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Simple Approaches to Data-Poor Stock Assessment

Simple Approaches to Data-Poor Stock Assessment. Rainer Froese rfroese@ifm-geomar.de March 9, 2011, Troutdale, Oregon. Overview. Some background Fecundity Size matters Recruitment Options for Management Length-only Semelparous species Revisiting Schaefer If biomass is known.

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Simple Approaches to Data-Poor Stock Assessment

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  1. Simple Approaches to Data-Poor Stock Assessment Rainer Froese rfroese@ifm-geomar.de March 9, 2011, Troutdale, Oregon

  2. Overview • Some background • Fecundity • Size matters • Recruitment • Options for Management • Length-only • Semelparous species • Revisiting Schaefer • If biomass is known

  3. NO RELATIONSHIP BETWEEN FECUNDITY AND ANNUAL REPRODUCTIVE RATE IN BONY FISHRainer FROESE, Susan LUNAACTA ICHTHYOLOGICA ET PISCATORIA (2004) 34 (1): 11–20 Maximum annual reproductive rate versus mean (solid dots) and minimum (open dots) annual fecundity.

  4. Fish and Fisheries, 2004, 5, 86–91Keep it simple: three indicators to deal with overfishingRainer Froese

  5. Reducing catch to Fmsy is good but insufficient • Stock size may increase seven-fold if fish are caught after multiple spawning, at around 2/3 of their maximum length • Large stock size means low cost of fishing

  6. Age-structure of North Sea Cod, with same catch but different minimum size Fmsy & Lopt Fmsy Current For a given catch, the impact on the stock is least if fish are caught at Lopt

  7. Same catch, better age structure Stock size can increase seven-fold

  8. The Hockey-Stick (Barrowman & Myers 2000) • Assumptions: • Constant R/S at low S • Constant R at high S

  9. The Smooth Hockey-Stick (Froese 2008) where A = ln Rmax • Assumptions: • Practically constant R at high S • Gradually increasing R/S at lower S

  10. Example Striped bass Morone saxatilis Parameters and accounted variance not significantly different Extrapolation VERY different

  11. Example: 12 stocks of Atlantic cod Gadus morhua Bold line is Smooth Hockey-Stick with n = 414, α = 4.5, Rmax = 0.85 Dotted line the Ricker model with n = 414, α = 3.1, Rmax = 1.4. Data were normalized by dividing both R and S by Rmax for the respective stock.

  12. Number of replacement spawners versus number of parents for 48 Pacific salmon populations. The fitted smooth hockey stick has a slope of 4.2 (3.6 – 5.2).

  13. Assesment and Management Options

  14. If nothing is known about the stock Management: • Get an estimate of maximum length (interviews; old photos; FishBase) • Get an estimate of length at first maturity (examine specimens; FishBase) • Set minimum length in catch and/or start of fishing season such that >90% of the specimens had a chance to reproduce before being caught • Give incentives to catch only fish with a length of 2/3 of their maximum length Justification: • Overfishing is theoretically impossible if all fish have a chance to reproduce before capture (Myers and Mertz, 1998). Impact of fishing on cohorts is minimized at about 2/3 of maximum length.

  15. If L∞ is known Assessment • Get length at first capture and mean length in catch • Derive reference length where F ~ M from • Derive reference length where Fmsy ~ ½ M from Management • Set minimum length in catch to LF~M, if larger than length where 90% are mature, else use that length • Set target length in catch to LFmsy

  16. If species die after spawning (salmons, eels, cephalopods)

  17. If Catch and Effort are Known

  18. If MSY and Bmsy are known(Data-rich Management)

  19. Generic Harvest Control Rules for European Fisheries Rainer Froese, Trevor A. Branch, Alexander Proelß, Martin Quaas, Keith Sainsbury & Christopher Zimmermann • Rules for sustainable and profitable fisheries based on • 1) economic optimization of fisheries • 2) honoring international agreements • 3) true implementation of the precautionary principle • 4) learning from international experiences • 5) ecosystem-approach to fisheries management • 6) recognizing the biology of European fish stocks • If these rules were applied, catches could increase by 63%

  20. Harvest Control Rule Schema

  21. Fisheries in 2007

  22. North Sea Herring 1960 - 1978

  23. North Sea-Herring 1979 - 2008

  24. ICES F-based Mangement

  25. North Sea Herring Once More F-based Management would not have prevented the collapse of herring.

  26. Critique of Planned F-based Management • Fmsy is taken as target, not limit, thus violating UNFSA and the precautionary principle • Fishing at Fmsy is less profitable than at Fmey • Fishing at Fmsy results in substantially smaller stocks, violating the ecosystem approach • Fishing at Fmsy results in strongly fluctuating catches with high uncertainty for the industry • Fishing at Fmsy provides strong incentives for overcapacity • Fishing at TAC = 0.9 MSY solves these problems

  27. Thank You Rainer Froese IFM-GEOMAR, Kiel, Germany rfroese@ifm-geomar.de

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