[FLR-list] Estimating Btrigger suggestions
Ernesto Jardim
ernesto.jardim at jrc.ec.europa.eu
Thu Aug 16 11:12:47 CEST 2012
Hi,
IMO you need to test the values by simulation anyway. Btriggers are not
estimated, it's a decision threshold that we compute based on the most
solid rationale possible, like LK's approach (which I like). So you need
to be sure (or at least confident) that it delivers what you need under
distinct conditions.
Best
EJ
On 08/10/2012 04:16 PM, Laurie wrote:
> 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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--
Ernesto Jardim<ernesto.jardim at jrc.ec.europa.eu>
Scientific Officer
FISHREG -- Scientific Support to Fisheries
IPSC Maritime Affairs Unit
EC Joint Research Center
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