long-term strategies NortheastAtlantic mackerel

The ICES has released its advice on the long-term management strategies for Northeast Atlantic mackerel on special request

The ICES has today released its advice on the long-term management strategies for Northeast Atlantic mackerel on special request from three parties.

The report was conducted after the EU, Norway, and the Faroe Islands requested advice on the long-term management strategies for Northeast Atlantic mackerel.

The ICES opinion is that mackerel stock can withstand a higher fishing pressure than previous evaluations have shown after they carried out a method revision in the winter of 2019. 

The ICES was requested to identify appropriate precautionary combinations in the Tables given in its response to the EU, Norway and the Faroe Islands request to ICES to evaluate a multi-annual management strategy for mackerel in the North East Atlantic (ICES, 2017), using:A range of Btrigger from two to five million tonnes with an appropriate range of target Fs 

  • A harvest control rule with a fishing mortality equal to the target F when SSB is at or above Btrigger 
  • In the case that the SSB is forecast to be less than Btrigger at spawning time in the year for which the TAC is to be set, the TAC shall be fixed consistently with a fishing mortality that is given by: F = Ftarget*SSB/Btrigger.

 All alternatives should be evaluated with and without a constraint on the inter-annual variation of TAC. When the rules would lead to a TAC, which deviates by more than 20% below or 25% above the TAC of the preceding year, the Parties shall fix a TAC that is respectively no more than 20% less or 25% more than the TAC of the preceding year. The TAC constraint shall not apply if the SSB at spawning time in the year for which the TAC is to be set is less or equal to Btrigger. 

The constraint mechanism shall be tested separately from and in combination with 10% banking and borrowing mechanism. 

Evaluation and performance criteria 

Each alternative shall be assessed in relation to how it performs in the short term (5 years), medium term (next 10 years) and long term (next 25 years) in relation to: • Average SSB • Average yield • Indicator for year to year variability in SSB and yield • Risk of SSB falling below Blim

The approach should follow the same full feedback methodology that has been recently used to evaluate stocks in the North Sea (ICES, 2019a). The evaluation should be conducted to identify options that are robust to alternative operating models including but not limited to: 

  1. Investigating alternative plausible recruitment dynamics and scenarios, 
  2. Alternative natural mortality assumptions, 
  3. The potential impact of density dependent growth. 

Following initial consideration of the request by ICES, the requesting parties confirmed that the strategy should also be evaluated with a banking and borrowing scheme representative of recent behaviour. The requesters furthermore confirmed that banking and borrowing should be suspended when SSB is below Btrigger, and that the implications of any future catch scenario that exceeds the advised catch should not be evaluated.

The ICES provides combinations of Ftarget and Btrigger that maximize the median annual yield in the long term and simultaneously minimize the risk of the Northeast Atlantic mackerel stock falling below Blim. Ftarget values are between 0.27 and 0.30, in combination with Btrigger values between 3 000 000 tonnes and 4 500 000 tonnes. Higher Ftarget values are associated with higher Btrigger values. 

The maximum sustainable yield (MSY) was estimated to be 970 000 tonnes in the long term, which correspond to an Ftarget = 0.29 and a long-term median spawning–stock biomass (SSB) = 4 500 000 tonnes when simulated with Btrigger = 4 250 000 tonnes. The simulations suggest that long-term yields within 1% of MSY can be achieved with a lower Ftarget. This would result in higher SSB and less variation in the yield and SSB in the long term. 

All management strategies considered precautionary in the long term were also precautionary in the short term. When additional management measures (limitation of TAC interannual variation and banking and borrowing) were applied in the harvest control rule (HCR), they had a limited influence on the median annual long-term yield or stock status. 

Two sets of simulations were carried out, based on alternative estimates of recruitment from 1998 onwards. Only small differences were found between these simulations. Including recruitment from years prior to 1998 (when recruitment was lower) impacts the precautionarity of the HCR. If future recruitment is lower than that assumed in the base case scenario, simulations show that this will result in both reduced yield and increased risk of SSB < Blim.

Conclusions 

The simulations revealed combinations of Ftarget and Btrigger that maximize the median annual yield in the long term while simultaneously minimizing the risk of the Northeast Atlantic mackerel stock falling below Blim. For these combinations Ftarget values are between 0.27 and 0.30, and Btrigger values are between 3 000 000 tonnes and 4 500 000 tonnes; higher Ftarget values are associated with higher Btrigger values. 

All management strategies considered precautionary in the long term (LT) were also precautionary in the short term (ST). When additional management measures (limitations of TAC interannual variation and of banking and borrowing, i.e. management strategies IAVcap, B&B_5%, Combo_5%, Combo_10%) were applied in the HCR, they had limited influence on median annual long-term yield or stock status compared to BaseHCR simulations. 

Two sets of simulations were carried out, based on alternative estimates of recruitment from 1998 onwards (BaseHCR and R_2). Only small differences were found between these simulations. Inclusion of recruitment from years prior to 1998 (R_3), when recruitment was lower, impacts the precautionary of the HCR. If future recruitment is lower than assumed in the base case scenario, simulations show that this will result in both reduced yield and increased risk of SSB < Blim.

The full report is available here.

 

Brian J McMullin Solicitors
MMG Welding Killybegs

Long-term management strategies for Northeast Atlantic mackerel

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