[FLR-list] Estimating Btrigger suggestions

[Axel G. Rossberg]

Hi Niels

Can you estimate the variability in B around Bmsy that one can expect
under "normal conditions" (e.g. as a result of recruitment
variability)?  That's only one scenario, so easy to simulate.  Then
you could choose a value for Btrigger as a low quartile of this range?

Cheers,
Axel

From: "Hintzen, Niels" <Niels.Hintzen at wur.nl>
Subject: Re: [FLR-list] Estimating Btrigger suggestions
Date: Fri, 10 Aug 2012 13:31:31 +0000
Message-ID: <8FE8A24E425793428F3C772FA61961A51FBFE7C8 at SCOMP0936.wurnet.nl>

> Hi Laurie,
> 
> Thanks for this, and I agree that F = Fmsy if B > Bmsy where Bmsy can be
> Btrigger. However, based on other criteria than biology alone, you could
> choose for different Fmsy values (upper or lower bound for example) which
> automatically will result in different Btrigger values. The problem is that I
> do NOT want to derive a Btrigger through MSE as you suggest, we’ve done that
> before, it works well, but if you are kind of in the dark where Btrigger
> should be, you are up for lots of scenarios, the one thing I try to prevent
> here. Hence, I’m looking for a smart way to get an estimate of Btrigger which
> is related to Fmsy without doing lots of scenario testing.
> 
> Any other suggestions?
> 
> Cheers, Niels
> 
> From: flr-list-bounces at lists.r-forge.r-project.org
> [mailto:flr-list-bounces at lists.r-forge.r-project.org] On Behalf Of Laurie
> Sent: vrijdag, augustus 10, 2012 15:17
> To: flr-list at flr-project.org
> Subject: Re: [FLR-list] Estimating Btrigger suggestions
> 
> Btrigger is a level of biomass that if the stock falls triggers management
> action to rebuild the stock as part of a HCR.
> 
> 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).
> 
> Laurie
> 
> On 10/08/12 14:32, Hintzen, Niels wrote:
> 
>     Dear all,
> 
>     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!
> 
>     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).
> 
>     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.
> 
>     Many thanks in advance for your suggestions.
> 
>     Kind regards,
> 
>     Niels
> 
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