| Summary: | Anthropogenic influences on riverine ecosystems have led to numerous impacts that have compromised fish communities. Large European lowland rivers have been amongst the most impacted aquatic environments globally. However, paradoxically, these environments have also been amongst the least studied, due the inherent difficulties of studying fish in large, turbid, dynamic environments. Common bream Abramis brama (L.) populations are amongst those impacted by reduced lateral and longitudinal connectivity that are a consequence of modifications to large lowland rivers in Europe. Thus common bream in the lower River Witham, Lincolnshire, UK, were selected as a case study to examine how fish utilize these river systems. A literature review was carried out to assess the influence of human intervention on riverine ecosystems and their impacts on fish communities. In addition the use of telemetry techniques for studying fish populations was reviewed. An ecological assessment of the lower River Witham was carried out using data available from The Environment Agency monitoring and hydrological systems and data specifically collected during this study. The history of this highly modified lowland river was reviewed to give perspective, timescale and context to the degree of modification that has been undertaken on the river and its consequences. Chemical and biological water quality indicators showed improvements in the last twenty years. Fisheries data identified three species that had become locally extirpated in the lower river since the 1800s. The river is now roach dominated, but common bream are still present, possibly due to available lateral spawning habitats that provide surrogates for, and are functionally similar to, the lentic floodplain waterbodies of natural riverine ecosystems. Hydroacoustic surveys show that fish communities are aggregated and favour the upstream half of the lower river where the channel is more heterogeneous. Recent changes in river character have made traditional fishery assessments by seine netting less effective. Data collected during fish tracking studies are most valuable when the tagged fish are behaving naturally, thus the effects of tag attachment should not impact on the behaviour or well being of the fish. Acoustic telemetry was used to investigate the impacts of tagging, surgical inter-peritoneal implantation, and translocation of common bream. Fish were recaptured following surgery and the incision site photographed to assess healing, which was clean in all cases. All recaptured fish appeared to be behaving normally as they were part of large shoals of fish. On five separate occasions fish were tagged when other previously tagged fish were present allowing their behaviour to be compared in terms of the distances moved and linear range; there were no significant differences between the two groups indicating the tagging procedure to have no detectable impact on the short-term behaviour of the fish at the resolution of the tracking undertaken. Three recaptured fish were translocated ~35 km downstream; these fish appeared to exhibit homing behaviour, returning to the capture site between 6-24 days. Their level of activity (in terms of distance moved) did not significantly differ from non-translocated fish, but their linear range was larger. The distribution and habitat use of adult common bream was assessed by tracking their longitudinal and lateral movements over forty-three months, producing a dataset of over 3.1 million detections. A positive relationship between activity and temperature was revealed, with common bream moving greater cumulative distances during the warmer months and occupying a greater longitudinal proportion of the main channel. The occupancy of tributaries was related to temperature, with common bream entering shallow tributaries during rising temperatures in the spring, whilst a deeper, slow-flowing tributary was used more frequently during the cooler autumn/winter months. During the autumn, occupancy of this deeper tributary was positively related to flow rate, suggesting that common bream use this as a refuge from high main channel flow. The home range concept is at the centre of theoretical models to explain the spatio-temporal behaviour of a wide range of animals including fish. There are lots of different ways of expressing home ranges, including areas, but since rivers are principally linear systems, most range is reflected linearly. Home ranges were calculated for 100% and 90% of locations. Seasonal differences were identified with fish occupying larger home ranges during the spring and summer opposed to autumn and winter. Artificial water level manipulations, management actions for the control of flood risk, affected home range size with fish occupying smaller ranges at reduced winter levels than at increased summer levels. Analysis of when and how often individuals within a population interact with one another provides a method to study the social organisation of animals with the potential to reveal ecologically significant aspects that would otherwise have remained hidden. Network analysis and social network theory were used to examine the social interactions of the tracked fish. The extent of sociality appears to follow a normal distribution, such that there was no evidence of discrete classes of social/non-social fish. Fish were more social immediately after tagging and less social as time progressed, as such little evidence could be found to support the hypothesis that these groups of fish remained in each others’ company for extended periods. However, more active individuals were found to be more socially connected, but home-range size did not significantly affect sociality. The functions of these behaviours are discussed along with potential management and rehabilitation strategies for the lower River Witham and other heavily modified lowland rivers in order to meet good ecological potential/status under the Water Framework Directive (2000/60/EEC).
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