Fish die-off mystery emerges in Ustevatn, Norway as hundreds of whitefish found dead

Ustevatn, located in Hol municipality, spans 12.2 square kilometers and reaches a depth of 90 meters. Recently, it has become the center of a puzzling fish-related mystery.

Last Wednesday, observers noticed unusual behavior in the fish, as they surfaced, gasping for air. The following day, the phenomenon intensified, culminating on Friday when hundreds of dead whitefish were discovered floating at three different locations within the lake.

Kjell Mykkeltvedt, a retired environmental manager and fish biologist, described the scene, stating that some fish swam erratically before eventually remaining motionless on the water’s surface. He expressed concern upon witnessing such behavior, stating, “It must be something serious.” Mykkeltvedt reached out to food safety authorities and veterinary specialists for investigation.

On Monday, Sofie Ottesen from the food safety agency examined the situation with local property owners and representatives from Hafslund E-CO, using boats and drones for a comprehensive survey. Initial examinations of some deceased fish showed no visible issues with their gills or internal organs, suggesting the cause was still unclear.

Mykkeltvedt hopes the Veterinary Institute will help determine the cause behind this fish die-off in the coming days. While he speculated that water quality could be a factor, he acknowledged the lake’s considerable size made it unlikely. Over the past few days, no additional dead fish have appeared. Investigations continue into potential toxins, diseases, or parasites, but there have been no reports of unusual discharges in the area.