346 Relations within the Species 



bers. For such species the growth of the population is measured in 

 terms of weight or volume, and, when plotted in corresponding units, 

 again follows a logistic curve, although the numerical values will, of 

 course, be different. Accordingly, in attempting to ascertain the 

 optimal tonnage of fish to catch we must consider the growth rate of 

 the population in terms of increase in weight rather than numbers. 

 If there are no further modifying considerations in respect to a par- 

 ticular fishery, the optimal catch may be obtained when the popula- 

 tion is maintained at the level at which the largest weight increment 

 will be added per unit of time, this increment being due both to an 

 increase in numbers and to the growth of the individuals (Fig. 9.15). 



Other practical considerations are the availability of the population 

 for exploitation and its suitability for the market. If game in a forest 

 or fish in a pond are scarce, more time, effort, and expense will be re- 

 quired to harvest the same number than if the population were dense. 

 With sparse populations the yield per unit effort is lower. In specific 

 instances, it may be better to allow the population to increase to a 

 somewhat greater density than that which represents the level of most 

 rapid growth in order to increase availability. Often minimum limits 

 and sometimes maximum limits of size exist in relation to market- 

 ability. Accordingly, the size composition of the catch must be con- 

 sidered as well as its weight or volume. 



Two additional considerations that also may modify the basic plan 

 for obtaining the optimal yield in respect to certain species are: the 

 number of breeders necessary to provide sufficient young, and the 

 rate of natural mortality. In species with a low natality such as most 

 large mammals and birds, the recruitment, or number of young an- 

 nually added to the population, is closely dependent upon the size 

 of the breeding stock. For each new young whale there must be one 

 adult female; but one fish, one oyster, or one termite can produce mil- 

 lions or billions of young in a season. The spat released by only a 

 few clams would provide an ample set for a large area, and, if al- 

 lowed to grow up, the young produced in one year could repopulate 

 a whole bay under completely favorable conditions. In such species 

 the recruitment usually depends more on the environmental condi- 

 tions during the early life of the new generation than on the number 

 of breeders, and the optimal level at which the population should be 

 maintained is determined by other considerations. 



We have seen that allowing populations to build up by avoiding 

 overexploitation sometimes has advantages in the greater size of in- 

 dividuals, the more concentrated populations, and the larger number 

 of breeders. On the other hand, allowing too great development of 



