Behaviour of fishes around engineered structures and in modified rivers

Abstract

Migratory freshwater fish populations have declined 76% over the last 40 years. One of the major contributors to this decline is the fragmentation of rivers by cross-channel structures like weirs and dams. These structures do not only alter riverine habitat by deepening and slowing the velocity of water upstream of the structure, which can result in changes to aquatic animal assemblages, but they also act as barriers to animal movement. The impact of weirs with associated fishways (commonly installed to mitigate against barriers to movement) on the upstream migratory behaviour of freshwater fishes was quantified in this thesis using telemetry techniques.

Weirs were found to cause significant delay, with passage often being significantly related to river stage where most passage occurred when the weirs were fully submerged. Fish were observed travelling back downstream after encountering a weir, increasing their energy expenditure and likely reducing their reproductive fitness as a result. Delays to migration and increased energy expenditure may become exacerbated in rivers which are heavily fragmented.

Experiments carried out in this thesis identified that weirs may select for larger individuals that have a greater probability of passage success. Fishways theoretically alleviate selective pressure, and fishway designs were shown to enable passage of smaller fish. However, fishways may continue to act as selective filters for phenotypic trait as a result of poor attraction efficiency due to fishway placement (e.g. entrance built adjacent to the riverbank, not the area of greatest discharge) and low attraction flow. This highlights the importance of considering where to place fishway entrances. Furthermore, certain behavioural traits may be selected for by fishways.

Passage behaviours and success at fishways varies between individuals, and may be partially driven by differences in individual behavioural traits. Bolder and more active individuals were found to have an increased chance of passage success, and were observed to make fewer passage attempts. Given the high cost of installing fishways, it is important that they function to their best ability in order to mitigate the effects of weirs and accommodate all behavioural and phenotypic traits.