Jellyfish blooms are expected to be a common sight this summer, with rising ocean temperatures one of the main causes of substantial population growths.
NIWA forecasters have identified a marine heatwave in the waters to the north of the North Island, and are keeping a close eye on sea surface temperatures around the country, which have all been above average since October.
NIWA marine biologist Diana Macpherson said jellyfish blooms occurred when water temperatures rose, which created an increase in the amount of food available for them. As numbers increased, winds and currents could gather them up into dense groups and strand them on beaches.
They were important in marine food webs, she added, with roles as predators, prey, or as decomposing scraps of food for suspension feeders in the water, or for bacteria on the seafloor.
Jellyfish blooms could have an impact on marine industries, such as salmon farming, however, and could invade beaches and sting swimmers. Scientists were discovering that human impacts such as overfishing, pollution and warming oceans were exacerbating the problem of jellyfish blooms, although jellyfish themselves were set to actually benefit from those impacts.
While there were about 223 species of Scyphozoa jellyfish worldwide, just 22 were found in New Zealand waters, although the term "jellyfish" was used to describe a diverse range of jelly-like creatures, including true jellyfish, hydromedusae, siphonophores, box and stalked jellyfish, comb jellies (Ctenophora) and salps (Chordata).
Macpherson described them as one of the most evolutionary primitive group of invertebrates.
"They have no brain, no bones and no heart, but they reproduce, eat and defend themselves, or catch prey, with astounding stinging cells called nematocysts, which work a bit like a harpoon that contains venom," she said. Those stinging cells were what sometimes gave jellyfish a bad reputation, but not all were stingers, and comb jellies and salps didn't sting at all. "They are all a natural part of the ecology of all oceans. They are resilient, and good at what they do," she said.
Her advice in the event of being stung by a jellyfish was to flush the area with seawater to remove the stinging cells, carefully pluck off any tentacles that might be stuck on, then apply heat to relieve the pain and deactivate the venom. Using urine to relieve a sting was a popular misconception — there was no way of telling its pH and chemical make-up, so it could actually make the pain worse.
Most stings in New Zealand waters were not serious, but jellyfish should be avoided when possible.
Species likely to be seen at New Zealand beaches include:
■ The lion's mane (Cyanea rosea), the biggest jellyfish species seen in New Zealand waters, with a bell that could reach two metres in diameter and tentacles up to 36 metres long. It is a pinky brown colour at the top of its bell and creamy to whitish at its margin, and has four frilly oral arms and eight clusters of tentacles, each cluster having more than 100 individual tentacles. They occasionally swarm in large numbers, and deliver a painful sting.
■ The moon jellyfish (Aurelia) has a discus-shaped, ghostly whitish-transparent bell with a scalloped margin that bears hundreds of short, fine tentacles. They are capable of only limited motion, primarily drifting with the current. They are usually up to 25cm in diameter, potentially up to 40cm or greater, and can be recognised by their four pinkish-purple crescent-shaped gonads, easily seen through the top of the bell. They have little or no sting. This species can occasionally bloom in large swarms, potentially posing a problem for salmon farmers as they can cause mass fish kills by suffocation from their mucus and irritation from their stinging cells.
■ Like the lion's mane, the spotted jellyfish (Desmonema gaudichaudi), known for dark polka dots on its body and 30cm tentacles, has four long curtains of oral arms, with its long tentacles arranged into eight clumps that hang from inside the bell (rather than the margin) in a straight line, delivering a painful sting.
■A NIWA e-guide to jellyfish can be found at https://niwa.co.nz/coasts-and-oceans/marine-identification-guides-and-fact-sheets/Jiggling-Jellyfish