[FLR-list] Estimating Btrigger suggestions

[Laurie]

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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