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

Abstract

1. Freshwater ecosystems are increasingly characterised by high levels of fragmentation that restrict the movements of mobile fauna. Yet studies also suggest the migratory behaviours of potamodromous fishes can be highly variable in barrier-free systems, where differing migratory behaviours enable populations to exploit a wide range of food and space resources. This intra-population divergence in spatial and temporal resource use is important to our ecological understanding of distribution patterns and population structure. 2. Common bream Abramis brama (‘bream’) is a potentially strong model species for testing the importance of divergent 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 migration 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 (~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. 3. 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 movements. Rates of movement and swimming speeds were highest in spring, with movements also generally occurring in the direction of tidal flows. 4. For fish tagged just prior to spawning, there was considerable diversity in their post- spawning movements, with some remaining in the immediate vicinity and others that moved to areas ~ 25 km away. These spatially discrete patterns remained until the following spawning period, with high individual consistency in movement behaviour between years. 5. These results suggest 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 migration behaviours of potamodromous fishes.