Brown Trout and the Chalgrove Brook

December 7, 2021

The Chalgrove Brook, one of the few chalk streams in the Thame catchment, with a newly installed berm on the left

The brown trout (Salmo trutta) is an iconic native trout species with its distinctive spotted patterns and unique reproductive lifecycle. One of the most genetically diverse species in the world, they live in a range of habitats from deep lochs to small chalk streams. They are closely related to the Atlantic Salmon (Salmo salar), sometimes hybridizing with them. However, unlike salmon, brown trout do not require a marine phase for their lifecycle. They do require clean, cold, highly-oxygenated freshwater and gravel river beds for laying their eggs. Due to these specific habitat requirements, the presence of brown trout can indicate good stream health. However, this also makes them sensitive to pollution incidents, disturbance, and habitat destruction.

Brown trout eggs, which take about 6 weeks to develop in southern chalk streams. The baby trout, called alevins, will then hatch and live off their yolk sac for another 6 weeks. (Photo courtesy of The Wild Trout Trust)

In the Thame Catchment, there is currently only one known waterbody with a population of brown trout, the Chalgrove Brook. Read our previous article on protecting chalk streams, like the Chalgrove Brook, here. While the presence of trout in the Thame Catchment is exciting, these populations are vulnerable to a number of threats. For one, prime habitat is scarce in the Chalgrove. Along many stretches, it was historically rerouted from its natural channel to make room for agriculture and power mills. Changing the course of a chalk stream often lengthens the distance the water must travel resulting in slower and less powerful flows. This allows for sediment in the water to settle out on the stream beds which buries the vitally important gravel beds needed for trout spawning. Slow flows also increase the temperature of the water which in turn decreases oxygen levels. These impacts are bad news for salmonid fish, like brown trout, who cannot survive in temperatures above 22ºC and require highly-oxygenated water.

The clear patch of gravel seen in this photo is an example of a redd, created by a female trout as a place to lay her eggs. Trout will only spawn in locations with raised gravel and accelerated flow that will keep silt from building up and smothering the eggs. (Photo courtesy of The Wild Trout Trust)

Due to their unique multi-staged lifecycle, brown trout populations require more than just good gravel beds to survive. Young trout need slower flowing refuges with ample hiding places to avoid predators. Adult trout, while strong enough to handle fast flow, prefer to preserve their energy and often establish feeding territories in areas of slow current next to a fast current that will carry their insect food to them like a conveyor belt. These diverse habitat requirements mean that many of our modified and degraded chalk streams, with their straight channels and homogenous riverbeds, provide very little space for brown trout to thrive. This is why RTCT has been building in-stream structures that will take these wide, shallow, slow-flowing stretches of the Chalgrove brook and narrow them to create diversity in the channel. These structures, called berms and deflectors, will not only speed up the water and scour sediment off the gravel beds but because they are built from natural woody material the structures themselves will provide hiding places for trout.

We have had nearly 50 different volunteers from Thames Water come out to work in the Chalgrove in October and November. While working instream, volunteers learned about the rarity and importance of chalk streams and did kick-sampling to explore the aquatic life that the project is benefitting.

In just two short months, we have installed 10 berms and 12 deflectors with the help of many hardworking volunteers, and funding from the Environment Agency and S.O.D.C Cllr Turner. We have improved nearly 200 meters of brook this autumn and will eventually add improvements to another 200 meters of the channel. However, we have paused all in-stream work in the Chalgrove for the winter to avoid disturbing trout during their spawning period. While it is unlikely that trout are currently spawning in the silted stretch of the Chalgrove we are working on, working in the stream kicks up sediment that could affect spawning gravels downstream. All of us at RTCT, along with our dedicated volunteers, look forward to the spring when we will resume this project.

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