Simple Approaches to Data-Poor Stock Assessment

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Simple Approaches to Data-Poor Stock Assessment

Rainer Froese

rfroese@ifm-geomar.de

March 9, 2011, Troutdale, Oregon

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Overview

• Some background

– Fecundity – Size matters – Recruitment

• Options for Management

– Length-only

– Semelparous species

– Revisiting Schaefer

– If biomass is known

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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.

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Fish and Fisheries, 2004, 5, 86–91

Keep it simple: three indicators to deal with overfishing

Rainer Froese

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• Reducing catch to F

_msy

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

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Age-structure of North Sea Cod, with same catch but different minimum size

For a given catch, the impact on the stock is least if fish are caught at L_opt

Current

F_msy

F_msy& L_opt

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Same catch, better age structure

Stock size can increase seven-fold

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The Hockey-Stick

(Barrowman & Myers 2000)

S R

₁

  

max

2

R

R 

Assumptions:

a) Constant R/S at low S

b) Constant R at high S

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The Smooth Hockey-Stick

(Froese 2008)

) 1

ln(

ln R A e

^e^A ^S

 







) 1

(

^max

max

R S

e R

R

 





Assumptions:

a) Practically constant R at high S

b) Gradually increasing R/S at lower S

where A = ln R_max

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Parameters and accounted variance not significantly different

Model α low up R_max 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

Example Striped bass Morone saxatilis

Extrapolation VERY different

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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 R_max for the respective stock.

Example: 12 stocks of Atlantic cod Gadus morhua

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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).

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Assesment and Management Options

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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.

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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 F

_msy

~ ½ M from

Management

•Set minimum length in catch to L

_F~M

, if larger than length where 90% are mature, else use that length

•Set target length in catch to L

Fmsy 15

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

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If Catch and Effort are Known

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If MSY and B _msy are known

(Data-rich Management)

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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%

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Harvest Control Rule Schema

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Fisheries in 2007

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North Sea Herring 1960 - 1978

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North Sea-Herring 1979 - 2008

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ICES F-based Mangement

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North Sea Herring Once More

F-based Management would not have prevented the collapse of herring. ₂₆

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Critique of Planned F-based Management

• F

_msy

is taken as target, not limit, thus violating UNFSA and the precautionary principle

• Fishing at F

_msy

is less profitable than at F

_mey

• Fishing at F

_msy

results in substantially smaller stocks, violating the ecosystem approach

• Fishing at F

_msy

results in strongly fluctuating catches with high uncertainty for the industry

• Fishing at F

_msy

provides strong incentives for overcapacity

• Fishing at TAC = 0.9 MSY solves these problems

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Thank You

Rainer Froese

IFM-GEOMAR, Kiel, Germany rfroese@ifm-geomar.de

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Simple Approaches to Data-Poor Stock Assessment