Shark maturity has a ring to it

By SIMON COLLINS

Sharks may reveal their age in a similar way to trees - by the rings in their backbones.

Auckland University student Silver Bishop says the rings in the cartilage that forms the sharks' backbones can tell you the story of their lives.

"Each year there is some kind of seasonal change," she says.

"If it's really hot, it means plenty of food and they will suddenly grow much denser, laying down calcium and strontium. That will be a band.

"But you can't assume that it's a year because it's really ambiguous."

Ms Bishop, 23, is into the second year of a master's thesis on mako sharks - the first detailed New Zealand study of what has been described as the world's most graceful shark.

She has collected samples from 140 mako caught in fishing competitions, ranging in size up to 4m for females and 2.5m for males.

Each fish is dissected on the spot. She cuts a 3-6cm chunk out of its backbone, or vertebrae, then takes a 0.6mm-thick section, photographs it with a digital camera and sits at her computer for hours counting age bands. She aims to study 50 more samples before completing her thesis next summer.

Ms Bishop is still not sure whether the ring-dating technique will work with mako, because it has been used overseas only with some shark species.

But already she has observed that the rings become closer together as the sharks age, suggesting there is less growth.

The technique does not work with fish that have bones instead of cartilage in their vertebrae.

Ms Bishop's study, supervised by the National Institute of Water and Atmospheric Research (Niwa) for the Fisheries Ministry, will provide the basic data needed if sharks are ever brought into the fishing quota system.

She will describe her work at the university's "Incredible Science" open day for primary and intermediate pupils and their parents on Monday.

Born in Australia, Ms Bishop moved to New Zealand at the age of 3 and grew up spending summer holidays at Great Barrier Island.

"I used to do a whole bunch of snorkelling at a very early age. I loved it, but I was terrified of sharks.

"Whenever I saw a shark, I got straight out of the water."

She overcame her fear when swimming with well-fed grey nurse sharks in an aquarium at Mooloolaba, Queensland. Sharks are now "just amazing - how ancient they are and how different they are ... "

"Yeah, they are dangerous creatures, but ... I don't think they are as aggressive and bloody-minded killers as people make them out to be."

The aquarium curator at Kelly Tarlton's Underwater World, Andrew Christie, said more than 100 million sharks were caught commercially each year - an unsustainable toll.

www.auckland.ac.nz/science