An investigation into thousands of dead and dying crabs and lobsters washed up on North Sea beaches last year did not identify a clear cause.

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.

The crustaceans washed ashore along parts of the north-east coast of England between October and December, with dying animals “twitching” and displaying lethargic behaviour and the inability to right themselves from on their backs.

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.

An 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.

A harmful algal bloom present in the area coincident with the event was identified as of significance

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

No “single, consistent, causative factor” was found, according to the report from the government agencies, though it 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.