Hundreds of sea creatures washed up on the sand in Saltburn, North Yorkshire on Wednesday.
High tides and “recent stormy weather” are to blame, according to the Environment Agency.
Those living nearby have been left baffled by the carpet of dead fish at their once-charming seaside town.
Dog walkers Rachel Forster and Jade Watkins, from Guisborough, said they were both “scared and worried” by the sight.
Jade said: “We had no idea. We just came along for a walk – I’ve never seen a starfish on the beach before. To see as many is quite shocking.”
The 32-year-old said she saw previous mass crab die-offs along the same beach.
Rachel, 27, said she “thought they were alive” when she walked towards the large black patch of sand.
She said she never swims in the sea because someone she knew became “violently ill” after surfing.
Away from the beach, Andrew Macmillan and Andrea Ornston sat on a bench watching the sea.
Andrew said he had noticed there were a lot of starfish, as well as numerous crabs.
The 60-year-old added: “We go to the seaside a fair bit – Andrea is from Marske. But we don’t go so much now with the dog – this puts you off.
“I believe something is killing [the starfish] – but it is only in one batch. I presume the tide has thrown it up.”
Andrew and Andrea, 57, agreed that they would not be returning with their dog, who is 14 years old, over fears he could get an infection.
Andrew, from Billingham, said: “I’m no expert but I’m surprised people are walking with their dogs.”
Stuart Marshall, who owns the colourful promenade beach huts, described the scene as “devastating.”
The 58-year-old said they do sometimes get bits of coal wash up on the sand, but “not anything like this.”
It comes months after a Defra probe ruled a “novel pathogen” was the most likely cause of the last mass die off in October 2021.
Authorities ruled out dredging, when silt is removed from the water, as a possible cause.
Tees Valley Mayor Ben Houchen said activists and labour politicians “jump to ridiculous conclusions” and “ignore all the scientific evidence and work done by the Environment Agency and others” every time there are rough seas.
“Mussels appear on beaches all the time,” he said.
Mr Houchen also later stated that the report was “clear” in its findings, where there was “than a 1% chance the die-off was caused by dredging or chemical pollution.”