Strategic Exe Weirs Project Officer, Tom Watts talks about the importance clean gravels in our rivers plays in the survival of our fish species.
As the leaves fall from the trees and colder, longer nights set in, our headwater streams and rivers are brought alive with the start of new life; the spawning of trout and salmon.
At this time of year, the frigid waters of our rivers are filled with Brown Trout (Salmo trutta) and Atlantic Salmon (Salmo salar). The females are busy finding nest sites also known as “Redds” whilst the males compete for the chance to spawn, displaying protruding mouths or “Kypes”.
Salmonid eggs need cold, clean and well oxygenated water to successfully hatch. Laying eggs at this time of year helps to ensures that they will hatch during the spring, giving the juveniles the best chance to feed and grow before the coming winter. Sadly, changes to the climate and the impact this is having upon our rivers, is limiting viable spawning conditions. This along with pollution events are drastically reducing the survival of eggs over the winter period.
Redds are found in areas with abundant and free moving gravels. Female fish use their tails to create a shallow depression in the substrate, where they deposit their eggs. The male then fertilizes the eggs within this nest. To protect the eggs, the female moves slightly upstream and once again uses her tail to sweep gravel over the redds, shielding the eggs from predators and preventing them from being swept away by the current.
Sadly, for salmonids in many of our West Country rivers, good quality spawning habitat is hard to find. For a redd to function effectively, the gravel must be clean and free of silt. Cool, good quality water must be able to flow through the gravel and eggs, delivering oxygen to the developing eggs and flushing the nest free of waste and toxins. Increasingly, these parameters are being lost due to modifications in sediment transport and water flow. Many of the suitable gravels have been washed downstream or are impounded behind weirs. What little areas of gravel that do remain are often smothered by silt input into the river through surface water runoff.
Gravel Cleaning earlier in the year is one way we can help spawning salmonids. By using hand tools to turn the riverbed, we can free gravel of fine sediment and ensure it moves easily. This increases water flow through the substrate allowing oxygenated water to reach the developing eggs when they are laid later in the year.
Salmon Fry on the river bed
Luckily for our trout and salmon, there are areas of good quality spawning habitat still present, namely in the upper reaches of our rivers, such as in the headwaters of rivers such as the Barle on Exmoor.
By surveying these areas and observing redds we can aim to assess the amount of spawning occurring in our headwater streams and rivers.
One group trying to do just this is the River Exe and Tributaries Association (RETA), joined by wider river community groups such as Tidelines and volunteers from across the Exe catchment. These passionate river groups, set out during the winter to cover eight spawning rivers and tributaries across the Upper Exe Catchment.
When a redd is spotted volunteers note date, time and location, this is collated and then inputted into the Strategic Exe Weirs GIS mapping database. This is a collaborative monitoring platform that uses geospatial data to inform habitat works throughout the Exe catchment.
Alistair Langford, a RETA member, said “The salmon don’t keep to a timetable, the areas they prefer are often remote and increasing rainfall events have meant that often the water was too coloured to see anything.
Nevertheless, it’s a task worth undertaking because when we do find evidence it adds to the bigger picture of what is going on with our rivers.”
Thanks to these dedicate citizen scientists and conservationists, we hope that we can continue to monitor spawning in the Exe catchment and continue to track changes and success to monitor the return of these incredible fish.