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Catch Curve (Length-Based)
Resources
Length-Based Assessment Methods Workbook
Primer for Length-Based Assessment Methods

This method utilizes length-frequency data (fish lengths) to estimate the fishing mortality affecting the fished population. Total mortality (Z) is estimated using the slope of the log transformed age-frequency histogram. Fishing mortality can then be calculated based on the difference between total fishing mortality (F) and M, the natural mortality (F = Z – M). Estimates of M can come from the literature, from MPA data (see MPA catch curve, Step 5.6), or can be calculated using the Barefoot Ecologist Tool at this link.

Inputs:

  • Length-Frequency data
  • Life history/ growth parameters (k, M, etc.)

Outputs:          

  • Estimate of fishing mortality (F)

Input Sensitivities, Assumptions and Caveats:

  • Assumes individual fish length measurements are accurate and format of measurements (i.e., total length, standard length, or fork length) are consistent
  • Assumes life history parameters (e.g., maximum length, length at maturity and size frequency) are accurate
  • Sensitive to accuracy of length-at-age relationships (Von Bertalanffy growth parameters)
  • Correct fitting of the curve (i.e., preferred fish size)
  • Length is related to age throughout life (i.e., growth is indeterminate – the species just keeps growing longer and longer as it ages until it dies)
  • Depends on reliably tracking population size structure changes, thus may be less accurate for small, fast-growing species
  • Recruitment is constant (i.e., juveniles are becoming adults at about the same rate each year) – this is a simplifying assumption that probably does not hold for any species
  • Mortality is constant – another simplifying assumption that probably does not hold for any species

Reference points:

  • TRP: M=F
  • LRP: F=2M

Recommendations:

  • Stakeholders set target and/or limit F/M ratio (i.e., management Reference Points) based on community objectives and thresholds of risk
  • F/M Reference Point is compared with F/M found through assessment
  • Effort/ fishing mortality is adjusted through harvest control methods (e.g. catch limits, seasons, or spatial closures) based on how far apart these values are