Overfishing Impact Calculator

The Overfishing Limit (OFL) is the maximum catch level at which a stock is considered to be experiencing overfishing. It is typically calculated using the fishing mortality rate at Maximum Sustainable Yield (F_MSY) applied to the current stock biomass. This calculator approximates it by comparing your current harvest rate to the sustainable MSY harvest rate you provide.

Stock collapse is generally defined as a population falling below 10% of its historical carrying capacity or unfished biomass. At this level, the stock can no longer sustain commercial fishing and may struggle to recover even with complete protection. The calculator highlights when — and if — current harvest rates would drive a stock to this threshold.

MSY is the largest average catch that can theoretically be taken from a stock indefinitely under constant environmental conditions. For a species with logistic population growth, MSY occurs when the stock is at 50% of its carrying capacity, with a harvest rate approximately equal to half the intrinsic growth rate (r/2).

Annual catch limits (ACLs) prevent cumulative over-harvesting by capping how much can be taken each year based on current stock assessments. Without ACLs, fishing effort can quickly exceed the stock's reproductive capacity, causing irreversible depletion. Science-based ACLs are the foundation of modern sustainable fisheries management in the U.S. and globally.

A complete moratorium stops all harvest, allowing the natural growth rate to rebuild the population toward carrying capacity. Recovery time depends on the intrinsic growth rate of the species — fast-growing species like anchovies may recover in years, while slow-growing species like orange roughy can take decades. This calculator's moratorium scenario models that recovery trajectory.

'Overfishing' is a rate descriptor — it means the current harvest rate is too high and is actively depleting the stock. 'Overfished' is a stock status descriptor — it means the population has already fallen below the minimum sustainable level (often B_MSY). A stock can be overfished without current overfishing occurring if past pressure depleted it, or overfishing can be happening on a stock not yet considered overfished.

Reducing harvest rates lowers the annual removal below the stock's reproductive output, allowing net biomass to increase over time. A 25% reduction may stabilize a moderately depleted stock, while a 50% cut can actively rebuild it toward healthy levels. The exact benefit depends on the species' growth rate and current depletion — the Management Scenario comparison in this calculator illustrates this effect.

Intrinsic growth rates (r) vary widely: fast-growing forage fish like anchovies or sardines typically range from 0.5–1.5 per year; medium-growth species like cod or haddock range from 0.2–0.5; slow-growing deepwater species like orange roughy or sharks may be as low as 0.02–0.1. Using an accurate species-specific value significantly affects the depletion projections.