Thousands more dead and dying shellfish have been washed up on a beach on a stretch of coast where there has previously been a mass crustacean die-off.

Visitors to Saltburn-by-the-Sea, just south of the River Tees, were met with the sight of hundreds of thousands of dead mussels on the shoreline, starfish – some of which were barely moving – crabs and razor clams.

Stuart Marshall, 58, who owns the colourful beach huts on the promenade, was mounting a clean-up on the sands.

He told the PA news agency: “It’s devastating.

“There’s starfish dead, clams, oysters and crabs.”

He said a large deposit of black debris on the beach – which some said was sea coal – had just arrived on the tide and was not normally there.

Mr Marshall, who has run the huts for almost seven years, said he does not believe the particles are coal, adding: “We do get bits of coal from time to time but not anything like this.”

He also questioned the possible explanation that rough seas had killed the creatures, as he said the waters had been calm lately.

Helen Whitworth, from Northallerton, North Yorkshire, was walking her border collie Drako on the beach.

She said: “This black area is not usually here.

“Usually it’s a really beautiful beach.

“There’s all sorts of debris here.

“I’ve come down for a lovely walk with the dog and it’s such a shame to see all of this today.”

The Environment Agency said its experts concluded the wash-up is “a natural event” and “normal for this time of year”.

A spokesperson said: “We can confirm the black substance washed up on the beach is coal deposit, which is not unusual for this stretch of coastline. This is likely to be in the intertidal system for a while given recent weather conditions so may continue to happen in the coming weeks and months.

“Creatures like starfish, razor clams and mussels occupy similar rocky habitat and are easily dislodged during storm events. And the physiology of juvenile flounder makes them vulnerable to being stranded on shallow gradient beaches such as Saltburn.

“While we know people are concerned, the combination of recent heavy swell, spring tides and onshore winds means natural wash ups will occur more often.”

The previous mass die-off on the North East coast in late 2021 sparked a series of investigations which have proved controversial to this day.

In January, a panel of independent experts convened by the Department for Environment Food and Rural Affairs chief scientific officer Gideon Henderson concluded: “A novel pathogen is considered the most likely cause of mortality.”

However, the panel was “unable to identify a clear and convincing single cause for the unusual crustacean mortality”.

Some campaigners have feared dredging for a new freeport on the Tees was linked to the die-off, but Tees Valley Mayor Ben Houchen has strongly disputed those claims.