Preliminary screening of tissue samples from both healthy and sick ice seals and walruses showed no radiation levels that would have directly caused the symptoms, the agency posted on its Alaska region website.
Radiation was considered because of the timing and size of the Fukushima nuclear power plant accident that followed a tsunami in March 11 in Japan. According to the NOAA announcement, marine animals and fish near the accident site in Japan were affected by radiation but there is no evidence to support any effects on animals in Alaska.
Sick and dead ringed seals started showing up in July on the Beaufort Sea coast near Barrow, the country's northernmost community. Strandings were reported as far west as Point Lay and Wainwright on the Chukchi Sea.
The affected animals had lesions on hind flippers and inside their mouths. Some showed patchy hair loss and skin irritation around the nose and eyes.
Stricken live seals were lethargic, allowing people to approach. Necropsies on the dead ringed seals found fluid in lungs, white spots on livers and abnormal growth in brains. Symptoms, but no deaths, were also observed in Pacific walrus.
Ringed seals are the smallest of Alaska's ice seals and are the main prey of polar bears. They give birth on ice and they are under consideration for a threatened species listing because of projected loss of snow cover and sea ice from climate warming.
NOAA's National Marine Fisheries Service in December confirmed more than 60 dead seals and 75 diseased seals in Alaska waters. Outbreaks were also reported in Russia and Canada.
The fisheries service in December declared the deaths an "unusual mortality event," giving researchers access to more money and expertise to find out what had happened to the animals.
John Kelly, a professor emeritus of chemical oceanography at the University of Alaska Fairbanks, who is working on the radiation assessment for the unusual mortality event, could not be reached immediately by phone or e-mail Friday.
NMFS officials said scientists will also investigate the animals for immune system diseases, fungi, man-made and bio-toxins, contaminants and stressors related to sea ice change.
The agency has received only a few reports of seals in Alaska since the end of November. In early January, according to the agency, hunters in Alaska's North Slope Borough killed three ringed seals that had complete hair coats and looked healthy but had small lesions on their flippers, suggesting that the disease may remain.
On the Russian side, Chukotka hunters did not report any sightings or harvest of sick or hairless seals in December or January, according to the agency.
NOAA Fisheries northern Alaska disease outbreak updates: http://bit.ly/A1zRKt
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