Partial nomadism in large‐bodied bream (Abramis brama)

The recent focus on a common movement ecology paradigm has increased awareness about the importance of distinguishing different movement patterns for understanding their specific impact on animal ecology and evolution. One specific form of movement is nomadism, where animals move between habitats in an arrhythmic fashion. Although not yet described in detail for freshwater fish, nomadism between lakes within drainage systems could affect local lake food webs in an otherwise unpredictable fashion. In this study, we used passive telemetry to describe inter‐lake movements of 1,280 individually tagged adult benthivorous bream (Abramis brama) over nine contiguous years in a drainage system with two paired lakes. The movements can best be described as partial nomadism, where a part of the population is neither resident nor migratory, but instead moves between lakes in a variable, arrhythmic fashion, often with long residency in one lake before moving to the other. We found extensive between‐year variation in the proportion and direction of bream moving, with higher movement propensity being partly associated with lower somatic condition. Notably, movements out of lakes were significantly more in direction of the neighbouring lake than away from the paired lakes, suggesting that at least direction was not random. Since bream are important ecosystem engineers, fluctuating densities of adult bream could affect ecological stability in lakes. Our study suggests that fish populations in interconnected lakes within drainages should not be treated as isolated units, and we discuss this in relation to lake ecology and the importance for lake ecosystem management