Life-history-specific thresholds for the Fishing Mortality Index: validation on 209 stocks

Kafkas M. Caprazli

The exploitation fraction E = F/(F+M) has served as a standard metric for assessing fishing pressure since Gulland (1971), with E = 0.5 widely adopted as a precautionary threshold. However, E compresses the relationship between fishing mortality (F) and natural mortality (M) into a single dimension, obscuring magnitude differences across life histories. We introduce the Fishing Mortality Index (FMI), which plots F against M on log-log axes, preserving both ratio and magnitude information. We validated FMI using 209 stocks from the RAM Legacy Stock Assessment Database, defining collapse as B/B_MSY < 0.5. A universal threshold (FMI = 1.25) achieved 89% sensitivity but only 39% specificity (AUC = 0.72). Life-history stratification substantially improved discrimination: optimal thresholds ranged from 0.45 for fast-turnover species (M ≥ 0.8) to 4.91 for long-lived species (M < 0.2), with specificity improving to 69% (AUC = 0.71–0.85). Unexpectedly, optimal thresholds increased with longevity, suggesting that long-lived species tolerate higher FMI values before collapse, while fast-turnover collapses are associated with low FMI and likely driven by environmental factors. We recommend life-history-specific threshold calibration for operational FMI applications.