Hammerhead sharks in north waters

The shark with the hammer-shaped head is a big eater and is potentially dangerous to humans.

Sphyrna zygaena, otherwise known as the hammerhead shark, and kakere or mango pare in Maori, is found in New Zealand coastal waters up to 110m deep, and on the continental shelf. It is more commonly seen around the North Island.

A few attacks on humans have been recorded, but they are very unusual.

"They bother spear fishermen, but they don't like the sound of scuba divers' bubbles," says National Institute of Water and Atmospheric Research (Niwa) shark expert and principal scientist Dr Malcolm Francis.

But what is that strange hammer-shaped head all about?

"It's thought that it might provide lift as the shark swims, by counterbalancing the downward thrust of the tail," Dr Francis said.

"If you come face to face with a hammerhead, you'll see the eyes and nostrils are on the tips of the 'hammer'. This wide spacing helps it to see its prey better and find food by smell."

Hammerheads have an extra sense. Scattered around the hammer are numerous pores connected to nerves. These detect the weak electrical fields created by the muscles of the shark's prey, including fish, shrimps, squid and crabs that bury themselves in the sand.

Hammerheads are olive grey to dark grey on top and white on the belly. They grow to 4m in length. The median fins are darker edged and the pectoral fins are sometimes dusky or dark tipped on their lower surfaces.

The teeth are directed sideways, creating a serrated edge for cutting food. This shark is a voracious feeder with an appetite for a wide variety of fish and other animals. It is social and sometimes travels in groups of 100 or more.

The hammerhead shark gives birth to live young. They are retained within the mother until fully formed and then born in shallow bays in litters of 20-50 pups, each about 55cm long.

The main source of information on hammerhead sharks in New Zealand waters is from research trawl surveys and observer records from commercial fishing vessels.

"There are several other tropical hammerhead species that occur in warmer waters north of New Zealand, and some of these probably turn up in New Zealand occasionally," Dr Francis said.

Juvenile hammerhead sharks up to 1.5m long have been caught around the entire northern North Island, with most captures in shallow water around the Firth of Thames, Hauraki Gulf and eastern Bay of Plenty.

This suggests they use warm coastal waters as a nursery ground. Large mature adults are uncommon in coastal waters, but sub-adults are common off the North Island's west coast beaches.

Ninety Mile Beach is notable for the number of juveniles offshore, but the young of the species are also widely distributed in the Bay of Plenty, East Cape, north of New Plymouth and Napier.

Supplied by Niwa