Wednesday, January 7, 2015

Potential of Stenella frontalis in the UK

While with the Sea Watch Foundation last year we encountered a lone dolphin during a dedicated survey from Aberystwyth on September 28th. Noted by all of us to be a little small, it would be impossible for it to have been any less than a juvenile bottlenose. I personally only saw it far off as a relative silhouette, and having spoken to many photographers who agree that dolphins are too photogenic and clog up memory cards I waited for it to approach closer.

Unfortunately I did not see it again.  A few of the ccrew at the front half of the boat said it had surfaced in front, out of my sight at the back, and that it was a bit more unusual than we had previously thought; it was covered in little dark speckles. It skipped towards the boat from afar, no more curious or inquisitive than any average dolphin. The exclaimed phrase "Stop!" in a most aggressive tone was passed down the boat, but not quick enough to satisfy our admirer. The animal didn't surface again, so it was clearly a timid creature, and, unlike most dolphins in the Bay, not highly tolerant of boat activity*. Then again, you may ask, if a boat was that close to you going at that speed, who wouldn't be at least a little afraid?

With several minutes having passed since it had last breathed from the surface, it was quite clear that it was sending us a message with the help of mocking ripples and the false-dolphin breaches of splashing waves and spindrifts. So long then, the ocean clicked between its teeth.

Clear that it was not returning in the time we had**, we set off and those who saw it well remained puzzled. Talking it over with Salomé, she made a rather startling suggestion. "It didn't look like a Tursiops," she affirmed. I couldn't argue; there was something unusual about the one that got away, though it looked identical to a bottlenose in every morphological aspect. But there was another group of dolphins that could look identical to a bottlenose in silhouette and that was the Stenella genus. Its closest member, the Atlantic spotted dolphin, normally inhabits latitudal seas beyond the Mediterranean. She wasn't immediately confident that it was one, and for good reason: it was a major rarity anywhere in the north Atlantic.

Of course they then look to me. "Did you get any pictures?" they ask, for which I can't offer an optimistic reply. I wondered if it could be an extremely marked bottlenose, and though I found a few examples of what we'd call well-marked bottlenose none of them were that similar. It was too bad none of those at the front were able to photograph it close up.



No internet information exists on this species presence in the UK, or even in France, though the latter's ocean coverage may be even less than our own. It was not the first possible spotted dolphin reported this year, though none of the reports had been confirmed or photographed, a fate that this one will also share. 

Considering the rare and vagrant Cetaceans known in the UK in the past, it is theoretically hardly impossible to see this species in UK waters, though improbable. There is nothing but open ocean in the way of their home seas, and it would be easy to merely swim this distance especially as an ocean-going creature. Birds are the best example of vagrancy, we've seen "megas" that have flown thousands of kilometres from their native range from western Asia, Africa and America. The UK's sole record of ancient murrelet, a central Pacific bird that will not fly over land except in hurricanes, would have traveled all up the Pacific Coast, along the Arctic Ocean, down the Atlantic through Greenland and all the way over the North Sea to reach the UK. So then the question is why has the spotted dolphin, something which by rarity is hardly comparable to some foreign birds, has not been "officially" seen in the UK before? 

This question is far more easy to answer. The extreme similarity of the Atlantic spotted dolphin to the bottlenose dolphin by default makes it susceptible to being highly overlooked. I saw our dolphin from no more than 100 metres off, albeit silhouetted, but I never would have given it a second look. If it wasn't for the markings, I doubt anyone would have raised an eyebrow. Most dolphins are seen far out to sea, and even binoculars or telescopes might not be enough to see the details. The only truly obviously diagnostic difference are in the spotted markings, which are tricky to see unless in close proximity, and you need a boat for that, and the sacred license to approach them in the first place. 

The issue with many sightings is that well-marked bottlenose, especially if only a dorsal view is seen, could be mistaken. This is supported by the issue that markings usually increase in density as they travel up the animal, so the back could easily be as spotted as the confusion species in question (but if it were to breach, then the rest of it would be obviously unspotted; spotted dolphins should be equally spotted all over except for juveniles).  

It is also worth noting that unlike birds, their complete territory is far harder to survey, and for the most part impossible from land. Salomé speculated that warming seas may relate to their eventual appearance here, which is more than plausible. Perhaps one day they will be seen for certain in the northern waters.

A map of the route taken from Aberystwyth (all i10 and i11-1 and i11-2) can be found here. If I obtain specific sightings data, I will upload those on the map later. The eBird report exists here.


*(bearing in mind this is a bit of a generalization, as many dolphin pods are in fact not tolerant of boats at all even in Wales).

**Dedicated surveys only allow a maximum of 5 minutes with each encounter before we must move on, whether it is to keep the survey going or to reduce interference with the animal's natural behaviour.

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