The constant environment assumption appears to be contra- 

 dicted by fluctuations observed in unexploited fish popu- 

 lations indicating that there are environmental factors which 

 can change the carrying capacity of an ecosystem for a par- 

 ticular species . Such changes may even occur over rela- 

 tively short periods of time, particularly if they are due 

 to factors such as increasing pollution levels. The syner- 

 gistic effects on a resource population of the combination 

 of fishing and other stresses due to a changing environ- 

 ment should be taken into consideration in predicting the 

 impacts of proposed fishing levels, but consideration of 

 combined stresses is not possible in the MSY framework 

 which assumes a constant environment. 



The applicability of the MSY concept is limited by the 

 underlying assumption of a constant environment. Another 

 limitation of the MSY concept is the lack of appropriate 

 consideration of any of the ecological relationships affect- 

 ing the target species , which results in incomplete consid- 

 eration of the impacts of a fishery on both target and non- 

 target populations . 



The only relationship between a target species and its 

 ecosystem that has been considered directly in the MSY con- 

 cept is the rate of natural mortality, i.e., mortality from 

 all causes except fishing, primarily predation, but also 

 disease, pollution, old age, etc. However, the treatment of 

 natural mortality is incomplete and hence unrealistic. The 

 rate of natural mortality is assumed to remain the same over 

 time and with or without a fishery. That assumption does 

 not allow for any feedback in interactions between the preda- 

 tor and prey populations. Instead it implies that predation 

 rate is independent of predator population size, and conse- 

 quently that the amount of prey consumed annually depends 

 only on the size of the prey population, whether predators 

 are abundant or scarce. 



The MSY concept does not allow for any predator popula- 

 tion responses to changes in the availability of target spe- 

 cies prey populations . 



However, the response of the size of a predator popula- 

 tion to changes in size of its prey population is a classic 

 part of predator prey theory, which also recognizes that 

 time is required for predator abundance to adjust to changing 

 prey abundance. This adjustment time is called a time lag. 

 Consideration of time lags such as the time required for preda- 

 tor populations to adjust to changed prey abundance and the 

 feedback of the predators on the prey during that adjustment 



27 



