A harmful algal bloom is the most likely cause of thousands of dead and dying crabs and lobsters washed up on North Sea beaches last year, officials said.

An investigation into the incident, which saw crustaceans washed ashore along parts of the north-east coast of England between October and December 2021, did not identify any “single, consistent, causative factor”.

But a harmful algal bloom in the area at about the same time was identified as being of significance, a report from a joint agency investigation into the incident said.

There were distressing scenes of large numbers of dead and dying lobsters and crabs on beaches, while fishing crews warned that sparse catches offshore following the event were “catastrophic” for their livelihoods.

Dying animals were observed “twitching” and displaying lethargic behaviour and the inability to right themselves from on their backs.

A spokesperson for the Environment Department (Defra) said: “Defra and partner agencies completed a thorough investigation into the cause of dead crabs and lobsters found washed up on the North East coast between October and December 2021 and concluded that a naturally occurring harmful algal bloom was the most likely cause of the incident.

“We ruled out a number of potential causes including chemical pollution, sewage, animal disease, and dredging.”

The  investigation by the Environment Agency, Centre for Environment, Fisheries and Aquaculture Science (Cefas) and the Marine Management Organisation (MMO) examined a range of potential causes.

They included licensed dredging, chemical contamination, activities related to offshore windfarms, the presence of algal blooms and aquatic animal disease.

The report concluded it was unlikely dredging, chemical or sewage pollution or animal disease were the cause.

It said: “A harmful algal bloom present in the area coincident with the event was identified as of significance.”

The presence of the harmful algal bloom – a rapid increase in the population of algae which can release toxins into the water and affect other wildlife – in late September was indicated by satellite images.

Tests on dead crabs and lobsters washed up in the incident, which affected a stretch of coastline stretching from County Durham and Teesside to Robin Hood’s Bay, confirmed they had been exposed to algal toxins.

The report said the significance of these algal toxins in the context of the deaths was not yet fully understood and Government-funded research will look into the issue.

It also said healthy crabs and lobsters are now being caught in the area and the investigation is closed.