Almost half a million smolt died at Leroy Seafood's Kjaerelva facility in Sunnhordland, Norway, earlier this month, the company said Monday.
The group said it suspects the deaths at the beginning of March were caused by "challenges with the water quality."
Leroy said it immediately implemented measures and moved the smolt into new vessels and a new water environment, and is working to find the root cause.
"The fish that have been moved seem to recover quickly and show an increased appetite," Morten Fjaereide, general manager of Leroy Sjotroll, said in a statement.
In total, the company had 1.9 million smolt at the facility and 490,000 died in the incident.
"We take this incident very seriously and have reported the matter to the Norwegian Food Safety Authority,"said Fjaereide.
Leroy said it now has the situation under control but that it is too early to go into more details about the cause.
The Norwegian Food Safety Authority will carry out an inspection of Kjaerelva this week.
"I look forward to the inspection, and emphasize the importance of continuing the good dialogue and cooperation with the Norwegian Food Safety Authority... both to identify the cause behind, but also to prevent similar incidents in the future," said Fjaereide.
