"To mistake a blue whale for a fin whale is impossible, this whale has all the characterizations of a fin whale in the ocean".
Dr Gill said the colouration, shape and size of the dorsal fin and colour of the baleen (mouth filter) were all characteristic of a blue whale.
"This whale, when you see it swimming in the ocean, it was like a fin whale", he explained.
Icelandic fishing magnate Kristjan Loftsson views the whale, believed to be a blue whale.
Photos of the massive mammal, which can grow to 33 metres long, were posted online by conservation groups claiming it was slain by Kristján Loftsson's whaling company, the Daily Mail reports.
"From the photos, it has all the characteristics of a blue whale; given that - notably the coloration pattern - there is nearly no possibility that an experienced observer would have misidentified it as anything else at sea".
No other nation, not even Japan or Norway, slaughters Fin whales, and there had not been a Blue whale harpooned for the last 50 years until this one.
However, the company does not have any legal authority to kill endangered blue whales, even within Iceland, Sea Shepherd said.
Sea Shepherd UK chief operating officer Robert Read demanded DNA samples be taken from the Hvalur 8's equipment, meat stocks, and storage to prove it illegally killed a blue whale.
Blue whales were nearly hunted to extinction last century and there are only 10,000 to 25,000 left alive.
"Iceland's whaling is rogue and archaic and should command diplomatic criticism at the highest levels".
"The killing of such a majestic creature, blue whale or hybrid, represents a significant crime against nature, given the rarity of these species and the threats to their survival today".
"This man must be stopped from ruthlessly violating global conservation law and bringing such disrepute to the nation of Iceland".
Mr Loftsson said there is "no way" his crew would have mistakenly killed a blue whale and that instead it was a hybrid of a blue whale and a fin whale, not protected under International Whaling Commission protocol.
Mr Loftsson said his crew spent an hour observing the whale before shooting it, and that they have never mistakenly taken a blue whale. 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.
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