known precisely, it is still clear that a significant reduc- 

 tion in food availability causes a reduction in the carrying 

 capacity of an ecosystem for predators. 



Another important aspect of the effects of a fishery on 

 ecosystem carrying capacity is the potential use of alternative 

 prey species , in this case other forage or small schooling 

 fish, by predators. Whether alternative prey are available 

 depends in part on the specialized food requirements of a 

 predator with respect to season, spatial distribution, density, 

 and behavior (such as schooling) of prey. If alternate prey 

 are available, then the impact on those populations of more 

 intensive predation (increased natural mortality) should be 

 considered, particularly if those populations are also harvest- 

 ed. 



Compared to detailed social and economic considerations 

 in the plan, predator prey relationships received a fairly 

 broad brush treatment. With the information on anchovy pop- 

 ulations in the plan, some analysis of the impact of the pro- 

 posed catch levels on predators could have been made. 



Ecosystem perspective example : One way to look at the 

 impact of a fishery on an ecosystem is to estimate the loss of 

 forage to predators other than the fishery. Traditional fish- 

 ery calculations include estimates for rates of fishing mortal- 

 ity (F) and natural mortality (M) . Natural mortality results 

 largely from predation in a healthy ecosystem, so can be used 

 to estimate the amount that the predators are eating. On an 

 annual total weight basis, fishing mortality displaces some of 

 the natural mortality, although the combination of fishing and 

 natural mortality exceeds natural mortality alone. The dif- 

 ference in estimated total annual natural mortality of a 

 fish stock with and without a fishery can provide some in- 

 dication of the short term impact of a fishery on forage 

 availability. To get a handle on longer term changes in 

 availability of forage, the aspect of average spawning biomass 

 or average standing stock must also be taken into consideration 



As an example, estimates of changes in natural mortality 

 anticipated in association with the proposed northern anchovy 

 fishery are presented here. The following analysis applies to 

 a fishery in which both natural and fishing mortality occur 

 concurrently (Ricker, 1975). The formulas used for estimating 

 biomass consumed by predators in this example are standard 

 ones for the assumptions used for gain and loss rates. It is 

 the emphasis on predator forage and the use of the formulas 

 to estimate changes in predator intake that are unusual. F is 



F-13 



