Implications of High Water Temperatures on Salmon and Trout Fisheries – A Scientific Perspective
Increasing water temperatures are having a serious effect on the ecology of the Wye and Usk and the fishery they support. As global temperatures inexorably rise, unless we change how we are managing our catchments to mitigate rather than exacerbate climate change the effects will become ever more pronounced and serious.
- Physiological Stress on Fish: Salmon and trout exhibit physiological stress responses when exposed to high water temperatures. As water warms it holds less dissolved oxygen, increasing the impact of any pollution that also lowers oxygen levels. If it doesn’t kill them, this stress compromises their overall health, affecting survival rates and reproductive success.
- Metabolic Effects: Increased water temperatures elevate the metabolic rates of salmonids. While this may enhance growth and extend growing seasons, this is not always good news as salmon that smolt after only 1 year are smaller than salmon that spend 2 years in the river. Smolt size is a major determinant in smolt survival. Sustained high temperatures can lead to increased energy expenditures for basic physiological functions, leaving less energy available for essential activities such as reproduction and predator avoidance.
- Feeding Behaviour and Foraging Efficiency: High water temperatures influence the feeding behaviour of salmon and trout. Warmer waters can decrease the availability of suitable prey species, impacting the foraging efficiency of these fish. Changes in feeding patterns may have cascading effects on the entire aquatic food web.
- Impacts on Reproductive Success: Successful reproduction is intricately tied to specific temperature requirements during spawning and egg incubation. A delay in the annual cooling of river water temperatures in late autumn caused the spawning failure in 2015. Rivers temperatures stayed too high for too long that winter. Elevated water temperatures can disrupt the synchronization of reproductive cycles and lead to fish emerging from gravel too early when their food is not there, reducing juvenile survival.
- Catch-and-Release Mortality: Catch-and-release is safe until water temperatures rise to a defined point after which mortality increases exponentially. For cold water species such as salmon trout and Pike this is around 19 degrees C, whilst for most coarse fish it is around 25 degrees C. Carp are the most resilient with little effect until water temperature gets to 29 degrees. The increasing summer water temperatures is why WUF has had to close fishing for on much of the passport for 80 days in 2023.
- Ecosystem-wide Consequences: The impacts of high water temperatures extend beyond individual fish. Changes in fish populations can disrupt the intricate balance of freshwater ecosystems, affecting predator-prey relationships and biodiversity. These ecological shifts have broader implications for the health and resilience of aquatic environments.
Scientific insights into the consequences of high water temperatures on salmon and trout provide a sobering perspective on the challenges ahead. As we grapple with climate change, adopting science-based management strategies becomes imperative for preserving the ecological integrity of freshwater systems and sustaining the delicate balance of these iconic cold-water fish populations.
Tuesday 7th August, 2018
Ten days ago the first rainfall of any note for three months arrived in this part of the UK. It was barely enough to raise the rivers but water temperatures were slightly cooled. Considering the extreme conditions, it is unsurprising that we have not heard of any salmon being caught on either river in July.
Quite a few fisheries have now decided to stop fishing, including some that let their fishing via the Passport. A few weeks ago we advised owners of Passport fisheries in the middle/lower reaches of both rivers to close for salmon and trout fishing.
There have been concerns about coarse fish too. Most anglers will know that cyprinids have more tolerance of high water temperatures and low oxygen levels than salmonids. But they too can suffer, especially if handled incorrectly during and after capture (see here for a catch and release guide for barbel).
The Foundation is taking regular water temperature readings up and down the river, which we will use to advise fishery owners. However, it is always the owners’ decision whether fisheries stay open or not.
Anglers too have a choice on whether or not to fish. Providing the right information is available, we believe that they are capable of making these decisions for themselves.
Stopping fishing, of course, is only a temporary and limited measure to an issue that is likely to occur more regularly in the future. The Foundation has always believed in tackling the causes (rather than the symptoms) of problems affecting the Wye and Usk. We are playing our part in trying to reduce greenhouse gas emissions and working with farmers to lock up large amounts of carbon in their soils. However, until everyone plays their part, temperatures will continue to rise. We must therefore also ensure that the rivers are more resilient.
The extra releases of cold water from the Usk reservoir and the Elan we agreed with Dwr Cymru/Welsh Water have been essential this year in keeping temperatures down and flows up in the upper reaches of both rivers.
There are other things we can do too. Natural Flood Management work enables more water to be absorbed by the land (rather than flowing straight into a river) to be released in times of drought. Our habitat improvement work in the upper catchments narrows and deepens streams, lessening their susceptibility to evaporation and heat.
Reducing abstraction and water conservation is also essential in keeping more water in rivers in conditions such as these. At long last the Government is bringing previously unlicensed agricultural “trickle” irrigation and “abstractions of right” into the overall control programme. Pressure must be kept up to ensure this is carried out, however, and that implementation is not allowed to drag.
Added together, these remedies provide the rivers with a further degree of protection against extremes of weather. Everyone needs to do their utmost to ensure this happens as well as making the fundamental changes required to reverse climate change.
