Fish Stock Assessment Calculator

It estimates the initial population size (N0) of a fish stock from how catch-per-unit-effort declines as cumulative catch accumulates over a season. The underlying logic is that a larger starting population declines more slowly in CPUE for the same amount of catch than a smaller one does, so the rate of CPUE decline reveals the population size. The method traces back to two foundational papers -- Leslie and Davis's 1939 study estimating a rat population from trapping removals, and DeLury's 1947 formalization of the same logic for fisheries -- and is still taught and applied today as one of the classic closed-population depletion estimators.

No. Total effort only feeds into the separate Avg CPUE output, a simple catch-divided-by-effort summary reported alongside the results. The Initial population, Remaining stock, and Exploitation rate all come from the CPUE-decline calculation, which does not use Total effort at all.

This calculator flags a stock as Healthy below 30% exploitation rate, Fully exploited between 30% and 50%, and Overfished above 50%. These are common rule-of-thumb thresholds in fisheries management, though actual regulatory thresholds vary by species, jurisdiction, and stock assessment method.

It assumes a closed population during the assessment period -- no new fish entering through recruitment or immigration, and negligible natural mortality alongside the fishing removals -- plus constant catchability, meaning your gear removes a consistent fraction of the remaining population per unit of effort throughout the season. Over a long season, new fish can enter the population through recruitment or immigration, which violates the closed-population assumption and makes the CPUE decline look smaller than the true fishing pressure would otherwise produce -- so the method is most reliable over a short, intensive season where the population can reasonably be treated as fixed.

The Leslie-DeLury method depends on CPUE declining as fish are removed, so if Final CPUE does not come in lower than Initial CPUE -- whether from recruitment offsetting the catch or the two figures being entered in the wrong fields -- the depletion math has no valid solution. Rather than reporting a misleading population estimate, this calculator flags Stock Status as undeterminable in that case; Avg CPUE still reports a real figure since it is computed directly from your catch and effort, independent of the depletion model.