Restoring our Rivers: Removing Barriers to Fish Migration

Earlier this week, I attended an online talk from Catie Gutmann Roberts, Bournemouth University. Catie’s research looked at the movement and diet of barbel on a stretch of the Severn/Teme which was impacted by barriers to flow.

Barbel are not native to the western flowing rivers of the UK and don’t tolerate salt water so hadn’t got round to the west coast. In the 1950s the Rivers Authority at the time brought barbel over from the River Kennet to the River Severn for angling opportunities (moved about 500 barbel). The barbel settled, moved naturally upstream in one of the tributaries of the Severn, the River Teme, and are now a recreationally important species.

The barriers in the study are the ‘Powick Weir’ which was installed in 1475 as a channel was dug to divert water to the Mill to grind flour to make bread. It became a hydropower plant in the 19^th century but was decommissioned in the 1920s. The 2^nd barrier is ‘Diglis Weir & Lock’ which was installed in 1844 to help boats navigate the river and deliver fuel to cities such as Worcester.

Weirs and dams are an international issue for migratory fish but Catie pointed out there is a lack of evidence of how barriers affect non-salmonids. Barriers to barbel migration might be structurally specific, so Catie investigated how non-native fish respond to restoration methods. Her research questions included whether the 2 weirs impact barbel equally, and how fish move upstream following weir removal.

Barbel are a lithophilic cyprinid, so they like gravel environments to spawn and stay within freshwater. They migrate upstream to get to ideal spawning gravel. Barbel are not a predatory fish, and their diet includes small fish that move from marine environments to freshwater environments to spawn, as well as those that live in rivers but move to marine environments to feed i.e. small elvers. As well as this they also feed on marine meal pellets which are used by anglers to attract fish. They have a downturned mouth specialised for bottom feeding.

Catie’s study used electrofishing to survey fish. 22 barbel were caught in the River Teme over a 24 hour period. Stable isotope analysis (SIA) was carried out involving removing a couple of scales from the fish. This helps indicate what type of food they’ve eaten, based on the ratios of carbon and nitrogen in the tissue in the scales. This was compared to natural local food sources and marine fishing pellets. Fish were tagged with a Vemco acoustic transmitter, to identify each fish uniquely. Fish movements were recorded on in-river receivers to measure where and when fish were mobile. Anomalies might occur when fish have been eaten by predators, or taken out by illegal angling.

Results showed variable movement without a clear pattern. Similarly, using the SIA to determine their diet, results did not show a clear correlation between their movement length and diet. Angler-caught fish and electro-caught fish were compared. Angler-caught had more pellets in their diet but no pattern on whether this can have a correlation on their movement.

From the 22 barbel tagged, and 18 with more extensive movement monitoring, about half of the fish which approached the Powick weir were able to ascend upstream, however none of the 6 fish which approached the concrete Diglis weir, were able to ascend. This shows Powick weir was a partial barrier to movement, and Diglis was a full barrier to barbel migration.

Catie went on to investigate the environmental conditions that might allow fish to ascend the weir. This included the impacts of river flow level, water temperature and day length (season/time of year). Fish like to lay eggs in waters of approximately 13⁰C. Only 2 of the 3 fish that moved upstream were tracked on their way down. 1 went over the top of the weir in winter high flow levels when the weir wasn’t visible, and the other appeared to use the mill leat to move downstream.

The weir was partially removed in 2017. A further 20 fish were tagged to see how this restoration would impact their movement. Fewer fish were recorded as passing the weir, however they made multiple ascents and descents indicating the upstream reaches might be better for spawning or feeding, and therefore the barbel incorporated this further migration into their home range of movement.

Catie investigated how flow impacts the ascent of the weirs. In 2015 more fish approached the weirs but did not ascend. Partial removal of the weir allowed barbel to move upstream in lower flows at Powick.

Check out this link to an interactive map for fish movement on this section of the Teme, looking at changes before/after river restoration. Play the animation.

Removing weirs also allows sediment to move more naturally in the river. Before the weir was removed, drone mapping and ground surveys were used to map the habitat. This can help identify spawning habitats for a range of species.

Diglis Weir (River Severn) is now undergoing a river restoration project to install a fish pass and create passage for a range of fish species. This project incorporates the UK’s first underwater fish viewing gallery! This is a great step in opening up this habitat and opening up the opportunity for anyone to have a look at fish as they are migrating.

To further this barbel investigation, Catie’s looking into if there are any differences in the sex of the fish that are moving. Catie is using ultrasound black & white imagery to investigate this!

Thanks to Catie at Bournemouth University and all those involved in running this seminar. Great to see what work has taken place to investigate our freshwater environments.