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MODELS FOR VARIABLE RECRUITMENT The Beverton and Holt model for yield-per-recruit applies to situations in which the influx of new recruits does not change from year to year, as in the cumulative yield that results from harvesting a single cohort during its fishable lifespan. The analysis can be used to determine whether there is growth overfishing. The yield-per-recruit analysis does not tell us whether there is recruitment overfishing . Not only must we allow the fish to grow and attain a good weight, we must also allow a sufficient number to spawn and thereby perpetuate the stock. If we catch too many fish before they have had an opportunity to contribute to later generations, then the stock is said to be experiencing recruitment overfishing. Also, please note that the yields predicted by the Beverton and Holt yield-per-recruit model may not be sustainable in the long term because the subsequent recruitment may be affected by the rate of fishing. F (/yr) Y/R (kg/fish) F max Do not confuse Y/R at F max from a yield-per-recruit analysis with the maximum sustainable yield . The Y(F max ) may not be sustainable!! To examine the problem of recruitment overfishing we need a different type of model. If recruitment is a variable, it is invalid to move it out from under the integral as we did in the derivation of the equation for yield-per-recruit. For most species of fish, the adult females produce vast numbers of eggs during their life spans. Most of these eggs do not survive. For any population to remain stable over time, each adult female on average must produce exactly two offspring (one male and one female) that survive and reproduce. If there were more than two successful offspring per female, the population would increase in number. If there were less than two, the population would decrease. Most fish populations exhibit tremendous variability in the number of young fish that recruit annually. It is not unusual for the largest cohorts at the age of recruitment to be 100 times more abundant than the smallest cohorts. Understanding recruitment variability is very important for the rational and successful management of any fishery. Recruitment must somehow depend on the size of the parent population because if No Fish ==> No Eggs ==> No Recruits. One method for examining the variation in recruitment is to plot on a graph the number of spawning adults on the horizontal axis against the subsequent number of recruits produced by these parents. This type of plot is often described as a
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This document was uploaded on 12/06/2011.

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