Grant it shark fins differ but they are very easy to pick out same way you can spot say a sunfish a mile away or pick a Malins fin without a 2nd glance.
I to argue on another site about this pic & convinced everybody it was a dolphin.
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I have never seen a Dolphin that big
I have, yesterday in fact you have to remember there there are many different species of dolphins think to many peeps think there's only flipper , through the species they differ by that I mean snout shape body shape etc some like the Spinner dolphin are streamlined which enables them to perform their spinner acrobatics then I've seen some dolphins close to 14 -15 ft long.
Also have a dolphin which I think is not that much smaller than a small pilot whale