Unravelling the secret lives of jellyfish using electronic tags
We now know that jellyfish in the Irish Sea are not just passive drifters but appear to live in distinctly different areas, with some close to the shore and others far out to sea. Perhaps the best example of this is the barrel jellyfish (Rhizostoma octopus) that forms enormous ‘blooms’ in large bays throughout the Irish Sea but is only rarely seen away from the coast. These blooms occur for many months (5 months at least) and in the same spots year after year suggesting they play an important, yet poorly understood role in coastal ecosystems. Importantly, these large jellyfish which can grow up to 35 kg, may impact on aquaculture industries such as mussels, oysters and clams by eating their larval stages or even indirectly by causing the development of red tides. On the upshot, these jellyfish may also offer a unique experience to divers as they represent a floating habitat or nursery grounds for juvenile fish such as whiting (Merlangius merlangus) and horse mackerel (Trachurus trahcurus).
To help assess these important impacts and opportunities, we have attached tiny dive computers or ‘tags’ to barrel jellyfish in Carmarthen Bay (Wales) during the summers of 2008, 2009 and 2010. These tags have recorded the vertical movements of jellyfish (up and down in the water column) for periods of up to several months (longest deployment was 29 days). We attach the devices around the stalk (or peduncle) of the jellyfish using cable ties and a small float. When the jellyfish dies (of natural causes) the dive computer will float to the surface and wash ashore. Each device has a return address on it, so all you need to do is pop it in the post and we will send you a reward for your time and effort. So far we have deployed ‘72’ tags and have thanks to your efforts recovered an incredible ‘27 (36%) of them! Tagging barrel jellyfish will resume in 2011.
In 2010, we decided to try to “tag” Lion’s Mane jellyfish off Dun Laoghaire (Dublin) in order to investigate the movement of these highly venomous jellyfish that cause problem to open-water swimmers and bathers in the Irish Sea. We successfully managed to attach small acoustic tags to 5 individuals (with the help of Oceandivers). This allowed us to track the movement of these animals for several hours (longest was 7 hours). It was the first time that this species was tracked! Preliminary results tend to indicate that the currents associated to tidal cycle is a very important factor in the transport of these jellyfish from one point to another. Tracking will resume in summer 2011.
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