the life history of the target species, or on the basis of ex- 

 perience with similar species, or the same species in a differ- 

 ent area. 



Traditional yield calculations are very much single species 

 oriented. They depend on the assumptions that the environment 

 remains constant and that natural mortality rate (M) also re- 

 mains constant. Natural mortality rate is that from all causes 

 except fishing. Natural mortality results from predation, dis- 

 ease, old age, pollution, etc. In a healthy ecosystem, preda- 

 tion is such a large component of natural mortality that M can 

 be used as an estimate of predation rate. In MSY calculations M 

 is assumed to remain constant. This implies that predation rate 

 is independent of predator abundance, an unrealistic assumption. 

 However, even if M does remain constant, the total food avail- 

 able to predators may vary. M is a rate analogous to interest 

 rate on a savings account. While the interest rate may remain 

 constant, accumulated interest for a year very much depends on 

 the amount of principal in the account. Half the principal earns 

 half the interest with the same interest rate. Similarly, even 

 if mortality rate remains constant for a fished population, the 

 fact that the fishery usually reduces the average standing stock 

 of the population means that the annual total consumed by preda- 

 tors may be likewise reduced. With reduced food the predator 

 population may shift to other prey or become reduced in size. 



Another problem with the constant M assumption is that it 

 does not take into account the time required for a predator pop- 

 ulation to adjust to changes in density of its prey. Marine mam- 

 mals, for example, might respond to a long term reduction in 

 available food through ' lower survival of pups and/or other age 

 classes, older age of first reproduction, longer intervals be- 

 tween births for adult females, and eventually a lower overall 

 population size. Such population responses are slow, not imme- 

 diate as assumed in the MSY concept. The time required for ad- 

 justment to changed environmental conditions is called a time 

 lag. During the adjustment process, the relatively overabundant 

 predators can put unusual stress on prey populations. For long 

 lived predators such as marine mammals, consideration of adjust- 

 ment time lags is essential for realistic management (May, et 

 al. , 1979) . 



The assumptions underlying MSY are particularly strained 

 when more than one stock is fished in a given area. Stock re- 

 cruitment relationships are affected both by the changes in the 

 environment and by fishing on other stocks in the same ecosystem 

 (Gulland, 1978). 



Ecological problems with MSY derive from its concentration 

 on only the stock, excluding the factors of competition or rela- 

 tionships between trophic levels , the impacts of symbiotic or 



G-4 



