Impact of limitation in interannual variations of cod yield on its stock dynamics. Anatoli Filin and Daniel Howell

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1 Impact of limitation in interannual variations of cod yield on its stock dynamics Anatoli Filin and Daniel Howell

2 Stability constraints Typically in the form of no more than X% percent change in quota per year (unless stock falls below some level Often described as a compromise Fishers gain greater stability But loose out on overall yields

3 Stability constraints: NEA cod 10% year to year constraint on quota change (Subject to a floor of F=0.3) (note, this is not included in these simulations) No equivalent ceiling But have not yet been in a situation where we might need one Suspended once stock reaches Bpa

4 Stability Constraints, single species Why do stability constraints reduce catch? During periods when the stock is rising, the catch rises more slowly During periods when the stock is falling catch also falls more slowly Miss both the peaks and the troughs Miss the highest catches during peak biomass Increased catches at low biomass don t compensate for this loss

5 Stability contstraints: beyond single species But we don t live in a single species world Do our expectactions still hold in a multispecies context?

6 STOCOBAR model Takes environmental factors Models cod stock development and capelin biomass Allows for cannibalism A proxy for variability productivity impacting on the cod population Includes a HCR Highly suited for this kind of question

7 Stocobar Water temperature Residual natural mortality Fishing pattern Harvest control rules Recruitment at age 1 Total stock biomass Total abundance Fishable stock abundance Fishable stock biomass Spawning stock biomass Fishing mortality by age Yield Weight by age in catch Weight by age in stock Length by age Maturation ogive Ration Fatness Cannibalism Cod biomass at age 1-5 Capelin stock biomass Input data Model outputs

8 STOCOBAR modelling Ran scenarios with 1: no restriction on catch variation 2: 20% maximum interannual catch variation 3: 10% maximum interannual catch variation

9 STOCOBAR RESULTS: increased stock variability with constraints

10 STOCOBAR RESULTS: increased variability in catch with constraints

11 STOCOBAR RESULTS: 10% limit Stock biomass, millions tons Fbar Modelled period, years 0 commercial stock fishing mortality rate

12 Oh Adding stability constraint on the catch gives: Greater variation in stock size Which gives greater variation in catches So we didn t achieve our stability goal, what about overall yields?

13 STOCOBAR RESULTS Parameter Limitation on interannual variations in TAC (±) no 50 % 40 % 30 % 25% 20 % 15 % 10 % 5 % TSB, mil. t 2,27 2,25 2,30 2,25 2,37 2,56 2,78 2,99 3,17 SSB, mil. t 1,08 1,07 1,09 1,06 1,13 1,24 1,38 1,49 1,63 TAC, thous. t 684,1 678,5 691,1 667,7 701,5 752,7 805,0 836,2 800,5 R, mil. specimens 700,4 690,2 704,7 680,4 707,8 733,9 743,5 764,1 755,2

14 STOCOBAR RESULTS Limitation on interannual variations in TAC (±) Parameter / Reference point None 50 % 40 % 30 % 25% 20 % 15 % 10 % 5 % F bar >F pa (0,40) 0,0 0,0 0,2 2,4 10,4 17,0 24,2 23,5 20,1 F bar >F lim (0,74) 0,0 0,0 0,1 0,1 0,3 2,4 6,5 8,7 8,3 SSB <B pa (460 тыс. т) SSB <B lim (220 тыс. т) 0,65 0,3 1,0 1,4 3,1 8,1 15,7 19,7 18,3 0,0 0,0 0,1 0,1 0,3 0,7 3,4 6,5 5,4

15 STOCOBAR RESULTS Limitation on interannual variations in TAC (±) Parameter / Reference point None 50 % 40 % 30 % 25% 20 % 15 % 10 % 5 % F bar >F pa (0,40) 0,0 0,0 0,2 2,4 10,4 17,0 24,2 23,5 20,1 F bar >F lim (0,74) 0,0 0,0 0,1 0,1 0,3 2,4 6,5 8,7 8,3 SSB <B pa (460 тыс. т) SSB <B lim (220 тыс. т) 0,65 0,3 1,0 1,4 3,1 8,1 15,7 19,7 18,3 0,0 0,0 0,1 0,1 0,3 0,7 3,4 6,5 5,4

16 Oh Average yield increased with catch constraints At the expense of increased risk of collapsing the stock Not surprising that the constraints increase the risk of collapsing the stock Extremely surprising that they reduce stability and increase overall yield The exact opposite of what we expected

17 Discussion: why does this work? Theoretically in a system driven by variable productivity: When a stock is rising, you should fish lightly (the stock is below Bmsy, let it rise rapidly) When a stock is falling (but above Bpa), you should fish hard (the stock is above Bmsy, fish it down) Stability constraints (inadvertently) achieve this But only if you consider multispecies effects

18 Discussion: why does this work? How STOCOBAR works Cod cannibalism is inversely related to capelin abundance in the model Capelin variations are modelled Can also model effects of cyclic temperature variations => this gives a variable carrying capacity/productivity proxy within the model

19 Conclusions Where stock productivity/carrying capacity is the key driver of stock level, stability constraints can actually increase the overall long term yield But there is a risk of going below Bpa/Blim

20 Below Bpa/Blim Going below Blim is officially a Bad Thing TM Spikes in F as the stock falls Although the 10% is suspended once you reach Bpa, you can shoot past it in one year with a high F A HCR with a stability constraint should also have a ceiling on F

21 Conclusions Using single species modelling to assess HCRs in general, and stability constraints in particular, may give the wrong answer In this case the conclusions would be wrong in direction, not just magnitude

22 Conclusions: general Need to be cautious about any one model, but: It is not obvious that stability constraints will stabilize or reduce catch in the real world They should always be used with ceiling on F We should be careful to examine multispecies interactions (where they matter) Ignoring them can give very wrong answers