Kray F. Van Kirk, SFOS, UAF, Juneau

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A Multispecies Age-Structured Assessment for the
Gulf of Alaska

• Kray F. Van Kirk, SFOS, UAF, Juneau
• Terrance J. Quinn II, SFOS, UAF, Juneau
• Jeremy S. Collie, GSO, URI, Narragansett
• ftkv_at_uaf.edu

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U.S. Commissions on Ocean Policy last year
recommended a move towards ecosystem-based
fisheries management.

• Why?

Collapse of multiple fisheries worldwide

Increased understanding of ecosystem
complexities and species interconnectivity
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Single-species models are a foundation
Whats going on down there?
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Elements of Ecosystem-BasedFisheries Management
Models
Relevant Dynamics
Predictive
Accurate
MODEL
Data Sources
Statistically Robust
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Virtual Population Analysis

• Begins with terminal cohort
• Abundance at oldest age Catch at oldest
  age
• Mortality (Z) Catch
• Backward calculation of abundance
• Assumes catch measured without error
• Underestimates abundance

Relevant Dynamics
Statistically Robust
Accurate
Data Sources
Predictive
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Cohort Analysis

• Begins with terminal cohort
• Backward calculation of abundance
• Mortality (Z)
• Fishery Mortality
• Fixed Natural Mortality
• Assumes catch measured without error

Relevant Dynamics
Statistically Robust
Accurate
Data Sources
Predictive
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Statistical Age-Structured Analysis

• Begins with Recruitment
• Forward calculation of abundance
• Mortality (Z)
• Fishery Mortality
• Fixed Natural Mortality
• Assumes catch measured with error

Relevant Dynamics
Statistically Robust
Accurate
Data Sources
Predictive
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Multispecies Models (MSVPA)

• Forward from recruitment or backwards from
  terminal cohort
• Involve two or more species connected by
  predation
• Fixed Predation Mortality
• (from gut studies)
• Assumes catch measured without error

Relevant Dynamics
Statistically Robust
Accurate
Data Sources
Predictive
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Mass Balance Models (Ecosim)

• Balances consumption and production
• Involve ALL species connected by predation
• Z Fishery Mortality
• Fixed Predation Mortality
• Fixed Natural Mortality
• Assumes catch measured without error
• Tunes assimilation parameters

Relevant Dynamics
Statistically Robust
??
Accurate
??
Data Sources
Predictive
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Our ApproachMultispecies Age-Structured
Assessment

• Three species with close predator-prey links
• (Walleye Pollock, Pacific Cod, Arrowtooth
  Flounder)
• Progress forward from recruitment
• Flexible predation mortality
• Based on gut studies
• Incorporates both species and age of prey
  preferred by a given predator
• Is responsive to predator and prey abundances
• Catch is assumed measured WITH error
• Gear selectivities are estimated by model

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Age-Specific Fishing Mortality
Residual Natural Mortality
Age-Specific Predation Mortality
YEAR ONE
Production from Spawning Biomass
YEAR TWO
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Catch-at-Age Gut Studies Spawner-Recruit Data
Data
Reproduction Fishing Mortality (F) Predation
Mortality (P) Residual Natural Mortality (RNM)
Dynamics
Catch measured with error Flexible Predation
Mortality Reproduction estimated to meet dual
demands of mortality and spawning biomass
Statistics
Forward progression allows predictive capacity
Predictive
Preliminary results produce RSS scores of less
than 500
Accuracy
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Preliminary Results

• Natural Mortality is estimable
• Predation Mortality is estimable

Pacific Cod
Arrowtooth Flounder
Walleye Pollock
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Next Step

• Assess the implications of model results for
  practical fisheries management.