Length based methods for data poor stocks

Outline

• Condition Operating Models for 5 contrasting species, sprat, turbot, pollack, bigeye and ray.
  • Use a sawtooth exploitation history, i.e. gradual increase in \(F\) until the stock is overexploited after which a recover and long-term management plan is implemented.
  • Implement scenarios for steepness, M, … to evaluate robustness to uncertainty
• Evaluate ability of length based indicators (LBIs) to detect overexploited and recovery.
  • Use ROCs and Euclidian distance from [0,1] to evaluate classification skill.
  • Use AUC to identify ranking skill
• Combine LBIs into a single index, and
  • Derive a generic rule, i.e. for all stocks, by finding indicator wts that maximise AUC.
  • Repeat for overfishing and recovery separately
  • Repeat by stock to look at the benefit of developing case specific rules
  • Conduct VoI, by setting wt to a single scenario to 0, i.e. what is the benefit of eliminating a scenario/hypothesis?

Operating Model Conditioning

Figure 1 Life history parameters.

Figure 2. Simulated time series.

Observation Error Model

Figure 3. Simulated length frequencies distributions with indicators, vertical lines show \(0.7F_{MSY}\), \(F_{MSY}\) and \(2.5F_{MSY}\).

LBIs

[1] "l25"   "lc"    "lmaxy" "lmean" "lbar"  "lmax5"

Figure 4. Time series of fishery dependent indicators relative to \(F:F_{MSY}\), vertical lines indicate 1 (green), 1.5 (orange) and 2 (red) times \(F_{MSY}\).

Figure 5. ROC curves to detect overfishing i.e. years 80 to 100

Figure 6. ROC recovery, power to detect reduction in fishing from 100 to 120

Figure 7. Indicators relative to reference years (80 to 82), note different y scales.

Figure 8. Euclidean distance from TPR=1 and FPR=0 for detecting overfishing.

Figure 9. Euclidean distance from TPR=1 and FPR=0 for detecting recovery.

Figure 10. Euclidean distance from TPR=1 and FPR=0

Figure 11. Area under the ROC curve.

Summary

References

Author information

Laurence Kell. laurie@seaplusplus.es

Acknowledgements

Software Versions

• R version 4.0.3 (2020-10-10)
• FLCore: 2.6.18.9002
• FLBRP: 2.5.8
• FLasher: 0.6.8
• FLife: 3.4.0
• mydas: 1.2.2
• Compiled: Mon Nov 15 14:07:17 2021