A team of international scientists have succeeded in satellite tagging ocean sunfish off the coast of Dingle. This is the first time that these fish have been tagged in Irish and UK waters.
Sunfish are the largest bony fish in the world and can grow up to 3.1m from tip (mouth) to tail fin, and weigh up 2235 kg (2 tonnes). They have a unique shape that is flattened from side to side and stretched from top to bottom. Their body is truncated (or shortened) to such an extent that they are literally all head and no body. Their German name ‘Schwimmender kopf’ means ‘swimming head’. They are related to puffers and porcupine fishes. These animals are fascinating to watch, spending large amounts of time at the surface with their large dorsal fin flipping from side to side. Amazingly, females can produce up to 300 million eggs, the largest number of eggs ever recorded in any fish. Unfortunately, almost nothing is known about the movements and behaviour of these ocean giants. However, it is thought that during the summer months, sunfish migrate into Irish coastal waters. The northern coastline of the Dingle peninsula (from Smerwick Harbour to Brandon Head) is one such area where they are regularly seen.
The team of scientists used techniques that were developed last year in the same waters to tag leatherback turtles that like the sunfish also feed on jellyfish. Taighde Mara Teo, an Irish fisheries research group, provided logistical and technical support. Additional support and local knowledge were provided by Pádraig Frank Eco-Tours based out of Ballydavid, Smerwick Harbour. The Marine Institute and Údaras Na Gaeltachta funded this research.
It was not known before how many sunfish were in these waters, but the researchers now believe the Dingle coastline may hold significant numbers of sunfish, with up to 20 individuals observed in a day. It is unsure why so many sunfish aggregate in this small area, but one strong possibility is that the northern coastline of Dingle may represent a nursery ground for sunfish, where juveniles can shoal and feed together. Similar nursery grounds are known in Bali (Indonesia), and off the coast of California.
The first sunfish was tagged on the 8th of August and the second on the 14th of August. Each tag will record temperature, depth, and light data, from which the scientists can reconstruct the movements and behaviour of these two fish. The first data is due to be transmitted via satellites in two months time. By then the sunfish may be located off the continental shelf or towards the Bay of Biscay and Portugal.
Dr Tom Doyle of the Coastal & Marine Resources Centre (University College Cork), who organised the expedition said, “This research has very important implications for Ireland as a whole as sunfish may represent a good biological indicator of climatic change, that is… if sunfish sightings increase dramatically it may be a clear sign that our climate is changing”.
Dr Jon Houghton of University of Wales Swansea said, “The northern coast of Dingle represents an important habitat for sunfish, and provides a fantastic opportunity to study the behaviour and biology of these ocean giants”.