The 2024 survey produced exceptional results, with near-record numbers seen across almost the entire surveyable main-stem Wye, and on the Usk we had by far the best year since our comprehensive electrofishing program began in 2016. The numbers blow any results seen in the last two decades out the water.
The surveys took place over 150 sites across the Wye and Usk, concentrating on juvenile Salmon and Trout (salmonid) populations. It covered an area of around 100km in 3 directions from Talgarth in Powys.
The process involves a series of 5-minute riffle surveys, where a low electrical current is passed over an area and the temporarily immobilized fish are caught, weighed , measured, recorded and returned. This type of survey not only improves understanding of salmonid spawning success trends, but significantly informs the direction of our future work to try to improve populations.
Main stem Wye
- Exceptional results.
- Near-record numbers seen across almost entire surveyable main-stem Wye.
- Blows any results seen in the last two decades out the water.
Major Wye Tributaries
Largely followed the same pattern as the mainstem Wye.
- Irfon, Ithon, Elan, Edw were all fantastic.
- Lugg, Arrow and Monnow were not good at all
Usk tributaries upstream from Brecon
By far the best year since our comprehensive electrofishing program began in 2016.
•Fry numbers bounced back to comparable levels to 2010-2015.
•Stand-out tributaries were the ones we’d expect: Crai, Senni, Cileini, Bran, Yscir.
Before 2016 numbers in both the Wye and the Usk were good but there was a catastrophic failure in 2016 and after that the numbers dropped dramatically and stayed mostly between 8 & 10. By contrast in 2024 the average count was 33!
2024 Average Salmon Fry Catch
– tributaries above Brecon vs Brecon-Crickhowell
Brecon 44 Crickhowell 1
Wye & Usk 2024 – Overall Patterns
We saw similar patterns on both the Wye and Usk.
Many parts of both catchments were fantastic, bouncing back to near-record levels of Salmon fry. However, both catchments had notable areas which did not show that same response: the Lugg, Arrow, Monnow on the Wye, and Middle/lower Usk and its tributaries.
Because we’ve conducted extensive Usk catchment scale monitoring we have a plethora of information to contextualize these results, and due to the consistency in patterns between both catchments, we can apply conclusions from Usk to the Wye.
Why was the bad, bad?
Likely case-by-case
Disappointing numbers recorded on Usk tributaries occurred at different times between 2010 and 2020. Does this point to different reasons for that drop-off? We believe we have enough data to start to paint a picture.
- Honddu – Citizen Science indicates notable ammonia and and soil run-off (turbidity) spikes. Plus issues with migratory barriers.
- Rhiangoll – Citizen Science and sonde data both indicate very high winter ammonia and turbidity spikes.
- Grwyne – Unidentified migratory barrier (now removed). Citizen scientist identified extreme ammonia spikes out of the Grwyne Fechan.
On the Main-stem of the Usk the numbers drop off where these tributaries flow in.
Why was the good, good?
More 2023 adult Salmon than we thought
As unlikely as it seems, more salmon must have run these rivers than the Usk and Wye rod catch numbers (of 51 and 238 respectively) suggest.
If we use an 8% catch rate, then we’re looking at 638 Usk and 2,975 Wye salmon. Those numbers simply cannot have produced the widespread, high numbers of 2024 salmon fry.
In addition the Usk fish counter, trialled for two weeks in both June and October, counted 414 salmon entering the river in just those four weeks. So 65% of the total salmon number suggested by rod catch to have run during less than 15-20% of the period we know salmon run the river, with good water levels throughout, is extremely unlikely.
- 2021 had good spring flows leading to 84% tagged wild smolt survival to tide (vs 24% in 2022 with poor spring flows). These smolts return as 2 sea winters fish. This class is the dominant part of both the wye and the Usk Salmon populations. The 2024 fry would be their progeny.
Excellent egg-fry survival
We have seen decent spring flows/smolt survival in some other recent years but they have not produced the same fry numbers as this year. This points to high survival from eggs last winter to fry this summer. To have this many sub-catchments across two separate rivers affected there has to be a shared major factor. That common element is likely to be the weather conditions from Autumn 2023 to Spring 2024.
So was it the temperature or the rain?
We tested degree day theory using sonde data and citizen science temperature data. Could it have been cooler winter/lower water temperatures? There is no evidence to back that up, in fact the winter of 23/24 winter was warmer than usual.
We tested winter rainfall, using historic rainfall and river level data. So was there less extreme rainfall last winter? No, but despite even higher than average total winter rainfall, the rainfall was exceptionally soft. There were no extreme or heavy rainfall events which means soil-runoff (turbidity) was significantly reduced. We know that turbidity damages gills, compacts gravel and covers redds.
Our citizen science data also backed this up; areas without major turbidity spikes had great fry numbers. Areas with high winter turbidity remained poor.
We need to get to a place where:
- Smolt survival doesn’t drop from 84% to 24% when there is low spring rainfall.
- Catchments are resilient enough that, even with more extreme rainfall, turbidity levels remain under control.
- We need the combination of perfect spring flows and exceptionally soft winter rainfall to re-create the conditions which gave us such exceptional results this year.
- …and this is what we are doing!
See the full data set used in our 2024 annual meeting presentation here
