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Limiting inter-annual variation in total allowable catch strategies. An application to ICES roundfish stocks

Kell, Laurence T.; Pilling, Graham; Kirkwood, Geoffrey P.; Pastoors, Martin; Abaunza, Pablo; Aps, Robert; Biseau, Alain; Korsbrekke, Knut; Kunzlik, Philip; Laurec, Alain; Mesnil, Benoit; Needle, Coby L.; Roel, Beatriz; Ulrich, Clara
Working paper
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URI
http://hdl.handle.net/11250/106505
Date
2003
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  • ICES CM documents authored by IMR scientists (1949-2011) [3139]
Original version
This report is not to be cited without prior reference to the authors  
Abstract
This study evaluated through simulation management strategy that stabilise catch levels by setting

bounds on the inter-annual variability in Total Allowable Catches (TACs). An integrated modelling

approach was used, which modelled both the ‘real’ and observed systems and the interactions between

all system components. The modelling framework therefore allowed evaluation of the robustness of

candidate management strategies to both the intrinsic properties of the systems, and the ability to

observe, monitor, assess and control them. Strategies were evaluated in terms of level of risk

(measured as the probability of spawning stock biomass falling below the biomass limit reference

level for the stock) and cumulative yield.

The simulation approach used provides a powerful tool for the examination of the performance of

candidate management strategies. It has shown that better management is not necessarily going to be

achieved by improving the assessement, since even with a perfect assessment (where the simulated

working group knew stock status perfectly) stocks may crash at fishing levels that standard stochastic

projections would suggest were safe. Also explicitly modelling the assessment process can result in

quite different outcomes than those predicted by the simple projection traditionally used by stock

assessment working groups. This is because the simple projection assumes that the status of the stock

in the current year is known without error and that the target fishing mortality can be achieved

without error. However, in practice the assessment is based on last years data and the effect of any

management measure on SSB is only manifest, following the implementation of the quota, at the end

of the following year.

The choice of target and fishing mortality levels and minimum stock levels results from ICES

interpretation of the precautionary approach. This lead to the definition of fishing mortality and

biomass reference points that are intended to prevent over-fishing and to trigger recovery plans when

a stock is overfished respectively. Although, fishing mortality and biomass reference points were

originally intended to be independent, a fishing mortality level implies a corresponding biomass level.

In the case of saithe a fishing mortality of 0.40 (i.e. the FPA level) would drive the stock to Blim,

suggesting that the choice of biomass and target reference points are not consistent for this stock.
Publisher
ICES
Series
ICES CM Documents;2003/X:7

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