Treasna na dTonnta
Are jellyfish increasing globally? Jellyfish reproduce rapidly and have fast growth rates; they predate many species, while few species predate them; and they feed via touch rather than visually, so they can feed effectively at night and in turbid waters. Around the world, jellyfish blooms have had critical socio-economic impacts e.g. limiting the potential recovery of already weakened fish stocks, clogging fishing nets, causing mass mortalities of farmed salmon, or blocking the cooling water intake of power stations. Last year, it was difficult to avoid jellyfish brushing against your paddle as your seakayak slid through the water. 2013 was a bumper year for jellyfish in Irish waters; on one summer evening paddle across Dublin Bay, they were so dense at one stage it was like paddling through jelly rather than water. Later, in October 20,000 farmed salmon were killed due to a jellyfish “bloom” (a sudden, massive increase of jellyfish numbers) off Clare island, Co Mayo. Anecdotal evidence from kayak anglers in the Irish Sea supports a general view that jellies are more common. Will it be the same this year, and what is happening in the sea to encourage the growth of creatures that have the power to both fascinate and repulse? Possible causes of Jellyfish expansion Jellyfish populations may be expanding globally because of overfishing of their natural predators (such as Tuna and turtles) and the availability of excessive nutrients due to land runoff. When marine ecosystems become disturbed jellyfish can proliferate because jellyfish thrive in warmer, less-oxygenated water. It may become difficult for fish stocks to re-establish themselves in marine ecosystems once jellyfish have dominated them, because jellyfish also feed on fish eggs and larvae.
Newsletter of the Irish Sea Kayaking Association