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Unformatted text preview: YIELDPERRECRUIT So far on this course we have been laying the groundwork for constructing the "real" models of fisheries science. We have a model for survival, a model for growth, and a model for the fishing process. Now we will put them all together. The Beverton and Holt Model for YieldperRecruit This model was developed in the 1950s by the English fisheries scientists Raymond Beverton and Sidney Holt for examining the problem of growth overfishing . One way to think about this problem is in terms of the graph of the biomass of a cohort. Cohort Biomass Time Biomass t 1 t 2 If we could catch all the fish at once, and if we harvested them when they were too young (say at t 1 ), we would waste potential growth. But, if we waited too long (say until t 2 ), we would lose biomass to natural mortality. In practice, we cannot usually catch all the fish at the same time. Because we cannot harvest all the fish at the same time, the graph of catch versus time will depend on the time (age) at which we start fishing. If we begin fishing at age t 1 we will get a different curve for catch versus time than if we start fishing at age t 2 . Also, the graph of catch versus time will depend on how hard we fish. Our catch will be a function of the instantaneous rate of fishing mortality F. The yieldperrecruit model allows us to consider the simultaneous effects of the age when fishing begins and the rate of fishing mortality. Phases in the Life of a Cohort Lest there be confusion about this model, let me introduce some terminology. PreRecruit PostRecruit Phase —————————————————————————————————————— PreExploited Exploited Phase ————————————————————————————————————— Age, t t r t e t λ t r is the age at recruitment . At this age the fish are present on the fishing grounds and are potentially available for capture. Fish younger than t r cannot be caught and therefore are of no concern in this model. A recruit is a fish that is age r. FW431/531 Copyright 2008 by David B. Sampson YieldPerRec1  Page 54 ——————————— M + F ——————————— t e is the age at entry to the exploited phase. The age at entry can be changed by suitable modifications of fishing practices, such as a change in the mesh size or a...
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 Fall '09
 Trigraph, Overfishing, David B. Sampson

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