 things are not as simple as in these experiments. With the pilchard 

 it is almost beyond question that predation would be a large influence. 

 Fortunately, there have been population experiments that included the 

 element of predationo In these it has been shown that the predator popu- 

 lation increases as the prey increases - with a time lag, to be sure. 

 Therefore, holding a prey population at a middling size would similarly 

 limit its predators^ especially with a species like the pilchard which 

 is in a dominant position among prey fishes. So the factor of predation 

 should not greatly alter the fact that replacement will be maximal in 

 a medium-size population. The time lag in the growth curves of composite 

 predator-prey populations does introduce fluctuations and may disturb 

 not only the general level but also the precise level at which the prey^ 

 population would otherwise have its maximal replacement. This, however, 

 does not affect the general concept. 



According to this idea, the optimal amount of fishing would be that 

 which lowers the population numbers to a level at which maximum increase 

 takes place. Such a fishery will remove only as many fish as will be re= 

 placed and the number removed will be the largest number that can be regu- 

 larly removed without further disturbing the population-size. 



