Salmon born within the River Test chalk stream are genetically unique. They are more vulnerable and less resilient than other salmon and need clean, well oxygenated water, the EA added.
Simon Moody, from the EA, said such chalk streams were “precious, natural resources”.
“Southern Water’s safeguards which should have protected this chalk stream failed one by one. That’s unacceptable,” he said.
“Chalk streams should be gin clear. Thorough maintenance and tested process would have prevented this.”
Following the court hearing, Southern Water said the incident happened “during a period of extreme rainfall and resulted in an asset failure”.
“This should not have occurred and fell below the standards we set for ourselves, and that our customers expect,” it said.
“We have pleaded guilty at the first opportunity to the pollution that resulted and are cooperating fully with the legal process.”
The company is due to be sentenced at a later date.