Life-history traits and mortality signals of Callista chione in the northwestern Adriatic Sea: implications under global change

The smooth clam Callista chione is an important commercial bivalve resource in the northwestern Adriatic Sea, yet updated information on its population biology, reproductive dynamics, growth, and sensitivity to environmental stressors remains limited. This study integrated fishery-dependent observations and fishery-independent surveys from the Monfalcone, Venice, and Chioggia districts to support adaptive management of the resource. Between 2017 and 2024, 17,907 individuals were measured for shell total length (TL), 1,493 were microscopically sexed, 359 were examined histologically, and 246 were aged using three shell-based techniques. Callista chione was confirmed as a long-lived, slow-growing species, with ages up to 20 years and a commercial size threshold (>40 mm TL) reached at approximately 4 years. Growth was consistent among districts and ageing methods and was adequately described by a common von Bertalanffy growth function (L∞ = 81.93 mm, k = 0.126 year−1, t0 = −1.56 years). Microscopic and histological evidence indicated year-round, asynchronous reproduction with multiple partial spawning events. Size at sexual maturity (TL50) was estimated at 26.7 mm TL (approximately 1.6 years), well below the current market size adopted by Adriatic management Consortia. The mortality observed in 2024 in Chioggia and Venice may have been associated with the exceptional environmental conditions that characterized summer 2024 relative to previous years, suggesting that C. chione is vulnerable to interacting environmental stressors. Overall, the results support management based on biological reference information, continuous environmental monitoring, and responsive measures following mortality events.