assessment of bigeye tuna thunnus obesus in the eastern pacific ocean
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ASSESSMENT OF BIGEYE TUNA ( THUNNUS OBESUS ) IN THE EASTERN PACIFIC OCEAN

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ASSESSMENT OF BIGEYE TUNA ( THUNNUS OBESUS ) IN THE EASTERN PACIFIC OCEAN. January 1975 – December 2006. Outline. Stock assessment Overview of assessment model Fishery data Assumptions Results of base case model Projections Sensitivity analyses Summary and conclusions Discussion.

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assessment of bigeye tuna thunnus obesus in the eastern pacific ocean

ASSESSMENT OF BIGEYE TUNA (THUNNUS OBESUS) IN THE EASTERN PACIFIC OCEAN

January 1975 – December 2006

outline
Outline
  • Stock assessment
    • Overview of assessment model
    • Fishery data
    • Assumptions
    • Results of base case model
    • Projections
  • Sensitivity analyses
  • Summary and conclusions
  • Discussion
overview of assessment
Overview of assessment
  • Age-structured, statistical, catch-at-length model (Stock Synthesis II).
  • Same type of model as A-SCALA or MULTIFAN-CL
  • Differences between SS2 and A-SCALA
bigeye fishery definitions

8

5, 13

9

3, 11

4, 12

1, 6-7

2, 10

Bigeye fishery definitions

Recent FLT (2-5)

Discards (10-13)

N Longline (8)

S Longline (9)

Early FLT (1)

Early & Recent UNA (6, 7)

FLT – Floating objects; UNA - Unassociated

data catch
Data - catch

Disc -N FLT

S LL

Disc – coastal FLT

Disc – Eq FLT

Disc - S FLT

N Offshore FLT

Early UNA

Recent UNA

N LL

S Offshore FLT

Equatorial FLT

Coastal FLT

Early FLT

data discards
Data - discards

Northern FLT

Inshore FLT

Equatorial FLT

Southern FLT

data effort
Data - effort

Disc -N FLT

Disc – coastal FLT

Disc – Eq FLT

Disc - S FLT

S LL

Recent UNA

N LL

N Offshore FLT

Early UNA

Equatorial FLT

Coastal FLT

Early FLT

S Offshore FLT

data cpue
Data - CPUE

Disc -N FLT

Disc – Eq FLT

Disc - S FLT

S LL

Disc – coastal FLT

N Offshore FLT

Early UNA

Recent UNA

N LL

Early FLT

S Offshore FLT

Equatorial FLT

Coastal FLT

data length frequency data
Data - Length frequency data

Quarter

1997

2005

2000

assumptions base case movement

DATA

  • Tagging records indicate little exchange of bigeye between E and W Pacific
  • Results from conventional and archival tagging indicate regional fidelity for bigeye in EPO
  • Different CPUE trends between EPO and WCP

ASSUMPTIONS

  • Single stock of bigeye in EPO
  • No net movement of fish between the eastern and western Pacific
  • SA for EPO and Pacific wide are consistent
Assumptions (base case) - movement
assumptions base case growth
Assumptions (base case) - growth
  • Von Bertalanffy – fixed parameters
assumptions base case cont
Assumptions (base case) - cont.
  • Age-specific maturity and fecundity indices
  • No S-R relationship (steepness = 1)
results base case
Results (base case)
  • Fit to the length frequency
  • Fishing mortality
  • Selectivity
  • Recruitment
  • Biomass
fishing mortality
Fishing mortality

Ages 5-8

Ages 1-4

Ages 9-12

Ages 13-16

Ages 17-20

Ages 21-24

Ages 25-28

Ages 29-32

Ages 33-36

Ages 37-40

size selectivity
Size selectivity

S Offshore FLT

Early FLT

Equatorial FLT

N Offshore FLT

Coastal FLT

Early UNA

S LL

N LL

Recent UNA

biomass
Biomass

Biomass of fish 0.75 + years old

spawning biomass
Spawning biomass

Population fecundity

comparisons to reference points
Comparisons to reference points
  • Spawning biomass depletion (SBR)
forward simulations
Forward simulations
  • Biomass
  • Spawning biomass depletion
  • Surface fishery catch
  • Longline catch
sensitivity analyses
Sensitivity analyses
  • Spawner-recruitment relationship (steepness = 0.75)
  • Growth
    • Growth estimated
    • Linf fixed (171.5 and 201.5)
  • Fitting to the initial equilibrium catch
  • Use of iterative reweighting of data
  • Time blocking of selectivity and catchability for southern longline fishery
  • Inclusion of new Japanese longline data
fit to cpue data1
Fit to CPUE data

Without iterative reweighting

With iterative reweighting

summary main results
Summary: Main results
  • Both total and spawning biomass is estimated to have substantially declined since 2000
  • Current biomass level is low compared to average unexploited conditions
  • The current effort levels are too high to maintain the population at level that will support AMSY
  • Yileds could be increased if more of the catch was taken in the longline fisheries
what is robust
What is robust
  • Fishing mortality levels are greater than that necessary to achieve the maximum sustainable yield
  • Two exceptions: Lmax fixed and time blocks
plausible sensitivities and uncertainties
Plausible Sensitivities and Uncertainties
  • Results are more pessimistic with the inclusion of a stock-recruitment relationship
  • Biomass trends are sensitive to the weighting of different datasets
  • Recent estimates are uncertain and subject to retrospective bias
conclusions
Conclusions
  • Current spawning biomass is unlikely to remain at or above the level required to produce AMSY.
  • In the most recent years the fishing mortality is greater than that required to produce AMSY.
  • Under average recruitment, the stock is predicted to be below the level that would support AMSY unless fishing mortality levels are reduced further than the current restrictions.
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