Resident fishes as ecological indicators in transitional waters

Transitional waters represent highly dynamic and vulnerable ecosystems, where the assessment of habitat quality requires the identification of reliable ecological indicators. In this context, resident fish species, which complete their entire life cycle within these habitats, are particularly suitable candidates. This thesis investigates the potential of resident fishes as ecological indicators, with a primary focus on the Mediterranean killifish Aphanius fasciatus and a complementary study on the sculpin Myoxocephalus scorpius. In the Mediterranean context, where transitional habitats are influenced by processes of marinisation alternated with extreme rainfall events that reduce the salinity wedge, A. fasciatus was investigated as an indicator of salinity-driven habitat shifts through laboratory experiments testing reproductive success from oligohaline to hypersaline conditions. In addition, the species was evaluated as a sentinel of environmental stress through the development of a novel method to quantify cortisol accumulated in scales, providing a reliable measure of chronic stress across different habitat types. Finally, a year-long field study demonstrated that A. fasciatus abundance correlates with habitat morphological complexity, supporting its use both as an ecological indicator and as an umbrella species, associated with typical lagoon fish assemblages. In an Arctic–boreal environment, M. scorpius was studied using acoustic telemetry to confirm its status as a resident species in Icelandic fjords, highlighting its potential role as an ecological indicator of environmental stressors in boreal transitional-like systems. This finding extends the applicability of the resident-species approach to a broader biogeographic scale. Overall, the results strengthen the case for using resident fishes as ecological indicators in transitional waters, providing methodological tools and empirical evidence that support their integration into conservation and management strategies.