Stable Landings, Falling Catch Rates: What to Advise the Regulator
A 12-year data series for a gillnet fishery targeting a sparid on a narrow continental shelf: total landings have stayed near 900 t for the last eight years, but registered effort rose from 300 to 700 boat-days and nominal CPUE fell from 3.0 to 1.8 kg per boat-day. Since 2019 part of the fleet has fished 20–40 km further offshore as inshore grounds were fished down. Mean length in the landed catch fell from 32 cm to 26 cm. Growth parameters are L∞ = 45 cm and K = 0.25 per year; length at first maturity is 24 cm; the current gillnet mesh is 60 mm. Managers propose raising the minimum mesh to 80 mm and closing the fishery for three months over the spawning peak. Industry argues that landings have been stable for eight years, so there is no problem, and that a larger mesh will only cut income.
1. Test the internal consistency of the data and give a defensible verdict on stock status.
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First, check whether the three series agree. If landings are constant and effort rose from 300 to 700 boat-days (×2.33), CPUE should have fallen to 43% of its starting value - i.e. from 3.0 to about 1.3 kg/boat-day. The reported fall to 1.8 kg/boat-day is only to 60%. The gap means at least one of the following: effort is under-registered (unlicensed boats, unrecorded days), landings are under-reported, or catchability has risen through technology creep, better gear, or fishers concentrating on the last remaining aggregations.
Verdict: overfishing is almost certainly occurring and the stock is likely overfished. The evidence chain is:
- A 40% CPUE decline understated fishing mortality; the true F increase is at least 2.3× and probably higher once catchability creep is allowed for.
- Mean length falling from 32 to 26 cm is a classic growth-overfishing signature; 26 cm is only about 22% of L∞, and well above L50 of 24 cm, but the truncation of the upper tail is the more telling signal than the mean.
- Spatial expansion 20–40 km offshore is serial depletion: the fleet is mining new ground to keep landings flat.
- Stable landings are the weakest of all indicators. They are consistent with hyperstability, where CPUE holds up while abundance falls, and with effort redistribution.
Recommend that the regulator treat the landings series as evidence of nothing and commission an independent fishery-independent index (travel survey or egg/larval survey) before the next quota decision.
2. Evaluate the 80 mm mesh change and the spawning closure using yield-per-recruit and recruitment-overfishing reasoning.
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Mesh increase to 80 mm. With L∞ = 45 cm and K = 0.25, an 80 mm gillnet selects fish roughly in the 26–30 cm range depending on body form, i.e. just above the 24 cm maturity point. That is a genuine conservation gain: it protects the first spawners and raises yield-per-recruit only if the stock is currently suffering growth overfishing (fish caught before they realise growth potential). The data suggest a truncated length distribution, so the gain is plausible. The risk industry raises is real: in a gillnet fishery, moving to larger mesh can reduce catch rates in the short term by 20–40% and creates a strong incentive to keep old nets. If compliance fails, the measure delivers cost without benefit.
Three-month spawning closure. The value depends entirely on whether effort is removed or merely displaced. If the fleet simply fishes harder before and after the closure (pulse fishing), total F is unchanged and the biological benefit is close to zero. A closure works when it protects a predictable spawning aggregation that would otherwise be targeted, and when it coincides with the actual spawning peak - which should be confirmed from gonad index sampling, not assumed.
Combination. The two measures are complementary but neither addresses the core problem, which is excessive effort. Mesh size fixes selectivity; it does not fix a fleet that is 2.3 times too large. Recommend adding a capacity measure (licence freeze or buy-back) and note that the closure should be paired with an effort cap, not used as a substitute for one.
Prediction to state openly: if landings remain 900 t after the measures, that is evidence of displacement, not success.
3. What monitoring would convince you in three years that the measures worked, and what would falsify that conclusion?
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Indicators to track, with pre-agreed thresholds:
- Length-frequency of the landed catch, monthly, ≥200 fish per quarter. Success: mean length recovering toward 29–30 cm and the proportion below 26 cm falling below 20%. Failure: mean length static or falling.
- Standardised CPUE, not nominal - correct for vessel, gear, season and area, because the fleet is changing how and where it fishes. Success: index stable or rising at constant effort.
- Effort, measured from logbooks plus port sampling, not from licence counts alone. Success: total boat-days at or below 700 and not displaced into a pulse around the closure.
- Proportion of catch taken offshore (the 20–40 km zone). Success: the offshore share stops growing; failure: continued expansion, which falsifies the claim that inshore stocks are rebuilding.
- Maturity ogive in the catch - proportion mature at first capture should exceed 90%.
- Independent fishery-independent index every second year - this is the only indicator not contaminated by fisher behaviour.
What would falsify success: stable landings with continued CPUE decline, an unchanged length distribution after two full cohorts (about 4 years at K = 0.25), a shift of effort into previously unfished areas, or an increase in the proportion of immature fish in the catch - the last being the clearest sign that the larger mesh is being circumvented by a smaller-mesh or trammel fleet.
Set these thresholds into the management plan now; deciding what counts as success after the data arrive is how failed measures get declared successful.