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Simple Approaches to Data-Poor Stock Assessment Rainer Froese [email protected] 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 NO RELATIONSHIP BETWEEN FECUNDITY AND ANNUAL REPRODUCTIVE RATE IN BONY FISH Rainer FROESE, Susan LUNA ACTA ICHTHYOLOGICA ET PISCATORIA (2004) 34 (1): 11–20 Maximum annual reproductive rate versus mean (solid dots) and minimum (open dots) annual fecundity. 3 Fish and Fisheries, 2004, 5, 86–91 Keep it simple: three indicators to deal with overfishing Rainer Froese 4 • 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 5 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 6 Same catch, better age structure Stock size can increase seven-fold 7 The Hockey-Stick (Barrowman & Myers 2000) Recruits (N) R2 Rmax R1 S Spawners (N) Assumptions: a) Constant R/S at low S b) Constant R at high S The Smooth Hockey-Stick (Froese 2008) R Rmax (1 e Rmax ln R A ln(1 e where A = ln Rmax Assumptions: a) Practically constant R at high S b) Gradually increasing R/S at lower S S ) e A S ) Example Striped bass Morone saxatilis S-R Model comparison for Morone saxatilis (striped bass) n=17 1982 --> 1998 [Stock: STRIPEDBASSUSA2] 25 20 15 Froese Ricker R B&H 10 observed 5 0 0 10 20 30 40 50 60 S Model α low up Rmax low up r2 B&H 3.67 2.60 4.73 24.9 17.3 36.0 0.834 Froese 3.40 2.64 4.15 17.4 13.5 22.6 0.843 Ricker 3.22 2.64 3.81 19.8 16.5 23.9 0.846 Parameters and accounted variance not significantly different Extrapolation VERY different Example: 12 stocks of Atlantic cod Gadus morhua Recruit abundance 10 1 0.1 0.01 0.01 0.1 1 10 Spawner abundance 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. Number of replacement spawners versus number of parents for 48 Pacific 12 salmon populations. The fitted smooth hockey stick has a slope of 4.2 (3.6 – 5.2). Assesment and Management Options 13 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. 14 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 15 • Set target length in catch to LFmsy Justification: The mean length Lmean of fishes in the catch is a function of the mean size at first capture Lc and of the fishing mortality F. The fishing mortality associated with Bmsy is typically smaller than the mean natural mortality rate of adult fish (M). If von Bertalanffy growth parameters L∞ and K are known, the mean length in the catch is given by Beverton and Holt 1957, p. 41, assuming that λ = tmax = ∞ and substituting age at first capture tp’ with length at first capture Lc : L FM Lmean L (1 (1 c )) FM K L If no reliable estimates of M and K are available, the M/K ratio can be assumed to be 3/2 (Jensen 1996) and the mean length in the catch where M = F can be obtained from LF M 3Lc L 4 Inserting F = ½ M mean length equation results in LFmsy 9 Lc 4 L 13 In Baltic cod, legal length at first capture is 38 cm and L∞ is 120 cm. The mean length in the catch resulting from fishing at Fmsy would then be 63 cm, which seems reasonable. 16 If species die after spawning (salmons, eels, cephalopods) Since the probability to die from fishing gets smaller if fishing starts later, and since the impact of a certain catch is smallest near the peak in cohort biomass, it still makes sense to target these species at a large size, shortly before spawning. The question then is how to define a reference point for escapement if no stock-recruitment data are available. The maximum annual reproductive rate for semelparous species equals the slope of the stockrecruitment function at the origin. A mean value across many populations is 4.2 (Froese, unpublished). The intrinsic rate of population increase rmax can then be obtained from rmax ln(4.2) 1.44 tm tm If we assume that Fmsy = ½ rmax we have Fmsy ln(4.2) 0.72 2tm tm Thus, for annual squids Fmsy = 0.7, for Atlantic salmon with average longevity of 5 years, Fmsy = 0.14, and for the European eel with 12 years, Fmsy = 0.06. All values seem reasonable. 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 0.5 B msy B msy 1.3 B msy MSY 1 0.91 MSY Depleted Zone Catch / MSY 0.8 Buffer Zone Overfishing Zone Target Zone 0.6 0.4 0.2 0 0 0.5 1 Biomass / B msy 1.5 2 21 Fisheries in 2007 0.5 B msy B msy 0.5 1 1.3 B msy 1.6 1.4 Catch / MSY 1.2 1 0.8 0.6 0.4 0.2 0 0 Biomass / B msy 1.5 2 22 North Sea Herring 1960 - 1978 1,200 1.3 B msy Landings (1000 t) 1,000 800 1960 1967 600 1962 1960 1962 400 1967 1977 1978 200 1978 0 0 500 1,000 1,500 2,000 2,500 3,000 3,500 Spawner Biomass (1000 t) 23 North Sea-Herring 1979 - 2008 1,000 1.3 B msy 1987 Landings (1000 t) 800 1985 600 2003 1985 2003 1987 2008 400 2008 200 1979 0 0 1983 500 1,000 1,500 2,000 2,500 3,000 3,500 4,000 Spawner Biomass (1000 t) 24 ICES F-based Mangement B pa 2 Catch B msy 1.8 F / Fmsy or Catch / MSY 1.6 1.4 1.2 F 1 0.8 0.6 0.4 0.2 0 0 0.2 0.4 0.6 0.8 1 1.2 Biomass / B msy 1.4 1.6 1.8 2 25 North Sea Herring Once More B msy 800 Landings (1000 t) 1966 600 F -based HCR 1971 1960 1967 1977 400 1978 Proposed HCR 200 1978 0 0 500 1,000 1,500 2,000 2,500 3,000 3,500 Spawner Biomass (1000 t) 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 [email protected] 28