The escape, from a farm owned by Alsaker Fjordbruk, was the fifth reported in the last two months.
Kurt Oddekalv, from Green Warriors of Norway, told Bergens Tidende that he believed salmon farmers hugely underreported escapes.
"Salmon farmers are now being fined for escapes, so they have good reasons for reporting as few as possible," he said. "Remember that there's only a net between farmed fish and the high seas. Farmed salmon is destroying wild salmon stocks and natural genetic diversity."