Biological Reference Points

Biological Reference Points

  • Definition: Biological reference points are numerical values representing metrics such as stock size or fishing mortality.
  • Purpose in Stock Assessments: Scientists estimate fish stocks and fishing mortality rates over time, utilizing these reference points to evaluate the population status relative to assumptions about growth, reproduction, and mortality.
  • Guidance for Decision Makers: Reference points assist in determining whether fish populations are at risk of being too small or under excessive fishing pressure, helping to set target populations and fishing rates.

Fishing Mortality Rate

  • Definition: The fishing mortality rate ( ext{F}) quantifies how quickly fish are removed from a stock through harvesting.
  • Analogy: Consider a fish stock as money in a bank account with a fixed interest rate (the growth rate). Excessive withdrawals (overfishing) lead to depletion of the principal (fish stock).
  • Goal: Maintain the withdrawal rate (fishing mortality) below the growth rate for sustainable production.

Maximum Sustainable Yield (MSY)

  • Concept: MSY is the highest average number of fish that can be consistently harvested from a stock under current environmental conditions, ensuring long-term stock productivity.
  • Notation:   - ext{B} denotes biomass (total weight of fish).   - ext{B}{ ext{MSY}} indicates biomass producing MSY.   - ext{SSB} represents spawning stock biomass (weight of mature individuals).   - ext{SSB}{ ext{MSY}} indicates the spawning stock size needed for MSY.   - ext{F}_{ ext{MSY}} is the fishing mortality rate that maintains MSY.   

Optimum Yield

  • Definition: The optimum yield is the catch amount providing the greatest overall long-term benefit to society, factoring in biology, economics, and public attitudes about risk and environmental protection.
  • Comparison with MSY: The optimum yield is always less than or equal to MSY; it can be set at a conservative level depending on socio-economic factors.

Targets vs. Thresholds

  • Targets: These are aspirational values for stock size and fishing mortality rates that managers aim to achieve based on biological and socio-economic factors.
  • Thresholds: Values to avoid, defined as specific fishing mortality rates or stock sizes that represent the risk of overfishing.
  • Control Rules: Utilize deviations from targets which can influence management policies.

Categories of Stock Assessment

  • Fish stocks categorized based on two distinct concepts: overfishing and overfished.
  • Overfishing: Occurs when ext{F} exceeds the fishing mortality threshold ext{F}_{ ext{threshold}} while stock size remains sufficient.
  • Overfished: Occurs when stock size ext{B} falls below its threshold ext{B}_{ ext{threshold}}.
Four Categories of Stock Condition:

  1. Overfished & Overfishing Occurring: Stock depleted and critical measures needed.   2. Overfished & No Overfishing: Stock is low but impacts are not ongoing.   3. Not Overfished & Overfishing: Stock is healthy while fishing is excessive.   4. Not Overfished & No Overfishing: Stock status is healthy overall.

Determining Thresholds

  • Stock Size Threshold ( ext{B}{ ext{threshold}}):   - Defined as a percentage of ext{B}{ ext{MSY}} (common standard: 50%)   - Alternatively, smallest stock size expected to grow to ext{B}_{ ext{MSY}} within a decade.
  • Fishing Mortality Threshold: ext{F}{ ext{threshold}} should be less than ext{F}{ ext{MSY}} and the natural mortality rate ext{M}.

Fishing Mortality Calculations

  • Yield-per-Recruit Analyses: Assesses fish growth, mortality, and yield based on age classes entering fisheries.
  • Fmax: Indicates the maximum yield per recruit achievable but often leads to unsustainable fishing, risking stock viability.
  • F0.1: A more conservative threshold based on 10% of the yield increase from adding one unit of fishing effort, ensuring lower risk.
  • Maximum Spawning Potential (MSP): Refers to reproductive biomass available in absence of fishing.

Risk Assessment and Uncertainty

  • Definition: Risk Assessment evaluates uncertainties in stock assessments, aiding decisions under unclear conditions.
  • Confidence Interval: The range within which the true stock value lies, indicated by a point estimate (e.g., 100,000 metric tons with a 95% confidence interval of 80,000 to 120,000).
  • Sources of Uncertainty: Include model choice, natural mortality, fishing effort, and recruitment variability; greater uncertainty suggests a need for conservative thresholds.

Terms at a Glance

  • F: Fishing mortality rate.
  • B: Stock size, measured in biomass.
  • F}_{ ext{MSY}}: Fishing mortality for maximum sustainable yield.
  • B}_{ ext{MSY}}: Stock size for maximum sustainable yield.
  • B}_{ ext{threshold}}: Stock size limit under which it is considered overfished.
  • F}_{ ext{threshold}}: Fishing mortality limit leading to an overfished state.