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

Abstract

The white anglerfish, Lophius piscatorius Linnaeus, 1758, is a demersal species of great ecological and commercial importance in Tunisian waters. This study uses data-limited assessment methods to evaluate the population dynamics and stock status of this species in northern Tunisia (GSA 12). Growth parameters were estimated from size-frequency data, collected at Bizerte fishing port in 2025, using ELEFAN functions (Electronic LEngth Frequency ANalysis) implemented in the TropFishR package. Lengths at sexual maturity ( L m50 and L m95 ) and natural mortality ( M ) were derived from empirical equations. Stock status was evaluated using the Catch MSY Monte Carlo model (CMSY++ model), which was applied to catch data from 1995 to 2024 and was complemented by length-based models (Length-based Bayesian Biomass “LBB”, the length-based indicator method “LBI” and length-Based Spawning Potential Ratio “LBSPR”), which used size data from 2024 and 2025. The best-fitting growth model (ELEFAN SA standard) yielded an asymptotic length ( L ∞ ) of 89.29 cm, a growth coefficient ( K ) of 0.369 year –1 and a growth performance index ( φ' ) of 3.469, indicating moderate growth. The estimated lengths at maturity ( L m50 = 47.1 cm; L m95 = 56.52 cm) suggest that L. piscatorius reaches sexual maturity at a relatively large size compared to its maximum size. Natural mortality ( M = 0.19 year –1 ) was low, signifying that population dynamic was strongly influenced by fishing mortality rather than natural processes. The stock assessment results from CMSY++, LBB, LBI, and LBSPR provided complementary evidence on stock status, but did not allow a definitive conclusion regarding the sustainability of the stock. Estimates of B / B MSY > 1 (CMSY++ and LBB) and F / F MSY = 0.904 (CMSY++) suggest that fishing pressure remains below critical thresholds. Demographic indicators from LBI showed a proportion of mature individuals ( P mat = 0.4652) and proportion of mega-spawners ( P mega = 0.1015) were below the reference point, a moderate proportion of optimally sized individuals ( P opt = 0.3975), suggesting that the size structure of the catch is not fully consistent with the reference pattern and that improvements in size selectivity may be beneficial. However, the LBSPR-derived spawning potential ratios (SPR = 39% in 2024 and 40% in 2025) are close to the recommended reference point of 40%, indicating that the stock is close to the limit of full reproductive sustainability, and any increase in fishing effort could compromise spawning biomass in the medium term. Overall, the L. piscatorius stock in GSA 12 should be considered to be operating close to several biological reference levels and a precautionary approach should be adopted to preserve the spawning stock and the population’s size structure.