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Slea Head Bloom

Monday, 12 May 2014 10:19
Basking shark with Common Dolphins of Slea Head,
Co. Kerry
Basking shark with Common Dolphins of Slea Head,<br>Co. Kerry
(c) Nick Massett and Lucy Hunt 2014

2014 The Story so Far: Field report from Nick Massett and Lucy Hunt from Slea Head,Kerry.

After a good run of warm spring weather here it came as no surprise when the first basking shark of the year was sighted just after dawn off Slea Head on April 14th. From that single shark in the early morning the count was soon up to six by midday; not only had they arrived, but in numbers. This was great news following last years poor showing in this part of the country due to the particularly cold spring of 2013. But this year the sea temps are up and there is an abundant plankton bloom triggering a whole food chain of activity. Shoals of sandeel are feeding on the plankton and they in turn are being preyed on by common dolphin and minke whales. So to find the sharks it’s been a case of searching amongst all the commotion of diving gannets and actively feeding cetaceans.

see image of  Basking shark with common dolphins.

From that first sighting the following three weeks then proved to be very consistent with on average 5-6 sharks showing most days and reaching peaks of 13-14 on a couple of days. The Kerry basking shark crew re-assembled, the rib Míol Mór was prepped to go and some boat work carried out. This principally involved dorsal fin photo ID and obtaining some underwater footage to ascertain sex.

Photo ID is useful when collaborating with other basking shark research teams highlighting any re-sightings and shark movements between territorial waters. For the most part basking shark dorsals aren’t that well marked but a percentage do have permanent nicks or distinctive features. Some other temporary features can also be noted such as the presence of the parasite pennella. Another parasite noted hitching a ride on the bulk of the shark is the lamprey. These more often though are on the underside of the animal and particularly in the genital area, this can make sexing the shark very difficult. Lamprey can look very similar to the male claspers of a shark in the murky soup that is plankton rich water. A lot of time is spent post trip poring over GoPro footage freeze framing the grainy images looking for, or the lack of, claspers.

One sharks dorsal image really did tell a disturbing story though, it had nearly been completely cut off by what was probably a propeller trauma injury. Two lateral incisions as well as other peripheral wounds were clearly visible. Albeit injured this shark appeared to be otherwise healthy though and was feeding in a normal manner. It highlights the susceptibility of these slow moving, surface feeding sharks to ship strike.  We would encourage all boat users to follow the Shark Trust code of conduct when in proximity of the sharks. This is available as a free download on the about us/downloads section of the website.

Slime samples were taken from five sharks with only a mildly invasive quick rub with a swab on a pole. This has proven to be a very efficient and simple method for getting DNA.

As in previous seasons nose to tail swimming and parallel swimming was observed, indicating possible courtship behaviour.  Also noted was the inter-species interactions between common dolphins and the basking sharks when they come into contact. The sharks don’t seem to alter their behaviour whatsoever and have carried on feeding but the dolphins have been escorting the sharks and effectively bow riding them. It doesn’t appear to be a mutual feeding strategy, nor is there any particular hydrodynamic gain. More like the highly intelligent dolphins are just taking a curious interest in the big sharks.

Sightings are ongoing despite some much fresher conditions of seastate 4-5 and it will be interesting to see how long a season we will have here. In the past it has been for just 4-6 weeks before the sharks move up the coast to Donegal and onto Scotland.

The other hotspot for sightings so far has been County  Cork where a constant stream of reports have also been coming in these last few weeks. Please report any sightings to the ‘report a sighting’ section of the website.