Brown trout in hot weather
Following the fly committee’s decision to suspend fishing at Gargrave, Buckden and Hubberholme we thought it would be useful to give some background on the issue of heat stress and brown trout.
It is clear that climate change is leading to a pattern of increasingly hot dry summers which will obviously impact summer river conditions, the low flow and high air temperatures leading to increased water temperatures. Most of the research into heat stress in salmonids seems to be about brown trout and salmon, but the same applies to grayling. Research has found that the maximum growth rate for brown trout is obtained at a water temperature of approx. 13°C, as water temperature rises growth rate declines until at 19.5°C growth stops. Brown trout will start to die if subjected to water temperatures above 25°C for more than seven days.
Between these two figures (19.5° and 25°) is a critical range where, although the fish may not die, there are subject to thermal stress and show stress behaviour – “reluctance to feed, sudden bursts of activity and rapid ventilation movements”, later becoming quiescent and lethargic.
Fish will seek out “thermal refuges” to survive hot dry periods. These will be in undercut banks, in amongst tree roots and amongst debris around fallen trees and branches. Unfortunately in periods of drought these refuges may not be available due to receding water levels.
So how are conditions on the primary BCAA game rivers at Gargrave, Buckden and Hubberholme? Buckden and Hubberholme will probably have lower water levels but are blessed with better tree cover. Hopefully we will have a report from Peter Ramsden, the bailiff, soon. At Gargrave the water temperature was measured at 22°C today so it is definitely in the critical zone and the water level is very low, lets hope that the fish are finding their thermal refuges. So far there have been no reports of stressed behaviour, I am sure our bailiff, Peter Bell, will be looking for any signs.
It is clear that as climate change starts to bite these weather conditions will become more frequent, we need to make sure we manage our waters to mitigate the effects of these changes. Much of the riverside work undertaken by the fly committee is aimed at just that, we have planted hundreds of trees in recent years, hopefully as they come to maturity they will provide much needed shading and thermal refuges. We need to consider additional ways to deal with these conditions because the effects can be more long term than just stressed fish in hot weather, they can impact reproduction and also make the fish more prone to disease
If you want to read more you might find these interesting:
https://freshwaterhabitats.org.uk/wp-content/uploads/2020/07/Trout-feel-the-heat-in-the-Forest.pdf
