Diagnosing and mitigating the adverse effects of extreme winter climate on salmonid spawning

Previous work in Wales and elsewhere has identified the sensitivity of rivers to climate change. Amongst all freshwater organisms, Brown trout Salmo trutta and Atlantic salmon Salmo salar are particularly vulnerable to extreme temperatures in both winter and summer, and potentially also to extreme flows. The geographical location of Wales at the Soth West edge of the European range for these two species means that populations here could be among the first to be seriously impacted or even extirpated should climatic tipping points be reached.

Apparently widespread impairment to salmonid spawning and/or recruitment occurred in Welsh rivers in 2015/16, manifest as dramatically reduced fry numbers during juvenile surveys during summer 2016. In some rivers, parr numbers were also reduced markedly. These patterns imply potentially effects on two year classes and followed unusually high temperatures during periods critical to spawning (December 2015) as well as exceedingly high flows during December 2015.

Working hypotheses to explain these effects implicate i) potential thermal effects on spawning and pre-spawning fish; ii) potential thermal effects on egg survival and development and iii) downstream washout or displacement of fry and/or parr.

Given the importance of salmonids in Wales, there is now an urgent need to diagnose the reasons for the impairment to salmonid spawning and/or recruitment in winter 2015/2016, and to identify the processes involved; to assess future risks to Welsh salmonids under possible climate-change future; and to guide possible management action.


This proposed project will inform these diagnostic and future planning needs by:

Examining long-term discharge (real discharge data) and temperature trends (real and modelled data) in Welsh rivers to place 2015/16 in a longer-term context. Also, examining long-term trends in fry and parr numbers from available sites in relation to putative climatic drivers of change based on:

Modelling the spatial pattern of thermal conditions across affected rivers in both surface water and interstitial environments using available data to assess how thermal patterns in main channels, tributary networks and locations at different altitude might explain spatial patterns in spawning impairment. Mapping and modelling thermal conditions and salmonid habitat availability across key Welsh catchments under different future climate scenarios to identify important refuge habitats.

To review and evaluate potential management options to protect salmonids in Welsh rivers under future climates (including extremes) and to increase resilience.