[FLR-list] Estimating Btrigger suggestions

[Hintzen, Niels]

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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