The first 21 whales killed were endangered fin whales, which the Icelandic government has permitted the company, owned by Icelandic fishing magnate Kristjan Loftsson, to slaughter despite an global moratorium on whaling, Sea Shepherd said.
Hard To Port said that the whale could be a blue whale or a rare hybrid of a fin and blue whale and called the killing a mistake in a statement.
"It was taken as a fin whale, but it [will] turn out to be a hybrid". In all countries, except for Iceland, whaling is strictly forbidden, but even in Iceland it is forbidden to take the life of members of the small group of the inhabitants of the Maritime fauna - blue whale. When you approach a blue whale, it's so distinct that you leave it alone'.
"Photographs point to the fact that it's a hybrid whale and we're nearly certain that it is one, but we can't be sure until autumn, when we get it DNA tested", he said.
While blue whales are protected, there is no designated status for hybrid blue-fin whales because they are typically extremely rarely encountered.
No other nation - not even Japan nor Norway - slaughters Fin whales, and there has not been a Blue whale harpooned by anyone for the last fifty years until this one harpooned by Hvalur 8.
Some have accused the company of using that loophole to deflect negative scrutiny of the killing.
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The government of Iceland confirmed that "blue whales are protected under Icelandic law with their capture prohibited".
The Icelandic government said it was taking the matter "seriously" and that the relevant authorities, which will use DNA testing to confirm the whale's true identity, were already investigating.
Sea Shepherd, which has been monitoring activity at Iceland's whaling station in Hvalfjordur, reported Wednesday that the blue whale was killed July 7.
The difference between these and blue whales is "easy to see", Loftsson said.
A moratorium on commercial whaling was passed in 1982 by the International Whaling Commission, taking effect from 1985 onwards.
"Now is the best time to see blues around Iceland, so I imagine that despite still being rare, their numbers around Iceland are at their highest during these months", she added.
This would be the first blue whale deliberately killed in 40 years, according to IUCN.
Despite the intense criticism Loftsson has faced - he has been dubbed a "rogue whaler" by some conservationist groups - he claimed most Icelanders supported whaling. Prior to the catastrophic commercial whaling of the 20 century it is estimated that there were in the region of a quarter of a million blue whales, but their populations crashed in the 1950s and 60s.