n 



estimates and assessments of potential yields. Consequently, great effort 

 was expended to refine resource inventory methods. 



At the same time, history had demonstrated that common-property fishery 

 resources would eventually diminish under increasing fishing pressure. 

 Ultimately, yields from those fisheries would exceed the capacity of stocks 

 to replenish themselves. 



Assessments, then, developed to predict the available yields under 

 differing amounts of fishing pressure. Because it was impossible to observe 

 a fish stock directly, the only means by which to gauge the short-term effects 

 on the stock related to catch and effort: as long as increased effort pro- 

 duced yield increases, the stock was considered healthy. However, when in- 

 creased effort produced the same yield or even yield reductions, the species 

 was considered maximally utilized or depleted. Under these conditions, 

 fishing mortality, combined with the assumed natural mortality*, exceeded 

 the ability of the stock to reproduce itself through spawning and recruitment. 



* The need to assume natural mortality may be the greatest uncertainty in 

 assessment theory; natural mortality must be assumed in any fish population 

 because direct observation is impossible. Assessment theory attempts to 

 determine yields through recommending a fishing harvest that, when combined 

 with natural mortality, does not exceed the reproductive capacity of the 

 stock. For example, assume that it is possible to remove 50 percent of a 

 chosen stock on a yearly basis without reducing the capacity of that stock 

 to reproduce itself to the same level the following year. If one assumes 

 that 30 percent of that stock is lost to natural mortality, then a manage- 

 ment goal would set an allowable harvest equivalent to the difference be- 

 tween the natural mortality and the assumed maximum allowable mortality. 

 Clearly, if the natural mortality in fact is 45 percent, a fishing mortality 

 based on the assumption of a 30 percent mortality will result in excessive 

 removals from the stock. Thus, an incorrect assumption regarding natural 

 mortality results in an incorrect yield strategy. Easily overlooked among 

 the other complexities of assessment science, the fact that natural mor- 

 tality must remain assumed indicates that yield estimates can only be 

 general . 



