The role of salinity in shaping the early life stages of Aphanius fasciatus

Salinity is a key environmental factor shaping the physiology, reproduction, and survival of estuarine and coastal fish species. Aphanius fasciatus , a euryhaline killifish inhabiting Mediterranean transitional water ecosystems, is listed in Annex II of the European Habitat Directive due to its ecological significance. Although highly tolerant to salinity fluctuations, its presence in oligohaline waters is limited. This study investigates the effects of salinity on A. fasciatus reproductive and early developmental stages by assessing (1) egg deposition rates, (2) hatching success across a salinity gradient, (3) egg mortality across a salinity gradient, and (4) larval survival probabilities. Results revealed that egg mortality is statistically higher and fry survival rate statistically lower in oligohaline conditions. Our findings contribute to understand how salinity influences reproductive success and early-stage survival, providing valuable insights into the species' ecological resilience. This knowledge is essential for conservation strategies in transitional waters, where salinity fluctuations are intensified by climate change. The discussion supports the use of ecological systems modelling as a valuable tool for studying and managing complex systems, such as transitional aquatic environments. • Early-life stages are vulnerable to salinity extremes. • Ecological resilience doesn't imply reproductive success. • Predictive modelling reveals critical survival thresholds. • Salinity-driven mortality explains species absence in oligohaline zones.