[FLR-list] Estimating Btrigger suggestions
Laurie
lauriekell at googlemail.com
Fri Aug 10 16:16:51 CEST 2012
This is a good starting pt
http://www.nafo.int/publications/studies/no32/sinclair.pdf.
i.e. BMSY is a limit, the biomass should be >BMSY. If it drops below
then reduce F. So BMSY could be the Btrigger. That would be too much for
ICES to get its head around, hence my suggestion of BMSY*60%.
Such things are discussed in
http://www.iccat.es/Documents/Meetings/Docs/2011_WG%20METHODS-ENG.pdf
I have a preference for taking a percentile of BMSY So that there is a
positive relationship between information and use, i.e. the less you
know the more precautionary you need to be. Something like 60% of the
75th percentile of BMSY would mean that the value of Btrigger would
increase as uncertainty increased.
Axel´s suggestion would be "un-precautionary" since taking a lower
percentile as the trigger means that as you become more uncertain about
BMSY then Btrigger would become smaller/, i.e. Reductio ad absurdum/ if
you knew nothing about BMSY then Btrigger would 0, if you knew it with
100% certainty it would equal BMSY
Laurie
On 10/08/12 15:53, Hintzen, Niels wrote:
>
> Sounds perfect! Any rational, papers that suggest this approach I can
> read up on?
>
> *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 15:42
> *To:* flr-list at flr-project.org
> *Subject:* Re: [FLR-list] Estimating Btrigger suggestions
>
> 60% of BMSY
>
> On 10/08/12 15:31, Hintzen, Niels wrote:
>
> 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>
> [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 <mailto: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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