It comes as Hampshire and Isle of Wight Wildlife Trust (HIWWT) urged people to keep out of the River Itchen to protect salmon eggs, which are buried in fragile gravel nests known as redds.
After often years at sea, adult Atlantic salmon swim thousands of miles to return to the chalk streams where they were born.
Shoals of juvenile salmon start their journey from the Itchen to the sea each spring.
The trust said just 133 adult salmon were recorded returning to spawn in 2022 – the lowest figure since monitoring began in 1990.
“Research has shown that chalk stream salmon are genetically unique and irreplaceable. They play a vital role in the ecosystem, supporting insects, birds, mammals, and other river life,” the trust said.
“Numbers have plummeted due to a myriad of different factors including over-abstraction, habitat degradation, pollution, impacts on redds, in-stream barriers, climate change, and pressures associated within the marine environment.”
The Department for Environment, Food and Rural Affairs said “Our chalk streams are one of Britain’s most nature rich habitats and are embedded in our plans to reform the water industry.
“We’ve banned unfair bonuses, secured record levels of investment and introduced landmark legislation to hold water companies to account – including jail time for water company executives who obstruct investigations.”
Southern Water’s 2025 Pollution Incident Reduction Plan said it was “making progress” in tackling pollution incidents and was “analysing the root causes and most effective actions, in the face of challenging weather conditions”.