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<div class="moz-cite-prefix">Btrigger is a level of biomass that if
the stock falls triggers management action to
rebuild the stock as part of a HCR.<br>
<br>
Choosing a value that is too small will result in higher risk of
stock collapse and loss yield since biomass will fall below BMSY,
Choosing a value that is too high will result in loss yield due to
fishing effort being reduced unnecessarily. Blim needs to balance
these objectives. The best way to do this would be to conduct an
MSE with appropriate uncertainty, i.e. try a variety of values for
Btrigger and choose the one that performs best. If you have an MSY
harvest control rule there is no need for an Ftrigger as you would
be setting F to FMSY if B>BMSY and F to that given by the line
joining (FMSY, Btrigger) to (Flim,blim).<br>
<br>
Laurie<br>
<br>
<br>
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On 10/08/12 14:32, Hintzen, Niels wrote:<br>
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<p class="MsoNormal"><span
style="font-size:10.0pt;font-family:"Verdana","sans-serif"">Dear
all,<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:10.0pt;font-family:"Verdana","sans-serif""><o:p> </o:p></span></p>
<p class="MsoNormal"><span
style="font-size:10.0pt;font-family:"Verdana","sans-serif"">Sometime
soon, I’m faced with another Management Plan Evaluation for
North Sea herring. Obviously we’ll be using FLR. However,
I’d like to gain a bit more insight into good practices to
‘estimate’ a specific biomass reference point, so it is a
bit off-topic but the expertise in this group is so high I
could not resist sending this question!<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:10.0pt;font-family:"Verdana","sans-serif""><o:p> </o:p></span></p>
<p class="MsoNormal"><span
style="font-size:10.0pt;font-family:"Verdana","sans-serif"">The
current HCR prescribes an Fbar of 0.1 below Blim (determined
on a biological basis) and increases linearly with SSB up to
a Btrigger biomass point (determined in 2008 based on a
performance basis, i.e. scenario testing, which scenario
performed best). Above this Btrigger, the HCR prescribes an
Fbar of 0.25 (considered Fmsy).<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:10.0pt;font-family:"Verdana","sans-serif""><o:p> </o:p></span></p>
<p class="MsoNormal"><span
style="font-size:10.0pt;font-family:"Verdana","sans-serif"">Within
the new evaluation, Blim, Btrigger and Fmsy will be a point
of discussion (because the stock assessment changed
considerably). I have some ideas on how to estimate Blim and
Fmsy, but with Btrigger I’m a bit in the dark. What I can
think of so far is to run stochastic equilibrium simulation
assuming constant Fs (over a range) and, as a first guess,
taking Btrigger as the median equilibrium SSB. Does this
make sense, and if not, what would you suggest to do. I need
some sort of approach to link potential Ftrigger points to
Btrigger points because else I will be running far too many
scenarios to be able to analyse and discuss later on.<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:10.0pt;font-family:"Verdana","sans-serif""><o:p> </o:p></span></p>
<p class="MsoNormal"><span
style="font-size:10.0pt;font-family:"Verdana","sans-serif"">Many
thanks in advance for your suggestions.<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:10.0pt;font-family:"Verdana","sans-serif""><o:p> </o:p></span></p>
<p class="MsoNormal"><span
style="font-size:10.0pt;font-family:"Verdana","sans-serif"">Kind
regards,<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:10.0pt;font-family:"Verdana","sans-serif""><o:p> </o:p></span></p>
<p class="MsoNormal"><span
style="font-size:10.0pt;font-family:"Verdana","sans-serif"">Niels<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size:10.0pt;font-family:"Verdana","sans-serif""><o:p> </o:p></span></p>
<p class="MsoNormal"><span
style="font-size:10.0pt;font-family:"Verdana","sans-serif""><o:p> </o:p></span></p>
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