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Age- vs. length-based selectivity for small pelagic fisheries: outside/inside model considerations for management. P. R. Crone, J. L. Valero, and K. T. Hill NOAA Fisheries Center for the Advancement of Population Assessment Methodology (CAPAM) Southwest Fisheries Science Center

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slide1

Age- vs. length-based selectivity for small pelagic fisheries: outside/inside model considerations for management

P. R. Crone, J. L. Valero, and K. T. Hill

NOAA Fisheries

Center for the Advancement of Population Assessment Methodology (CAPAM)

Southwest Fisheries Science Center

8901 La Jolla Shores, Dr., La Jolla, CA 92037, USA

slide2

Pacific mackerel assessment - selectivity evaluation

  • Phase 1
  • Motivation: ongoing sensitivity analysis with stock assessment model for management
  • Question: choice of biological data and selectivity assumptions impact on management quantities?
    • e.g., MSY, SPBcurrent, Depletion (SPBcurrent/SPBunfished)
  • Evaluation summary
    • Outside model considerations
    • Fish vs. fishing dynamics
    • Inside model considerations
    • Examine alternative model scenarios involving combinations of biological data and selectivity
      • Biological data
      • Age compositions vs. length compositions
      • Selectivity
      • Age-based vs. length-based
    • Conduct simulations/estimations involving alternative model scenarios
    • Results
    • Examine central tendencies, precision, and bias of management statistics
    • Identify potential areas of parameter tension (misspecification) in assessment model
    • Phase 2
    • Increasingly add process (estimated parameters) to assessment model and repeat evaluation
    • Evaluate other species/assessments (P. sardine and bigeye tuna)
slide3

Pacific mackerel assessment - selectivity evaluation

  • Outsidethemodelconsiderations
  • What underlying factors govern selection?
    • Extrinsic, e.g., gear design/operation (contact- and retention-selection)
    • Intrinsic, e.g., fish biology (available-selection)
  • Gear
    • Commercial: purse seine fisheries that operate seasonally across states/countries
    • Recreational: hook-and-line fishery operates year-round in CA
  • Biology
    • Driving mechanisms: size (length) vs. time (age)
    • Examine biological compositions for consistency
    • Fish grow rapidly and realize full selection in the fisheries by age 1
slide4

Pacificmackerel

Outsidethemodelconsiderations

OR-WA

Distribution

Spawning Area

Fisheries

Monterey

San Pedro

San Diego

Ensenada

Bahia

Magdalena

slide5

Pacific mackerel assessment - selectivity evaluation

  • Outsidethemodelconsiderations
  • Consistency across biological compositions (or not)
slide6

Pacific mackerel assessment - selectivity evaluation

  • Insidethemodelconsiderations
    • Current assessment model
    • Stock Synthesis model
    • Multiple fisheries and indices of abundance
    • Age and length data
    • Age compositions and age-based selectivity
    • Sensitivity analysis is not robust
    • Tension between selectivity and growth/natural mortality/S-R relations
    • Assessment model simplified for Phase 1 evaluation
    • 1 composite fishery: USA (com and rec) / MEX
    • 1 index of abundance: recreational CPUE
    • Age or length data
    • Other data omitted and most other parameters fixed
    • Alternative model scenarios evaluated (biological data/selectivity combinations)
slide8

Pacific mackerel assessment - selectivity evaluation

  • Insidethemodelconsiderations
    • Design and methods
    • Construct alternative model scenarios
      • 4 hypothesized ‘states of nature’ based on combinations of biological data and selectivity
      • Biological data
      • Age compositions vs. length compositions
      • Selectivity
      • Age-based vs. length-based
      • Each model scenario represents ‘true’ population and fishing dynamics
    • Produce 500 data sets from each model scenario (simulation)
      • Parametric bootstrap procedures (Stock Synthesis)
    • Evaluate each model scenario based on true (simulated) vs. assumed (estimated) selectivity
      • 4 (simulated) model scenarios x 2 (estimated) selectivity assumptions
      • Biological data / True selectivity / Assumed selectivity
      • Age / Age / Age
      • Age / Age / Length
      • Age / Length / Length
      • Age / Length / Age
      • Length / Length / Length
      • Length / Length / Age
      • Length / Age / Age
      • Length / Age / Length
slide9

Pacific mackerel assessment - selectivity evaluation

Biological Data

‘True’ Selectivity

Assumed Selectivity

500 Simulated models

500 Estimated models

Age - AAA

Age - AA

Age - AA

Length - AAL

Age - A

Length - ALL

Length - AL

Length - AL

Age - ALA

Length - LLL

Length - LL

Length - LL

Age - LLA

Length - L

Age - LAA

Age - LA

Age - LA

Length - LAL

slide10

Pacific mackerel assessment - selectivity evaluation

Results– General selectivityforms

AA

LL

slide11

Pacific mackerel assessment - selectivity evaluation

Results– General fitstobiologicalcompositions

AA

LL

slide12

Pacific mackerel assessment - selectivity evaluation

Results

Black: True Selectivity = Assumed Selectivity

Red: True Selectivity ≠ Assumed Selectivity

AAA

AAL

ALL

ALA

LAA

LAL

LLL

LLA

slide13

Pacific mackerel assessment - selectivity evaluation

Results

Black: True Selectivity = Assumed Selectivity

Red: True Selectivity ≠ Assumed Selectivity

slide14

Pacific mackerel assessment - selectivity evaluation

Results

AAA

Black: True Selectivity = Assumed Selectivity

Red: True Selectivity ≠ Assumed Selectivity

AAL

ALL

ALA

LAA

LAL

LLL

LLA

slide15

Pacific mackerel assessment - selectivity evaluation

Results

Black: True Selectivity = Assumed Selectivity

Red: True Selectivity ≠ Assumed Selectivity

slide16

Pacific mackerel assessment - selectivity evaluation

Results

Black: True Selectivity = Assumed Selectivity

Red: True Selectivity ≠ Assumed Selectivity

AAA

AAL

ALL

ALA

LAA

LAL

LLL

LLA

slide17

Pacific mackerel assessment - selectivity evaluation

Results

Black: True Selectivity = Assumed Selectivity

Red: True Selectivity ≠ Assumed Selectivity

slide18

Pacific mackerel assessment - selectivity evaluation

  • Preliminaryconclusions
  • An objective approach to evaluate ‘risk’ (uncertainty in management terms) of misspecification of assumed selectivity in the assessment model
  • Basic research that could contribute to a diagnostics/selectivity/management section in a good practices in stock assessment modeling guide
  • Age-composition data more robust to selectivity misspecification than length-composition data
    • Implication of using age compositions (vs. length) as the basis for the assessment is that growth parameterization is more certain, which may be misleading
    • No substitute for careful scrutiny of ageing techniques/consistency outside the model
  • Instability of some model scenarios can lead to poor convergence
  • Further develop assessment model and repeat evaluation and need to apply evaluation to other species/assessments
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