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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
[email protected]
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
Single-species models are a foundation
What’s going on down there?
Elements of Ecosystem-Based
Fisheries Management Models
Relevant
Dynamics
Accurate
Predictive
MODEL
Statistically
Robust
Data
Sources
Virtual Population Analysis



Begins with “terminal
cohort”
Abundance at oldest age
= Catch at oldest age
Mortality (Z) = Catch
Backward calculation of
abundance
Va  Va 1  Ca


Assumes catch measured
without error
Underestimates abundance
Relevant
Dynamics
Statistically
Robust
Accurate
Data
Sources
Predictive
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
Statistical Age-Structured Analysis



Begins with Recruitment
Forward calculation of
abundance
Mortality (Z) =
Relevant
Dynamics
Statistically
Robust
Fishery Mortality +
Fixed Natural Mortality

Assumes catch measured
with error
Ni ,a 1,t 1  Ni ,a ,t e
 Z i ,a ,t
Accurate
Data
Sources
Predictive
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
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
Our Approach:
Multispecies 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
Age-Specific
Fishing Mortality
Residual Natural
Mortality
Age-Specific
YEAR ONE
Predation Mortality
Production from
Spawning Biomass
YEAR TWO
Data
Dynamics
Statistics
Catch-at-Age
Gut Studies
Spawner-Recruit Data
Reproduction
Fishing Mortality (F)
Predation Mortality (P)
Residual Natural Mortality (RNM)
Catch measured with error
Flexible Predation Mortality
Reproduction estimated to meet dual demands of
mortality and spawning biomass
Predictive
Forward progression
allows predictive capacity
Accuracy
Preliminary results produce RSS
scores of less than 500
Preliminary Results
1. Natural Mortality is estimable
2. Predation Mortality is estimable
LN Annual Biomass
15.4
Pacific Cod
15.2
Arrowtooth Flounder
15
14.8
14.6
14.4
14.2
14
Walleye Pollock
13.8
0
2
4
6
8
10
12
14
16
18
20
Next Step
Assess the implications of model results for
practical fisheries management.