In October 2016, a collaborative project between the South East Rivers Trust, Kent High Weald Partnership and the Environment Agency successfully removed three redundant weirs from a 1.5 km stretch of the Lesser Teise. The objectives of the project were to improve fish passage (their ability to migrate), remove the ‘canal like’ impoundments created by the weirs, and restore the natural processes like erosion and deposition, all of which are important for a healthy river. You can read the blog here!
This video clip was taken at the location of one of the weirs in May 2018, and shows chub spawning on the rehabilitated gravels.
Coarse fish ‘gravel spawners’
Dace, chub and barbel are classed as ‘Rheophilic’, meaning they prefer flowing water to spawn, and are usually found in fast-flowing, well-oxygenated stretches of river where the river bed is characterised by sand and/or gravel. Coarse fish, such as dace, chub and barbel, are known to undertake spawning migrations, triggered by increases in water temperature, to spawning habitats in the form of gravel riffles.
For example, a multi-year radio tracking study of chub in the River Spree (Germany) showed that chub migrated and spawned twice each year, starting their first spawning migration in late May in both years of the study. They arrived at three different spawning grounds within a day and left after 1 to 6 days to return to their original sites. In mid-June chub started their second spawning migration, with the same pattern as the first. Chub mostly moved between 1 and 13 km upstream to the nearest spawning ground. Some exceptions were observed: a few chub did not use the nearest spawning grounds, some chub used different spawning grounds at first and second spawning, or moved downstream to spawning grounds. Spawning grounds were characterized by 0·4 m/s current velocity, shallow depth (0·1–0·8 m) and a stony bottom, where eggs adhere. But one message is clear, the chub needed to migrate within the river to spawn.
Nutrients and spawning gravels – a rolling stone gathers no moss
In the last few decades, nutrient levels in our rivers have reached high levels, fuelling algal and diatom growth, both of which have the potential to degrade spawning gravels and therefore reduce spawning success of both coarse and game fish. Like with salmonids (salmon and trout), the quality of spawning substrate can have a large influence on the survival of coarse eggs and any resulting larvae/fry. Fouling of spawning gravels with silt, algal and diatom growth can reduce the water flowing past the eggs and thus limit the essential oxygen being delivered.
Coarse fish will be more susceptible to the fouling of spawning gravels by algal and diatom growth because they spawn in a season when the nutrient levels are high. By contrast, trout and salmon, spawn in the wintertime, so will be less affected.
The natural processes of geomorphology and sediment transport within a river can have a positive influence on the quality of spawning gravels. Gravel riffles that are ‘active’ geomorphologically are in a constant state of flux, which can reduce the build up of algal and diatom growth, improving the survival of fish eggs.
Weir removal enhances geomorphology
Rivers are naturally dynamic systems with erosion and deposition occurring in balance and can shift their position quite dramatically during a flood event. However humans have been impounding rivers with weirs for centuries to facilitate human society and ‘control’ rivers, so they don’t go and do something unexpected! Unfortunately for fish, it is this very dynamic nature of a river that creates the habitat feature fish have evolved to exploit.
A dynamic river channel will sort eroded sediment (e.g. gravel) into riffles, and the newest, freshest riffles will have the least silt deposited within the spaces between the grains of gravel, and will therefore be the most productive spawning sites. Fish can sense this, and will actively seek out the best quality spawning sites to maximise the survival of their eggs and potential offspring.
This is why, weir removal is a common first step in river restoration. Returning impounded sections to free flowing active river channels allows the river to start creating these high quality habitats once again, bringing benefits to freshwater fish and other aquatic wildlife, as well as freeing migratory pathways for fish of many different species.