Unforeseen consequences of trout stocking in alpine lakes – the introduction and spread of bait fish in Retezat National Park, Romania

The introduction of native and alien trout into naturally fishless alpine lakes has long been associated with significant ecological consequences. This study focuses on an unforeseen effect: the accidental introduction and spread of bait fish, particularly minnows (Phoxinus sp.) and stone loach (Barbatula barbatula), in alpine lakes within Retezat National Park, Romania. Fish stocking, initiated in the 1960s and 1970s for recreational purposes, had potential cascading effects on aquatic biodiversity by facilitating the colonization of bait fish. Between 2022 and 2024, 25 alpine lakes were inventoried for fish presence. We used visual surveys and umbrella-traps in the littoral areas for smaller lakes, and additionally aerial and aquatic drone surveys for larger lakes. We observed a decline in trout populations in lakes that had been stocked in the past. In addition, bait fish were found in 12 lakes, all of which had either native fish present or had previously been stocked. The findings suggest that bait fish introductions are associated with fishing activities, with their successful establishment attributed to their ecological plasticity and high reproductive potential. This study highlights the unintended ecological consequences of fish stocking and the challenges in managing introduced species in alpine ecosystems.