ICES MSY Approach MSY ranges. Baltic AC 11 June John Simmonds ICES ACOM vicechair

Transcription

1 ICES MSY Approach MSY ranges Baltic AC 11 June John Simmonds ICES ACOM vicechair

2 M S Y Maximising Yield from a Sustainable stock Maximising useful catch

3 Age 1 50 fish in the first year - Fishing Mortality = 0.5 Small stock, catch and stock dominated by small fish Year Recruitment 2 3 Spawning Stock Fishing Catch/ Mortality Yield = Ratio of Catch/Stock Landings Catch Catch

4 Age 50 fish in the first year - Fishing Mortality = 0.25 Medium size stock, with spread of ages Year Catch

5 Age fish in the first year - Fishing Mortality = 0.1 Large stock more older fish in the catch Year Some fish die naturally so at low Fishing mortality less fish in total are caught Catch

6 Spawning Stock Biomass (SSB) 5 Mt Example North Sea Plaice (Biomass) The harder you fish the smaller the stock Fishing Mortality

7 Landings MAXIMISING YIELD Example North Sea Plaice yield, (average landings) The Yield (landings) increases as you fish harder up to MSY, then decreases as you take fish before they have grown Maximum Yield at F= Fishing Mortality

8 MSY Ranges ICES provides precautionary F MSY ranges that are derived to deliver no more than a 5% reduction in long-term yield compared with MSY. All ranges are precautionary less than 5% prob of SSB<Blim Ranges specified with and without ICES Advice rule (AR)

9 How ranges are set without Advice Rule (AR) MSY F and yield estimated from long term simulations. To estimate what F delivers maximum long term yield Test to check FMSY is precautionary Reduce FMSY if necessary Estimate F upper and F lower based of F where yield is 5% less than the maximum yield Test to check Fupper is precautionary, reduce Fupper if necessary

10 SSB Yield Intervals without AR Example from ICES March 2015 Saithe FMSY unchanged Relative median landings 5% prob SSB<Blim Note fall in yield F upper reduced with PA Considerations Total catch F Relative median SSB Note SSB at F= Fishing Mortality (F) Total catch F

11 How ranges are set with AR AR gives F reduced when SSB<MSY Btrigger, F=FMSY if SSB>=Btrigger F=FMSY x SSB/Btrigger if SSB<Btrigger MSY F and yield are estimated from long term simulations. To estimate what F delivers maximum yield Test to check FMSY is precautionary including the reduction in F when SSB is less than MSY Btrigger Reduce FMSY if necessary Take Fupper and Flower from evaluation without AR where yield is 5% less than the maximum yield. Test to check Fupper is precautionary with AR included, (reduce Fupper if necessary)

12 MSY - Maximize long term average yield Limited by precautionary considerations AR is a safeguard against low SSB ICES MSY Advice Rule (AR): MSY B trigger more caution needed F upper F MSY F lower SSB 2016

13 Intervals with AR SSB Yield FMSY unchanged F upper reduced with PA Considerations Relative median landings 5% prob SSB<Blim Limited fall in yield because true F is lower Total catch F Relative median SSB Note SSB at F= Fishing Mortality (F) Total catch F

14 Stock MSY MSY F F MSY F upper lower with AR MSY B trigger (thousand t) MSY F upper with no AR Cod in Subdivisions Herring in Subdivisions and 32 (excluding Gulf of Riga herring) Herring in Subdivision 28.1 (Gulf of Riga) Herring in Subdivision 30 (Bothnian Sea) Herring in Division IIIa and Subdivisions (Western Baltic Spring Spawners) Sprat in Subdivisions (Baltic Sea) a)

15 Additional Considerations There are considerations other than average long-term yield for fishing above or below F MSY. In a single-species context fishing above F MSY implies reduced stock biomass and this may be substantial where F upper is much higher than F MSY. So in utilizing F MSY ranges there are more advantages to fishing between F MSY and F lower than between F MSY and F upper. With higher fishing mortalities the following occurs: A need for increased fishing effort; Higher dependence of stock and yield on recruiting year classes and increased variability on catch opportunities; The size of the fish in the stock and the catch will be smaller on average; Greater probability of SSB being less than MSY B trigger ; Though there may be lower probability of density-dependent effects such as reduced growth or increased cannibalism.

16 Additional Considerations For some mixed fisheries it may be difficult to reconcile the Fs on different stocks. An approach for maximizing long-term yield could be to attempt to reconcile F on a mixed fishery using Fs between F lower and F MSY. If this cannot be accomplished, F between F MSY and F upper could also be used in the short term. However, using F >F MSY for the same stock in the long term implies that there are structural changes required in the fishery to avoid the consequences listed above. Moreover, in line with the request, F MSY and the upper and lower ranges are calculated based on current fishery selectivity with the possibility of higher yields if selectivity is altered through changes in gear design, fishing area, or season.