A dense pack of jellyfish drifted into salmon cages and killed the fish.
A large number of Mauve Stingers (Pelagia nocticula) recently wiped out the only salmon farm in Northern Ireland, and a second pack of jellyfish was sighted on 23 November 2007 off the coast of Scotland. These large swarms of stinging jellyfish are unusual in British waters, and it has been suggested that their arrival can be attributed to global warming.
Jellyfish Swarm
The jellyfish arrived off the coast of Northern Ireland in a dense swarm which was 35 feet deep and covered 10 square miles. Workers struggled to get out to the caged salmon, but the density of the jellyfish soup meant that by the time they arrived the fish were either dead or dying from the stings.
Red Water
Pelagia noctiluca in dense swarms turn the sea red by day and emit flashes of purple light in the dark. They are usually restricted to warmer waters, and when the sea looks reddish in the Mediterranean sensible bathers take to the land!
Sting
Most jellyfish have nematocysts (sting-cells) which inject a poison into anything they touch, and while some jellyfish have very dangerous stings, the Mauve Stinger is not considered dangerous – their sting can be as bad as a wasp’s, but is usually less troublesome. The problem comes when there are lots of them, and this is why they caused so much damage at the fish farm.
Normal Prey
Mauve Stingers normally feed on other small jellyfish and pelagic ascidians (small sea-squirts which float in the plankton), but they will sting anything they touch and are not too fussy about what they eat. Their prey must be small enough to pass into the gut, so large fish (and bathers) are stung ‘by accident’ – but this is not much consolation!
Reproduction of the Mauve Stinger
Most jellyfish produce planktonic offspring which swim away and attach to the sea bed. These grow into ‘hydroids’ (which look like small sea-anemones), and it is the hydroids which give rise to the jellyfish. (This phenomenon is called ‘alternation of generations’.) Mauve Stingers are not like this – in autumn the adults produce larvae that grow into young jellyfish directly in the plankton, and since these young cannot move away very efficiently this can give rise to dense ‘swarms’ or ‘packs’. These dense packs will only form where there is a lot of plankton, but once formed they will drift with the ocean currents.
Climate Change
Mauve Stingers live in warm water and are not normally common around the British Isles, so the appearance of large swarms this year might well be an effect of Global Warming.
sources: Jellyfish attack Irish salmon farm and Times Online
see also January 2008 blog
Other articles by John Blatchford
