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MODELS FOR VARIABLE RECRUITMENT
The Beverton and Holt model for yieldperrecruit 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 yieldperrecruit 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 yieldperrecruit 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
yieldperrecruit 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 yieldperrecruit.
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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 Fall '09

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