Goblin girl on... a mysterious fin!

Last week I got an e-mail from two sport fishermen who had been fishing on the west coast of Sweden, quite near the Norwegian border. They were fishing for Atlantic mackerel when a fin showed up. They managed to snap a few shots before the dark shadow disappeared. The question now is: what is it? Some say shark other whale... Most people doesn't even dare do hazard a guess! So... what's your opinion? Let me know!