commensal relationships , and environmental changes such as cli- 

 mate, pollution, or other human influences which affect changes 

 in carrying capacity (Holt and Talbot, 1978). 



The concept of MSY as a function of stock size alone was 

 developed to provide an approximate description of the response 

 of a stock to exploitation. It has been useful as a teaching 

 aid and in providing a very preliminary conservation criterion 

 for avoiding or correcting overexploitation . The MSY concept 

 has been a useful simplified model for understanding wild popu- 

 lations. However, it has become institutionalized and is play- 

 ing a more absolute role than is justified. It was never in- 

 tended as the sole conceptual basis for management. Unfortu- 

 nately, MSY has become an obstacle to accepting concepts derived 

 from more detailed ecological information that could provide a 

 more adequate management basis. In particular, a more adequate 

 basis is necessary when human impacts on the biosphere are in- 

 creasing and diversifying (Holt and Talbot, 1978). 



The MSY concept has shortcomings, but it has focused man- 

 agement on long term and sustainable yields, considering the fu- 

 ture, as well as on an exploitation level that definitely cannot 

 be exceeded without depleting the resource stock. The MSY concept 

 is not applicable in situations of harvesting both predator and 

 prey. In such cases time scales and trophic interactions become 

 very important (May, et a_l. , 1979). 



If enough assumptions are made about population dynamics , 

 recruitment and mortality rates, and fishing effort on a stock, 

 then population models exist which provide formulas for the cal- 

 culation of sustainable yields (MSY or EY) Ricker , 1975). However, 

 maintenance of existing population sizes or highest sustainable 

 yields may not always be appropriate management objectives. 



Acceptable Biological Catch (ABC) may be greater or less 

 than MSY on the basis of biological considerations such as fluc- 

 tuating recruitment in stock or allowing depleted stocks to in- 

 crease. ABC estimates depend on scientific or managerial judge- 

 ment in addition to formulated calculations . 



MSY is a single species management concept. One problem 

 that has been apparent with MSY is the difficulty of estimating 

 and attaining it in practice. Such difficulty has arisen in part 

 from incomplete data, in part from the tendency of fisheries to 

 become overcapitalized and hence reluctant to remain at specified 

 catch levels, and in part from failure of the oversimplified as- 

 sumptions underlying MSY calculations. However, there are con- 

 ceptual as well as practical problems with MSY. In a multi-species 

 fishery situation with various ecological interactions among re- 

 source populations to be taken into account, MSY for each species 

 is no longer an appropriate management goal . 



G-5 



