The cause of a poison outbreak that led to the deaths of two dogs remains a mystery more than a year on, investigators revealed.
A total of nine animals fell ill after eating washed-up fish along the Norfolk and Suffolk coast in January 2018.
Eastern Inshore Fisheries and Conservation Authority (EIFCA) said it was “highly likely” both dogs died from paralytic shellfish poisoning (PSP).
Tests will continue along the coastline until August 2021, a spokesman said.
The illnesses were reported over a two-week period in January 2018, and the deaths recorded at Holkham, Norfolk, and Felixstowe Ferry, Suffolk.
Since then there have been no further cases, and scientists say there remains a “very low risk” to humans.
A joint report by the IFCA and the Centre for Environment, Fisheries and Aquaculture Science (Cefas) said the toxins found were typically associated with molluscs – mussels, clams, oysters and scallops – which washed-up on the eastern beaches after winter storms earlier in the month.
As filter feeders, the molluscs absorb PSP toxins which are produced naturally by microscopic algae.
However, the algae does not bloom during winter – when the animals died – leaving the source of the deadly toxins at that time of year a mystery.
Tests also found high levels of PSP in starfish from Norfolk and Suffolk, and in crabs and flatfish to a lesser degree.
Julian Gregory, chief executive of EIFCA, said a monthly regime of sample collection and analysis would continue.
“The mystery here is why the toxins from the algae emerged in the middle of winter,” he said.
“At that time in early 2018 there were significant storms which could have released something, given that high levels were found in washed-up starfish.
“Our primary concern is that the fisheries remain viable.”
The teams have received almost £250,000 in funding to continue the research for two years.