MetaboRain: Diet and Metabolism in Trout Naturalization

Rivers and streams across Europe are increasingly under pressure from invasive fish that threaten native species and aquatic ecosystems. One of the most widespread and controversial examples is the rainbow trout, a fish originally from the Pacific coast of North America. Brought to Europe over a century ago for aquaculture and recreational fishing, rainbow trout now live in the wild across much of the continent. In recent years, self-reproducing populations have begun to spread rapidly in Central Europe, raising serious concerns about their impact on native fish such as the brown trout. This project seeks to understand why some rainbow trout populations are able to adapt, reproduce, and thrive in European rivers, while others fail to establish themselves. The answer may lie in the way their physiology and behaviour interact with changes in food quality and temperature. In nature, fish must rely on wild prey rather than the rich, pellet-based diets provided in fish farms. Many of the essential nutrients found in hatchery feed (particularly omega-3 fatty acids such as DHA) are scarcer in natural environments, especially during colder warmer seasons when the diet of fish depends more on terrestrial insects and leaf litter. These nutrients are crucial for brain development, energy metabolism, and behaviour, and may influence how well trout can hunt, avoid predators, and compete for resources. The project combines field studies in Austrian streams with controlled laboratory experiments to explore how rainbow trout cope with natural diets and changing temperatures. Research will examine differences between hatchery-reared and wild-born fish in their energy metabolism, growth, and behaviour. They will also investigate how climate change, by warming alpine rivers and reducing the availability of omega-3 fatty acids, affects the physiological performance and resilience of trout. By studying the energy balance and metabolic efficiency of these fish, the project aims to uncover the mechanisms that determine whether rainbow trout populations can successfully reproduce and spread in new environments. Understanding these processes is essential not only for protecting native biodiversity but also for anticipating how climate change and human activities may reshape aquatic ecosystems in the future. The findings will help us to better predict and manage invasiveness of rainbow trout and potentially other non-native species. Ultimately, this research contributes to a broader understanding of how living organisms respond to rapid environmental change, providing science-based knowledge that can inform sustainable conservation and fisheries management strategies across Europe.