Norwegian salmon farming giant SalMar has reported a significant mortality incident at its Senja smolt production site on 13 November.
An estimated 30,000-40,000 salmon fry were killed, according to Mattilsynet information seen by WeAreAquaculture.
The company informed Norway's Food Safety Authority that an error made during routine maintenance at its smolt facility resulted in thousands of fish being flushed into a central drainage pump.
In the Norwegian-language report, provided to WeAreAquaculture by Mattilsynet, SalMar said: "An error in connection with biofilter washing led to overpressure in the return pipe from the tank to RAS. The centre sieve in the tank that was under sorting during initial feeding has been pushed off at the center of the tank."
"Fish have come down the centre drain and been pumped through a pump used for tank drainage. The fish have thus been run through the pump and into a fish trap," the report said.
SalMar said it had acted immediately on detecting the problem. At the time of reporting, workers at the site were still netting out fish that had entered the RAS system, and cleaning out the fish trap and sludge.
The company stated: "At the moment, it is unknown exactly how many fish have been retrieved from the various locations. But we have so far taken out between 30-40 thousand fish that have either been pumped into the fish trap or taken up in RAS/Sludge."
The affected fish had an average weight of around 7 grams, and most died due to injury. The remaining injured or disoriented fish were euthanized under anaesthesia, the company reported.
Salmar’s RAS facility in Senja island, located about 200km west of Tromsø, is designed to supply smolt to all of the company’s sea sites in northern Norway. The RAS consists of two units with a combined capacity of close to 29 million Atlantic salmon smolt of 120g per year.
In its report, SalMar said that they would now thoroughly check all centre strainers and attachment points, as well as checking programming with the supplier to ascertain why the filter wash was not blocked when the "drain pump" was running.
The company intends to conduct an internal review when it has a more complete overview of the incident, it said in its report to Mattilsynet.
Local reports also suggest that SalMar suspects some of its salmon fry escaped due to the incident.
SalMar has been approached for comment by WeAreAquaculture, but has not replied or released any information at the time of publication.
