Movements of common bream Abramis brama in a highly connected, lowland wetland reveal sub‐populations with diverse migration strategies

Abstract Studies suggest the migratory behaviours of potamodromous fishes can be highly variable in barrier‐free systems, where differing movement types enable populations to exploit a wide range of food and space resources. This intra‐population diversity in spatial and temporal resource use is important to our ecological understanding of fish distribution patterns and population structure. Despite this, freshwater ecosystems are increasingly characterised by high levels of fragmentation and degradation that restrict mobile fauna, and limit opportunities to study natural, unconstrained movement behaviour. Common bream Abramis brama ( bream ) is a potentially strong model species for testing the importance of diverse migration patterns in lowland rivers, but existing studies have been largely restricted to spatially confined and/or anthropogenically modified systems. This study's principal focus was to examine the diversity of bream movement behaviour in a highly connected, lowland system using passive acoustic telemetry, which provided continuous, multi‐year data on the movements of 181 bream across a tidally influenced, lowland wetland in eastern England ( c . 60 km of continuous river length plus numerous interconnected shallow lakes and dykes). Tracked bream were grouped according to their initial location and timing of tagging. Bream migratory behaviours varied considerably between tagging groups, but with greater consistency within groups. There was little mixing of groups outside of spawning periods, with season and tidal phase being significant predictors of movement. Rates of movement and swimming speeds were highest in spring, with movements also generally occurring in the direction of tidal flows. For fish sampled just prior to spawning, there was considerable diversity in their post‐spawning behaviour, with some remaining in the immediate vicinity of the sampling location and others that moved to areas c . 25 km away. These spatially discrete patterns remained until the following spawning period. These results suggest that this lowland fish population is comprised of several distinct, semi‐independent subpopulations that only share space resources in their spawning period. This indicates the importance of connectivity in lowland freshwater systems for enabling and maintaining high phenotypic diversity in the movement behaviours of potamodromous fishes.