It has been native to Britain for thousands of years and was heralded as the national fish on the BBC’s Springwatch, but a government report suggests the brown trout risks being wiped out in large parts of England within decades.
The first national temperature projections for English rivers by the Environment Agency forecasts that by 2080 the water will be too warm almost everywhere in England at the height of summer for the Salmo trutta species to feed and grow.
It is further suggested by the chief scientist’s group’s report that the important temperature threshold of 12C for the survival of brown trout eggs during the winter spawning season from November to February will be exceeded by 2080 at more than 70% of the sites where temperatures were modelled.
The analysis suggests the temperature would exceed the critical threshold of 13C at 25% of the 4,082 geographically distributed sites modelled.
The forecast is based on the highest emission scenario envisioned by the Met Office, under which the average water temperature of the warmest summer month in England is projected to rise by about 0.6C each decade.
“The projections indicate that adult brown trout will be under threat from high summer water temperatures at almost all sites by 2080 as their upper growth and feeding temperature range of 19.5C could be exceeded,” the report says. “Salmonid eggs [including that of the brown trout] survive best at winter temperatures below 12C. This threshold could be exceeded at over 70% of sites by 2080.”
In 2016, the results of a UK national fish vote were covered by the BBC’s Springwatch. Of nearly 7,000 respondents to a survey, 21% voted for brown trout as their favourite fish.
There is an increase in reports of egg mortality, smaller size in hatching and increased levels of defects when the water temperature is above 12C.
The modelling of the impact of a rise in water temperature suggests that “the initial sites affected appear few and mostly in the south of England” but that the affected catchments will include “greater numbers of more northerly sites as the decades progress” with “a significant number of sites reaching 12C and 13C across this period, particularly after 2040”.
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Water temperature is crucial for adult brown trout growth because it affects their metabolic rate, while increased temperatures raise the impact of pollutants and the susceptibility of fish to pathogens.
The report recommends further research but warns that adult brown trout would be “under threat” from the high summer temperatures envisioned. “The increasing number of sites projected to experience water temperatures above critical ranges for brown trout (19.5C) is of particular concern,” the report concludes.
