Fish at three aquaculture sites in Nova Scotia have died and a so-called superchill is suspected, the provincial Department of Fisheries and Aquaculture said Tuesday.

Cooke Aquaculture's sites in the Annapolis Basin, Shelburne Harbour and Jordan Bay are reporting mortalities, officials said.

A fish health veterinarian visited the Annapolis Basin and Shelburne Harbour sites and is expected to visit the Jordan Bay site in the next few days to investigate the cause of death, Fisheries and Aquaculture Minister Keith Colwell said in a statement.

"Our provincial fish health veterinarians investigate mortality events to rule out diseases of concern," he said.

The department said a preliminary investigation has found a superchill happened, meaning sustained cold temperatures dropped the temperature of the water to the level that fish blood freezes, which is around –0.7 C.

Tides in late February and early March also tend to be high, the department said, contributing to to lowering temperatures in sea cages by flooding more shallow areas than usual. Low air temperatures cool the water and receding tides flush the cages with superchilled water.

The Department of Fisheries and Aquaculture said superchills happen every five to seven years and the deaths do not pose a risk to the environment.