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


Age specific fishing mortality
Age-specific fishing mortality

1993-2006

1975-1992


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











Growth curves
Growth curves growth curve


Assessment of bigeye tuna thunnus obesus in the eastern pacific ocean
SBR growth curve





Iterative reweighting
Iterative reweighting growth curve



Fit to cpue data
Fit to CPUE data growth curve


Residual plot
Residual plot growth curve



Fit to lf data base case
Fit to LF data – base case southern longline fishery


Spawning biomass ratio6
Spawning biomass ratio southern longline fishery


Fit to cpue data1
Fit to CPUE data southern longline fishery

Without iterative reweighting

With iterative reweighting


Residual plot1
Residual plot southern longline fishery



Biomass2
Biomass southern longline fishery


Recruitment2
Recruitment southern longline fishery


Assessment of bigeye tuna thunnus obesus in the eastern pacific ocean
SBR southern longline fishery


Comparisons between models
Comparisons between models southern longline fishery


Summary main results
Summary: Main results southern longline fishery

  • 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 southern longline fishery

  • 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 southern longline fishery

  • 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 southern longline fishery

  • 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.


Comparisons between models1
Comparisons between models southern longline fishery