Fish are disappearing from Slovenian waters: one otter eats a kilogram of fish a day

Slovenian rivers face mounting pressure from multiple environmental factors, including pollution, climate change, and human interventions such as dam construction. Recent incidents have highlighted the deteriorating conditions of fish populations, with over 28 tons of grayling dying in the Studenčnica River near Ptuj due to fecal contamination. Experts warn that the situation is worsening, particularly in the Middle Savinja River, where the health of fish stocks is declining. These developments underscore the complex interplay of pressures affecting aquatic ecosystems. The decline in fish populations is influenced by several interconnected factors. Watercourse regulation, the building of hydroelectric dams, and other water barriers contribute significantly. Climate change, droughts, loss of spawning grounds, degradation of habitats, occasional pollution, diseases, and the spread of invasive species all play roles. Managing these challenges has become increasingly difficult as their effects overlap and intensify. Pollution is not the sole factor impacting fish populations. Natural predators also exert considerable pressure. Among the most significant predators in Slovenia's inland waters is the Eurasian otter (Lutra lutra). Its population has been successfully increasing due to improved water quality and long-term conservation efforts. The otter’s impact must be considered alongside other factors influencing fish populations. As a highly adaptable predator, the otter primarily feeds on fish, which constitute more than 80% of its diet. Its prey selection depends largely on the availability of fish in specific watercourses. The successful return of the Eurasian otter represents one of the most significant nature conservation success stories of recent decades. Due to improved water quality and sustained protection measures, the otter has returned to more than 70% of Slovenia's territory, with its range continuing to expand. Data from fisheries management agencies show a clear trend of species expansion, confirmed by national monitoring results. This resurgence raises new questions about how to balance conservation goals with the preservation of native fish species. Over 80% of the Eurasian otter's diet consists of fish. An adult otter consumes approximately one kilogram of food daily, translating to around 300 kilograms of fish annually. The otter typically does not target the largest fish, preferring smaller and medium-sized individuals, as this hunting strategy is more energy-efficient. Given its rapid metabolism, the otter must hunt almost daily, making the availability of fish crucial for its survival. Smaller salmonid watercourses serve as vital habitats for native fish species while also being suitable environments for the Eurasian otter. The otter's diet adapts to the availability of prey, meaning it can impose additional pressure on fish populations in areas with small or already stressed fish communities. This effect is most pronounced in smaller salmonid watercourses, fish farms, and regions with endangered or isolated fish populations, such as the Soča grayling. Here, predatory pressure combines with other stressors, including habitat loss, climate change, and watercourse regulations. Similar challenges are observed throughout Europe. Researchers across the continent are paying increasing attention to monitoring the impact of otters on sensitive salmonid watercourses and fish farms. In the Czech Republic, for example, the damage caused by otters in fish farms has raised concerns. Efforts are underway to better understand and manage these interactions, ensuring both the conservation of otters and the sustainability of fish populations.