I WROTE about Salmon leaping the falls at Stainforth Force on the river Ribble. That made me wonder about migrating Salmonids (Salmon and Trout) in the river Wharfe. Apparently, increasing numbers of Salmon have been entering the river Ouse and, from there, some make their way into the lower reaches of the Wharfe. Unfortunately, their journey upstream in the Wharfe is limited by weirs that are difficult or impossible to pass. Such weirs also render fish more susceptible to predation, make the downstream progression of juvenile fish more difficult, and isolate fish populations, increasing the likelihood of genetic issues (see https://www.wildtrout.org/wttblog/why-presume-remove-weirs-river-dove-case-study).

As well as being good for fish, good for anglers, and an exciting spectacle, Salmonid migration along a river brings other ecological benefits. Nutrients are taken upstream, and food is provided for other species. Although it should be noted that some argue river barriers can provide a defence against the spread of invasive species.

There have been reports of incoming Salmon attempting the weir at Otley (this video from Nov. 2015 provides a possible example: www.youtube.com/watch?v=lspy9J9vG6s). Since then (in 2016), as part of conditions associated with building a hydroelectric turbine adjacent to this weir, a Larinier fish pass and a Baulk fish pass have been installed to assist migration (see https://www.youtube.com/watch?v=4k8aKy1y5iw). Larinier fish passes have baffles in the lower section, to slow the flow of water and encourage fish to make the ascent. Baulk fish passes are simpler and diagonally ascend the downstream face of a weir. Unfortunately, the passes at Otley are not monitored, so there are no records of the frequency with which they are used (https://www.wildtrout.org/assets/reports/Wharfe_OtleyAC_AV.pdf). Of course, their presence also means the chances of observing Salmon attempting to leap/swim up the weir are less.

Any fish that successfully navigates Otley weir next faces the weir at Burley. The series of substantial steps incorporated into this structure are thought to be impassable. When the flow of water is strong enough to encourage upstream movement, there is a standing wave at the bottom to help fish jump, but, above that, there is limited respite and no assistance. I spent some hours watching, last October (2022), and saw several fish attempt the weir (see photograph). On each occasion I think they managed just one leap (nowhere near sufficient to get past it), after which most were quickly washed back down to their starting point.

If a fish pass could also be installed at Burley weir and, in the longer term, the Environment Agency are looking at this, perhaps we would be able to watch migrating Salmon leap the small step beneath the Old Bridge in Ilkley!