By Alison Ballance
It’s in the middle of our capital city, but Frank Kitt’s lagoon on the Wellington waterfront is home to a remarkable marine wildlife spectacle, with up to a dozen eagle rays congregating in the lagoon during summer.
“We have three common rays in New Zealand,” says NIWA shark expert Malcolm Francis. “Two of them are stingrays, the short-tailed and long-tailed stingrays and there’s the eagle ray. Eagle rays are quite easy to identify – they have pointed wings and swim by flapping their wings up and down. The stingrays have more rounded wings, and they swim by undulating the wings.”
So, just remember that eagle rays flap like a bird, while stingrays undulate.
“They’re quite broad in their dietary requirements,” says Tim Riding, a marine biologist with the Ministry for Primary Industries. “They’ll eat crabs and bivalves, and they’ll eat them off the rocks or dig them out of the sediment in estuaries and harbours but they’re also detritivores, and they’ll consume things like dead kahawai and other dead stuff floating around, so quite opportunistic.”
Malcolm adds that “in a lot of the northern harbours, where there’s a large area of sand flats, you can see at low tide that they’ve excavated pits that are about 30 cm across. And that they do that by taking in water from the top of their head, through little holes called spiracles at are behind the eyes, and they jet that out through the gills that are on the underside of the body. And that just creates a water flow that winnows away the sand, and exposes any shellfish or other invertebrates that are sitting in the mud.”
Tim carried out research for his Master’s degree tracking eagle rays in small harbours on Northland. He wanted to find out how the rays navigate in these shallow, narrow and often very muddy or sandy water bodies, where visual cues wouldn’t work. He decided that the sharks were using a “rheotactic response – so they were essentially orientating to water flows, coming into the estuaries on incoming tides and leaving on the outgoing tide.”
Both eagle rays and stingrays frequent the Wellington waterfront, but it’s usually only eagle rays that come into Frank Kitts lagoon. The eagle rays seem to arrive in October or November, and stay in the area until the water begins to cool in about April. They have been seen feeding, and also mating. It is not known where the eagle rays go in winter. Genetic studies show that the population of short-tailed stingrays in northern New Zealand is distinct from the population around the lower North island and Marlborough Sounds.
The largest eagle rays that Tim has seen had a wing span of 1.5 metres, while Malcolm comments that “historically, short-and long-tailed stingrays have been recorded at up to 4 metres long, including the tail. Those animals were massive, presumably decades old although we don’t know how long they live for. We don’t see those big old animals anymore, so we’ve probably removed them from the population by catching them, particularly in nets. It’s a bit of a concern, that the smaller rays just don’t get an opportunity to become big old rays anymore.”
Orcas, dolphins and hammerhead sharks are known to hunt rays. As a defence mechanism rays have a barb at the base of their tail, which is covered with proteinaceous slime. If a ray feels threatened by a shadow from above it will strike with the barb, and if the barb punctures the skin of a human it can cause an unpleasant wound. However, Malcolm is quick to point out that rays are not aggressive and only do this in self-defence. Tim suggests a good way to avoid accidentally standing on a ray in shallow murky water is to shuffle rather than walk.