Ioannis Leris

Conservation translocations are essential for reversing biodiversity loss. Here, we report a translocation initiative on the endangered Evrotas chub ( Squalius keadicus ) to re‐establish it in the Vassilopotamos River, within its historical range in Greece. Our approach combined feasibility assessments, invasive species control, demographic modelling and staged releases. Three years after the last release, the species persists, and individuals reach maturity; yet, reproduction remains undetected. Does that mean failure? Perhaps not, but persistence, while encouraging, is not enough. This effort highlights the promise and limitations of translocations and shows that long‐term monitoring and flexibility are critical for success.

Freshwater biodiversity is under immense pressure globally, with invasive species among the primary drivers of extinction risk. We tested whether urban aquatic habitats can serve as refugia for threatened freshwater fish from invasive species' pressure, focussing on the translocation of a critically endangered killifish (Corfu killifish, Valencia letourneuxi ) to a spring‐fed peri‐urban pond in northwestern Greece. Urban habitat suitability was assessed using the REVIVE‐POLIS feasibility analysis tool, which incorporates conventional ecological criteria (water and biological quality, species‐specific habitat requirements, and invasive species pressure) alongside urban‐specific criteria including hydrological stability, food web structure and surrounding land use. The tool supported the suitability of the release site and indicated its utility in urban conservation planning. The translocation involved a two‐stage transfer of a total of 84 adult killifish spawners into the recipient pond across 2021 and 2022, corresponding to an estimated introduction density of 1.12 translocated inds m ⁻² . Post‐release monitoring in 2022 utilizing electrofishing confirmed establishment at the release site (density 0.83 inds m ⁻² ), as evidenced by the presence of larvae and juveniles. Subsequent monitoring in 2024 and 2025 using underwater cameras recorded cohorts of approximately 30 individuals (density 0.71 inds m ⁻² ). These spanned multiple age classes on both occasions, suggesting persistence and recruitment for ≥3 years after the initial release. As a single‐site case study, these findings should be interpreted cautiously; nevertheless, they indicate that carefully selected and managed urban aquatic refugia may complement conventional approaches for threatened fishes exposed to invasive species, provided key constraints (biosecurity, habitat area and long‐term viability) are addressed. Practical implication . This study delivers a user‐ready feasibility assessment tool that helps practitioners identify urban aquatic habitats suitable as refugia for threatened fish, provides methodological guidance for designing, implementing, and monitoring threatened‐fish translocations, and shows that where invasive‐species eradication is unfeasible, well‐managed urban refugia can complement or in some contexts, substitute for conventional in situ translocations.

Invasive fish species are a major driver of freshwater native fish biodiversity loss and their spread and impacts on the native fish are expected to increase within the current freshwater salinization and global warming crisis. In the current study, the upper thermal and salinity tolerance of the geographically range-restricted, threatened killifish Valencia robertae and its alien competitor, the globally invasive Eastern mosquitofish Gambusia holbrooki are compared in an experimental setting. Fish were exposed, after acclimation, to a continuous, dynamic temperature or salinity increase until predefined sub-lethal end points (loss of equilibrium and/or loss of buoyancy). The critical thermal and salinity maxima (CTMax and CSMax) were then calculated as the arithmetic mean of the combined thermal or salinity points at which the endpoint was attained. Finally, thermal and salinity safety margins for the two species were also calculated using abiotic data. Mosquitofish (females and males pooled) showed an average CΤmax of 35.85 °C and the killifish 36.27 °C (sexes pooled). Mosquitofish (male) showed an average CSmax of 40.25‰ and (male) killifish 42.64‰ (sexes also pooled). Killifish safety margins are much higher than those of the mosquitofish. Future impacts of global warming and salinisation on these species and on their interactions under current climate change scenarios are discussed.

Conservation translocations are increasingly applied for the protection of threatened species, but their success depends strongly on pre‐release preparatory work and strict implementation methodology, followed by post‐release monitoring. The aim of this work was to implement a conservation translocation that involved (a) the design and application of a pre‐release feasibility assessment tool for the selection of the translocation water bodies; (b) the translocation of 77 individuals of the highly threatened freshwater Corfu killifish Valencia letourneuxi in 2015, 2016 and 2017; and (c) a 5‐year post‐release monitoring with multiple methods. After initial pre‐screening, a feasibility assessment was conducted on two potential release water bodies (R‐WBs) and on four potential source water bodies (S‐WBs), using a custom‐made feasibility assessment tool that incorporated 14 criteria for the potential R‐WBs, related to the species' habitat requirements, human pressures and technical issues. Two criteria were applied to the potential S‐WBs, on source population availability and on its genetic compatibility to the release area. Post‐release monitoring (2017–2022) was conducted with fish sampling methods, underwater video recording and/or using environmental DNA (eDNA). The feasibility assessment tool gave one R‐WB high, positive score, and one S‐WB met both criteria, thus these selected as the translocation water bodies. Despite the low number of founders, the establishment of a self‐reproducing population was finally confirmed in 2022 in the R‐WB, initially with video recording and subsequently by fish sampling and eDNA. This study highlights the importance of pre‐release feasibility assessment, the use of multiple post‐release monitoring methods and actions to secure the long‐term viability of the translocated population. The feasibility assessment tool and the methodological protocol can easily be modified for successful application to other freshwater fish species, with different habitat requirements, current and future human pressures and genetic constraints.

This contribution presents the first record of the noble crayfish Astacus astacus (Linnaeus, 1758) in Euboea (Evia) Island, Greece. The crayfish was found during a fish monitoring expedition in the upper Mesapios River, in a reach that maintains flow throughout the year, while most of the river length desiccates completely during the dry season. The finding of A. astacus in an intermittent river of Euboea Island expands the species geographical range and calls for immediate conservation actions to protect its habitat and preserve this vulnerable population.

To survive, animals must respond appropriately to stress. Stress responses are costly, so early-life experiences with potential stressors could adaptively tailor adult stress responses to local conditions. However, how multiple stressors influence the development of the stress response remains unclear, as is the role of sex. Trinidadian guppies (Poecilia reticulata) are small fish with extensive life-history differences between the sexes and population variation in predation pressure and social density. We investigated how sex and early-life experience influence hormonal stress responses by manipulating conspecific density and perceived predation risk during development. In adults, we sampled cortisol twice to measure initial release and change over time in response to a recurring stressor. The sexes differed considerably in their physiological stress response. Males released more cortisol for their body mass than females and did not reduce cortisol release over time. By contrast, all females, except those reared at high density together with predation cues, reduced cortisol release over time. Cortisol responses of males were thus less dynamic in response to current circumstances and early-life experiences than females, consistent with life-history differences between the sexes. Our study underscores the importance of early-life experiences, interacting ecological factors and sex differences in the organization of the stress response.

Outcome transparency and the weight given to social information both play important roles in decision making, but we argue that an overarching influence is the degree to which individuals can and do gather information. Evolution, experience, and development may shape individual specializations in social decision making that carry over across contexts, and these individual differences may influence collective behavior and cultural evolution.