An investigation into the deaths of thousands of crabs and lobsters which washed up along the North East and North Yorkshire coastline did not find a clear cause.
The report into the incident did however identify a harmful algal bloom in the area at about the same time as being of significance.
Large numbers of dead and dying crustaceans washed up on a stretch from County Durham to Robin Hood's Bay between October and December.
Fishing crews warned that sparse catches offshore following the event were "catastrophic" for their livelihoods.
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.
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 the dead crabs and lobsters 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.