is essential to sound ecosystem level management (May, et 

 al. , 1979) . The MSY concept does not incorporate any theory of 

 predator prey interactions , and consequently fails to con- 

 sider feedback known to occur within an ecosystem. 



Accounting for the impacts of harvesting on nontarget 

 species is one of the objectives of ecosystem oriented man- 

 agement. It has been argued that MSY management does not 

 affect the biomass or quantity of prey (target) species 

 available for consumption by predators, and consequently 

 that no impacts on natural predators are anticipated, because 

 an estimate of natural mortality is included in MSY calcula- 

 tions. This inferred absence of impact can be contradicted 

 from two perspectives. First, even if all the conditions 

 of the MSY concept are fulfilled, and the rate of natural 

 mortality remains the same with or without a fishery, the 

 annual total biomass of prey taken by predators is still 

 expected to be reduced. Under MSY management, a reduction 

 in standing stock, from a carrying capacity level to a lower 

 population size level at which MSY is theoretically produced, 

 is a management goal. Thus with a reduced average standing 

 stock of target or prey species, the total annual biomass 

 of prey taken by predators can be expected to be similarly 

 reduced, even if the rate of natural mortality remains con- 

 stant, because total biomass consumed under MSY assumptions 

 is a function of the prey standing stock as well as of the 

 predation rate. The reduction in prey availability may af- 

 fect predator population size, or may result in increased 

 predation pressure on alternate prey species , some of which 

 may also be harvested populations. Predators may respond to 

 threshold densities of prey, and responses may be expressed 

 through behavior, through change in age of first reproduction 

 or fecundity, or in other ways. Second, the treatment of 

 predator prey interactions in the MSY concept is too simpli- 

 fied even to incorporate the interactions of classical preda- 

 tor prey theory, so does not predict impacts of fishing on 

 the predators of target species. It seems reasonable to 

 conclude that single species, MSY oriented management does 

 not account for either maintaining annual total food supply 

 for predators or the potential responses of predators to 

 changes in food availability. 



In addition to the conceptual problems with MSY, there 

 have been practical difficulties with MSY application and, 

 as a consequence, many fishery resources and marine mammal 

 stocks have been overexploited . One difficulty with the ap- 

 plication of MSY has been the collection of data on which 

 the estimation of MSY is based. With incomplete data, esti- 

 mates may be inaccurate and consequently management goals 



28 



