Officials are heading to Marovo lagoon in the Western Province of the Solomon Islands to investigate why schools of fish are dying.

For over a week locals have reported seeing dead creatures including fish and crocodiles in the lagoon.

Deputy Director from the Ministry of Fisheries, James Teri, says a team is heading there today to try and get to the bottom of the mystery.

"Fish are dying and even crocodiles were seen floating along the surface of the water. We're estimating it covers an area of 40 kilometres square," said Mr Teri.

"We'll be sending a team down today comprising the Ministry of Environment and Public Health to confirm what the source is."

James Teri says some locals believe someone has dumped chemicals in the water, but officials suspect harmful algae bloom is the source.

He says people should not eat fish from the lagoon until the situation is cleared up.