Keeping Tabs on Jellyfish
Ben - We now have John Houghton on the line. Thanks very much for joining us. You are studying turtles so why is it that you're tagging jellyfish?
Jon - Leatherback turtles are the ones that eat jellyfish and they cause a bit of a problem for us because they're not like a typical migratory species that moves from one spot to another. They just fan out through the entire ocean. We, for a long time, haven't actually known where they were feeding or what they were feeding on. [For] a couple of years we did big surveys of the whole Irish Sea. What we found was not what we thought we were going to find. We thought the jellyfish were just going to be randomly everywhere. What we found was in four or five main bays. You get these hundreds of thousands of giant jellyfish that are there year after year after year. When we modelled the distribution of leatherback turtles we actually find they're tied up in the same place. That wouldn't be very exciting if you work on land but when you work on an animal that lives beneath the sea and you can never blimmin' find it actually just a simple thing of tying predator and prey is very good.
Ben - How do these electronic tags work? I'm guessing these are not the things that report you're not in your home when you should be?
Jon - No but they're not a million miles away from it. They're data storage tags and they're tiny - they're about the size of your little finger. The ones we're gonna do this year are quite simple. They're just going to record depth and temperature and light levels. We just put it on to a jellyfish. It records all the information and then eventually we retrieve the tag.
Ben - I've seen plenty of jellyfish washed up on the beach. They're very squidgy sort of fluidy things. How on earth do you attach an electronic tag onto something that's so amorphous and blobby?
Jon - That's true but there's jellyfish and there's jellyfish. I think the ones you're describing would be called aurelias. They're common jellyfish. They're tiny and floppy and wobbly and they would be almost impossible to tag. The ones we're going after are called barrel jellyfish and they're massive. They're nearly a metre across and weigh 27-28kg. They are actually quite big, tough animals. They're very strong swimmers, they can swim against a current. Actually if you think your jellyfish are looking like a mushroom. You've got the stalk part coming out underneath what we call the bell. Quite simply all you do is you just tie a time and depth recorder to a plastic cable tie, swim up to the jellyfish: tie it around. It takes about ten seconds.
Ben - Does this affect their behaviour? They must not like having something stuck round their body.
Jon - They're very simple animals and they do react. That's true. We did trials off the West of Ireland last year. Not surprisingly, when you attach a tape to a jellyfish it just swims to the seabed and tries to get away from you. What we found, after an hour or so, they'll just move back up in the water and get on with jellyfish business. As long as you ignore those first few hours then it's fine. You're talking about a device that is 0.1% of the whole animal's body weight. It doesn't really affect it that much.
Ben - How long are they going to keep these tags on? When are you expecting to get this data back?
Jon - That we don't know the answer to. The particular jellyfish we're going after - they're unusual. Most jellyfish boom and bust for a couple of months in the summer. These guys seem to be around all year. We're going to put the tags on probably in August and I'm sure they could be turning up anytime between say two months to maybe even a year down the line. So yeah, any time over the next year.
Ben - How do you actually collect this data? Does it just float up from where the jellyfish were?
Jon - What we've got attached to the time-depth recorder, a little dive computer, is just a tiny fishing float. On the fishing float is just a little label with a reward on it. Once the jellyfish dies the whole device just detaches itself from the jelly and floats to the surface. We're putting them on in big bays where we know they will wash ashore. If you find one on the beach just pick up the reward label and give us a call.