Anglerfish Stock in the Central Mediterranean Is Living Dangerously Close to Its Limits

In the cold, dim waters off northern Tunisia, one of the Mediterranean’s most grotesquely charismatic predators is quietly approaching a biological tipping point. The white anglerfish, Lophius piscatorius, the ambush hunter famously nicknamed the monkfish, has become an increasingly valuable target for bottom trawlers working the continental shelf of the central Mediterranean. Now, the first comprehensive stock assessment for this species in Tunisian waters reveals a population that is not overfished, but is running so close to several critical reference levels that any increase in fishing effort could undermine its reproductive future.

The study, published in Acta Ichthyologica et Piscatoria, focused on Geographical Sub-Area 12, a region corresponding to the northern Tunisian coast where fisheries data have long been too sparse and fragmented for conventional assessment methods. Researchers from Tunisian institutions tackled this data-poor problem by deploying a battery of complementary data-limited models, combining thirty years of catch records from 1995 to 2024 with nearly 1,900 individual fish measured at the port of Bizerte during 2024 and 2025. Sampling was conducted during night-time landings, when bottom trawlers return from their fishing trips, across 111 dedicated surveys spanning two years.

The biological portrait that emerges is of a species perfectly adapted to patience and ambush, and correspondingly vulnerable to exploitation. The best-fitting growth model, using the ELEFAN simulated annealing algorithm implemented in the TropFishR package, yielded an asymptotic length of 89.29 centimeters and a growth coefficient of 0.369 per year, indicating moderate growth with rapid early development that slows as fish approach their maximum size. Notably, the inclusion of a seasonal growth component did not improve model fit, suggesting that anglerfish in these deep, thermally stable Mediterranean waters grow continuously throughout the year rather than in seasonal bursts.

Perhaps the most consequential finding concerns reproduction and survival. The researchers estimated that half of the population reaches sexual maturity at 47.1 centimeters, with 95 percent mature at 56.52 centimeters, a relatively large size compared with the species’ maximum dimensions. Natural mortality came out remarkably low at 0.19 per year, based on the Hoenig estimator that links mortality to maximum observed age, which for this species reaches 24 years. This combination of late maturity, long lifespan and low natural death rates means the population’s fate rests almost entirely on fishing mortality rather than natural processes. Even modest changes in fishing effort can therefore ripple through the age structure and spawning biomass with disproportionate force.

The catch statistics themselves tell a striking story of an expanding fishery. Reported production rose from a mere 8 tonnes in 1995 to 93 tonnes in 2024, with a peak of 144 tonnes in 2019. Bottom trawling accounts for roughly 98 percent of the landings. Statistical analysis confirmed a significant upward trend of about 3.5 tonnes per year, corroborated by both linear regression and the non-parametric Mann-Kendall test, though the coefficient of variation of 73 percent reveals wild interannual swings that complicate any simple reading of the trajectory.

The heart of the assessment lies in four complementary models applied to these data. The CMSY++ Monte Carlo approach, which reconstructs stock biomass and fishing mortality from catch time series, estimated a biomass ratio of 1.18 relative to the level producing maximum sustainable yield, with fishing mortality at 0.904 of its sustainable benchmark. On paper, those numbers suggest a sustainably exploited stock. But the confidence intervals tell a more unsettling tale: the uncertainty range for the fishing mortality ratio extends from 0.445 all the way to 2.00, meaning that fishing mortality may in fact already exceed the sustainable threshold. The authors urge caution rather than celebration.

Length-based models added a demographic dimension to the picture. The length-based indicators method showed that only 46.5 percent of caught fish had exceeded the maturity length, far below the 100 percent target, while the proportion of mega-spawners, the large old females that contribute disproportionately to population renewal through higher fecundity and better egg quality, stood at just 10.15 percent against a precautionary reference of 20 percent. The Length-based Bayesian Biomass model reinforced the concern about size selectivity: fish are first caught at around 36 centimeters, well below the optimal capture length of about 47 centimeters, meaning the fishery harvests individuals before they have had their full reproductive opportunity.

The Length-Based Spawning Potential Ratio model, meanwhile, placed the stock right at the edge of the reproductive cliff. The spawning potential ratio was estimated at 39 percent in 2024 and 40 percent in 2025, hovering exactly at the 40 percent reference point generally regarded as the lower limit of full reproductive sustainability. In practical terms, the stock retains just enough spawning capacity to sustain itself, but possesses essentially no safety margin. The model also detected that fish become vulnerable to capture at slightly smaller sizes in 2025 than in 2024, a subtle signal worth monitoring. Sensitivity analyses showed the LBSPR results were robust to alternative biological inputs, but revealed that the LBB model’s encouraging biomass estimates could swing dramatically, from a biomass ratio of 1.60 down to 0.18, when different published growth parameters were substituted, underscoring how much rides on getting the biology right.

How does the Tunisian population compare with its relatives elsewhere? The Mediterranean fish grow faster but reach smaller maximum sizes than their northeastern Atlantic counterparts, where asymptotic lengths can exceed 200 centimeters and growth coefficients fall as low as 0.05 per year. This pattern fits the temperature-size rule: warmer, nutrient-poor Mediterranean waters accelerate early growth but cap ultimate size. Intriguingly, the estimated theoretical age at zero length of -0.33 years aligns closely with values from the Irish Sea, Norway and Iberian waters, suggesting the fitted growth curve accurately captures the species’ early life phase even in this under-studied region.

The overall verdict is deliberately nuanced: the stock shows no signs of severe depletion, yet the evidence does not allow it to be classified as sustainably exploited. The authors recommend a precautionary approach centered on three pillars: holding fishing mortality at current levels rather than allowing any increase, improving gear selectivity so that smaller individuals escape, and protecting the large spawners whose loss would erode the stock’s resilience. Concrete options include raising the minimum landing size above the current first-capture length of 36 centimeters and strengthening data collection on effort and catch composition. For a species whose survival strategy is to wait motionlessly on the seafloor for prey to wander within reach, the white anglerfish now finds itself in a similar position, one wrong move by the fishery could leave it without the spawning biomass it needs to recover.

Subject of Research: Population dynamics and data-limited stock assessment of the white anglerfish Lophius piscatorius in the central Mediterranean Sea

Article Title: Population dynamics and stock assessment of the white anglerfish, Lophius piscatorius ( Actinopterygii , Lophiiformes , Lophiidae ), in the central Mediterranean Sea (GSA 12)

Article References: Agrebi, S., Ennouri, R., Alayet, F., Hedfi, A., Louiz, I., Ben Ali, M., Mahmoudi, E., & Mili, S. (2026). Population dynamics and stock assessment of the white anglerfish, Lophius piscatorius (Actinopterygii, Lophiiformes, Lophiidae), in the central Mediterranean Sea (GSA 12). Acta Ichthyologica et Piscatoria, 56, 441-459. https://doi.org/10.3897/aiep.56.209443

Image Credits: AI Generated

DOI: 10.3897/aiep.56.209443

Keywords: white anglerfish, Lophius piscatorius, stock assessment, data-limited fisheries, Mediterranean Sea, Tunisia, CMSY++, LBSPR, length-based indicators, fisheries management, demersal species, spawning potential ratio