2 or 3 of 6-8 porpoises feeding for about 30 minutes then moving south along the coast. First image just shows countershading. Last image shows tailstock during dive
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The relatively triangular dorsal fin indicates porpoise. In dolphins the dorsal fin is far more swept back with a very curved leading edge. Though your first photo looks to be of an animal with a curving leading edge to its dorsal fin it is not curved enough to be a dolphin.
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There is some variation in dorsals - I think the triangular idea is to contrast with the big dolphin "sickles", as you say. Because dorsals are not being used for individual ID in this species (unlike bottlenose), I think we don't look at them often enough.
I have seen this little group before I think - the one that rolls high and shows the lighter underside has been around for a few months I think, though I can't prove it really is the same individual.
Lat/Lng: 54.1, -5.9
OS grid ref: SB4684