The brown trout, a species native to Britain for thousands of years, faces the risk of disappearing from large parts of England within decades, according to a government report.

The Environment Agency’s first national temperature projections for English rivers predict that by 2080, water temperatures during peak summer months will be too warm for brown trout (Salmo trutta) to feed and grow in most parts of England.

The report from the chief scientist’s group also warns that the crucial temperature threshold of 12°C, necessary for the survival of brown trout eggs during the winter spawning season (November to February), will be exceeded at over 70% of the monitored sites by 2080. Additionally, temperatures are expected to surpass the critical limit of 13°C at 25% of the 4,082 sites analysed.

These projections are based on the highest emission scenario from the Met Office, which anticipates that the average water temperature of the warmest summer month in England will rise by about 0.6°C per 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.”

Concerns are mounting due to increasing reports of egg mortality, smaller hatchling sizes, and higher defect rates when water temperatures exceed 12°C. The analysis indicates that while the initial impacts will be felt mainly in southern England, affected areas will gradually expand northward over the coming decades. A significant number of sites are projected to reach critical temperatures of 12°C and 13°C, particularly after 2040.

Water temperature is vital for adult brown trout growth as it influences their metabolic rate. Warmer temperatures also heighten the effects of pollutants and make the fish more vulnerable to pathogens.

The report calls for more research but cautions that adult brown trout will be “under threat” from the projected high summer temperatures. It highlights growing concerns about the increasing number of sites likely to experience temperatures above the critical range of 19.5°C for brown trout survival.

The potential decline of brown trout is particularly concerning given its cultural significance; in 2016, it was voted Britain’s favorite fish by nearly 7,000 respondents in a poll featured on Springwatch.