The Hunting of the Shark

An interesting email came in from one of the staff at Scottish Natural Heritage on 25 February about a satellite tag that had been fitted to a basking shark by a research group in Ireland. It had apparently come off the shark and had been washed ashore early in the year on the small islet of Luinga Mhor west of Arisaig. Apparently a local sea paddler had already been to look for it but had been unable to find it. Having had some experience using a hand held radio receiver to home in on the transmission signals of golden eagle satellite tags and retrieve them, I thought that I might be able to help here.

The weather forecast was really good for the weekend of 14/15 March and so Alastair, Fiona, Margaret, Mick and I headed south and spent Saturday night in a bunkhouse in Mallaig. On Sunday morning we all paddled out to Luinga Mhor armed with the information that the tag was bright green, about nine inches by six and lately the overnight transmissions were variable, suggesting that it might be stuck in an active zone and possibly getting seaweed dumped on top of it. We were also armed with garden rakes and forks to turn over and search through piles of weed and debris on the shore. Landing in the bay identified by the location fixes that the tag had been downloading during the past couple of months, we diligently trawled through piles of seaweed to no avail.

 

Knowing that a compromised view of the sky (i.e. stuck under a rock or covered in weed) would be likely to reduce the location accuracy, we increased our search area by two or three hundred metres either side. This took in much of the island as it was only eight hundred metres from end to end anyway. Despite this, the whole of Sunday spent searching piles of seaweed, rock crevices and pools and the various strand lines failed to achieve the objective, and by late afternoon we had to admit defeat.

Having brought the radio equipment that I’d used in the past to find golden eagle tags, I was reluctant to go home without trying it, in addition to which it might be the only way to find a radio tag that was hidden from view.  The snag was that it meant staying on the island overnight in the hope that it would transmit a radio signal. But, unlike me, the others had to go to work on Monday morning. So we paddled back to Arisaig while I thought this all through. The weather conditions the next day were my main concern – particularly if I was out by myself. A quick phone call to a friend to look up and relay the forecast suggested that although not calm the weather should be OK – at least until mid-day. So, quickly scavenging everyone else’s left-over lunch, buying a couple of tins of cooked chili con carne and loading up with warm clothes, I paddled back to Luinga Mhor as it was getting dark.

Making myself comfortable (mainly putting another three layers on), I spent some time learning my way around the island in the dark, navigating between the boat and the search site using the lights of Arisaig five kilometres to the east and the constellation of Orion in the sky to the west. Every now and then I’d switch my radio on, tuned to the specific download frequency I’d learned from the Irish Basking Shark Project’s research coordinator.

And then at midnight it started transmitting with short bursts of radio signal every two minutes or so. Wandering around by the light of a failing headtorch, I spent the next few hours moving along the shore line in and out of the bays and inlets trying to work out where the signal was strongest. In the end I decided that it was almost certainly coming from the next inlet along to the east from where the downloaded location information suggested. Even then it was difficult to home in on accurately, but in the end – at quarter to four in the morning – I found it partly covered by grass and debris and above the spring tide strand line. Almost certainly at least one of us had walked very near it during our team search the previous day but failed to pick it out visually. And our interpretation from the radio signal variability that it was buried in seaweed turned out to be incorrect and misleading.

We were extremely lucky with the weather conditions and very pleased to have been able to help the Irish Basking Shark Project recover one of their satellite tags – not that we really need any excuse to paddle in such a beautiful part of the west coast of Scotland.

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