18-EquilYield2 - TOTAL EQUILIBRIUM YIELD (continued)...

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Fishing Mortality F Yield-per-Recruit F max TOTAL EQUILIBRIUM YIELD (continued) Harvest Management policies Based on Yield Analysis We have seen in the yield and yield-per-recruit models that equilibrium yield rates (catch per year) are controlled by the age-at-entry t e and by the rate of fishing mortality F. In practice it has been difficult to require fishermen to change their gear (e.g., trawl mesh size) to adjust the age-at-entry. Instead, many management agencies attempt to regulate their fisheries by means of restrictions on the rate of fishing mortality. Often they do this by imposing annual or seasonal quotas on the landings. These quotas are generally based on two things: an assessment of the current stock biomass and a target value for the rate of fishing mortality. The objective of management (although it is often not explicitly stated as such) usually is to prevent over-fishing, not to fish efficiently. The objective is to keep F less than a target level F*. We can use the following relationships to determine F*. Y F av B ( ) = ==> F Y av B ( ) = where av(B) is the time-averaged biomass. To keep F F* we must set Y F* · av(B) . Often the target fishing rate F* is based either on an analysis of yield-per-recruit, if recruitment is
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18-EquilYield2 - TOTAL EQUILIBRIUM YIELD (continued)...

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