Impacts of low flows on salmonid river ecosystems

In the first of a new series from students actively involved in research relevant to wild trout, Jessica Picken from Queen Mary University of London summarises the aims of her PhD working with the Game & Wildlife Conservation Trust and with CEFAS.

Climate change is considered to be the most critical disturbance imposed on natural systems on a global scale. Climate models predict that average temperatures in the UK will increase over the course of the next 50 years with the greatest warming in the south of England during summer months, and that annual average precipitation will reduce. However, the reduction in precipitation is expected to be more pronounced during summer than winter, whereas extreme winter precipitation is expected to become more frequent. In other words, there is likely to be an overall shift towards drier summers but wetter winters.

River itchen jess pickens qmul gwct cefasFlow regime is widely recognised to be the master variable controlling a number of key riverine processes, including levels of dissolved oxygen, sediment transport and deposition, water quality, and habitat type and distribution. These processes in turn influence the spatial and temporal distribution and abundance of critters! Chaotic variability in river discharge (due to climate variations) can adversely affect migratory and resident salmonids at different stages of their life-history. However, in the relatively more stable chalk streams of southern England, drought and associated low river discharges (often exacerbated by human influences such as abstraction, flow regulation and land use), are thought to be the biggest threat. In the future, chalk streams are likely to suffer from periods of intense summer drought more frequently, with potentially disastrous implication for their fish populations.

We are studying three carrier streams of the River Itchen where complete control of the flow is achieved by altering sluice gates at the top of each stream. During the summer, the sluice gates will be lowered in order to experimentally induce low flow conditions. Each stream will experience a four week low flow treatment, reduced by 90%, 45%, or not altered (as a control). The following summer, the individual streams will be subject to a different flow treatment so that at the end of the three years, each stream will have experienced all the different flow treatments.

Samples of the riverflies and other invertebrates  will be collected from each stream throughout the year to enable any response in fish food to be identified, in terms of species composition and density. Salmonid stomachs will be flushed to determine their diet, and to enable the construction and assessment of food webs in each stream under each treatment. It is possible that reduced flow may result in the loss of certain habitat types that are required by some invertebrates (e.g. decreases in stoneflies may occur where fast flowing habitats are lost) which, in turn, will affect salmonid diet as they respond to the change in prey availability by becoming increasing reliant on alternative food sources. We may see a shift in stream food web dynamics under different low flow conditions.

The movement and behaviour of salmonids may also change in response to reduced food availability, with fish increasing their foraging range to seek out food of a better quality or more suitable instream habitat. However, salmonid movement may be restricted due to the reduced amount of available habitat during the low flow period which could cause and increase in foraging competition due to fish density increases. To help me tease all this apart, I will monitor behavioural responses using Passive Integrated Transponder (PIT) tags inserted into individual fish to monitor habitat choices of salmonids before, during and after flow reduction in all three streams.

Results from this three year project will increase our understanding of the impact of short term summer drought events, which is vital to safeguard salmonid populations in these streams and to define future catchment and water resource management strategies for the south of England.

If you have any questions or comments, I would love to hear from you, and I hope to reappear on these pages to update you as the project unfolds.

Jessica (, or @jesspicken)