Study Highlights Trees’ Importance To Fish As Climate Warms

Planting trees along stream banks could play an important part in protecting sensitive fish such as salmon and trout from rising temperatures as the climate warms, a new study has found.

Salmon and trout are among the most temperature-sensitive of Britain’s native cold-water species, typically preferring water temperatures below 20°C. Temperatures greater than this cause changes in fish behaviour, growth and fertility, and could have knock-on effects for their distribution and survival in southern England.

Climate change predictions indicate a rise in average summer temperatures across southern England of 2 to 6 degrees centigrade by 2080. It is expected that this might result in similar rises in water temperatures in small streams, causing them to frequently exceed the range that some native fish species can tolerate.

Now results from a new study in lowland streams in southern England suggest that planting riparian woodland (trees on the margins of streams) could play an important part in preventing summer water temperatures exceeding dangerous limits.

The findings come from a three-year study by Samantha Broadmeadow and Dr Tom Nisbet from Forest Research, the scientific research arm of the Forestry Commission, and Professor Terry Langford, Pete Shaw and John Jones, fisheries specialists from Southampton University.

Using The Ober Water and Dockens Water streams in the New Forest, the scientists monitored stream water temperatures over three years, and assessed how they related to the riparian tree cover. The authors also considered the potential for using riparian shade to manage stream water temperatures for the benefit of fish such as salmon and trout.

Water temperature fluctuates through the day in small, rain-fed streams such as those of the New Forest. The study found that riparian woodland had a marked effect on the water temperatures, with daily peaks in water temperature typically five degrees cooler in the shade than in open water. This was enough to significantly reduce the number of days on which stream water temperature exceeded critical thresholds for trout.

Critically, the study indicates that achieving about 20 per cent canopy cover along at least 500 metres of small, rain-fed streams could be effective in preventing current summer maximum water temperatures from exceeding potentially life threatening levels for native cold-water fish.  However, higher proportions of riparian woodland are likely to be needed to address future climate warming.

Ms Broadmeadow said,

“Most anglers are aware that fish such as brown trout are very sensitive to the water temperature, and will be concerned about the possible effects of climate change on fish populations in their rivers.

“Although more work is needed to refine its findings. However, this study will provide some reassurance that there are relatively simple, natural and sustainable measures that can be taken to protect fish populations and the biodiversity of our cold-water stream habitats from rising temperatures.”

Further research is being conducted to build on these findings, with the intention of enabling future riparian tree planting to be targeted in catchments known to support trout and salmon fisheries.

Copies of the study report, entitled The Influence Of Riparian Shade On Lowland Stream Water Temperatures In Southern England And Their Viability For Brown Trout, are available from .