A population of a given size can have either few births 

 and deaths (low turnover) , or many births and deaths (high turn- 

 over) . The turnover rate is a replacement rate for a popula- 

 tion. A high turnover rate generally implies more production 

 available to the ecosystem of which the population is a part, 

 since most deaths are due to predation and provide food for 

 other elements of the ecosystem. 



The density of a stock is its standing population size per 

 unit area. When the density of a population falls below carry- 

 ing capacity, according to the theory underlying MSY, birth 

 rate (not necessarily the total number of births) and survival 

 rate increase and exceed the death rate. The rate at which new 

 individuals in a population enter a given age class (e.g., 2 year 

 olds or adults) or size class (e.g., longer than 10 cm.) is 

 called recruitment to that class and depends on the combination 

 of birth rate and rate of survival to the given age or size. When 

 recruits outnumber losses for an age or size class, either the 

 surplus can be removed by harvesting and the population level 

 will remain below carrying capacity, or the surplus will in- 

 crease standing population size until recruitment and death 

 rates are once again in balance. If harvesting removes ex- 

 actly the surplus of recruits over deaths, then the average pop- 

 ulation size will not change, the surplus will be the same the 

 following year, and that level of harvest can be sustained. The 

 population level at which the surplus of recruits over deaths 

 for the total population is greatest, is the population level 

 producing MSY. This approach assumes that environmental condi- 

 tions remain constant. 



When the size of a population is not changing, it is in 

 equilibrium with the combination of environmental conditions and 

 harvesting pressure. Equilibrium yield (EY) , also known as re- 

 placement yield, is the level of harvesting that removes the 

 surplus of births over deaths without changing population size. 

 Depending on the size of the population, EY can be greater than, 

 less than, or equal to MSY. EY is sustainable yield for the 

 population at a particular abundance level; EY varies with time 

 because population size varies with time. MSY is achieved at 

 only one of a possible range of population sizes, and, concep- 

 tually, is constant for a given carrying capacity. 



Using highly simplified birth, survival and death rate as- 

 sumptions, the MSY population for "r selected" species is esti- 

 mated to be half of carrying capacity. Frequently the MSY level 

 is assumed to be between 40% and 607 o of carrying capacity (Holt 

 and Talbot, 1978.) The relative size of MSY level and carrying 

 capacity appears to vary with the species being harvested. For 

 marine mammals, for which simplified sigmoid population growth 

 curve assumptions are not appropriate, MSY population levels 

 have been estimated to be higher than 607 o of carrying capacity 

 (Fowler, et al . , 1978). When enough information is available 



G-2 



