Summer Series 1: Hammerhead - the shark with a hammer for a head
The shark with the hammer-shaped head (Sphyrna zygaena) is a big eater and is potentially dangerous to humans. It has been found in New Zealand coastal waters, in up to 110 metres of water, and on the continental shelf. It is more commonly seen around the North Island.
"They bother spear fishermen, but they don't like the sound of scuba divers' bubbles," says NIWA shark expert and principal scientist Dr Malcolm Francis.
But is the hammer useful for anything? "It's thought that it might provide lift as the shark swims, by counterbalancing the downward thrust of the tail," says Dr Francis.
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 their hammer-like head are numerous pores, connected to nerves at their base. These detect the weak electrical fields created by the muscles of animals, including fish, shrimps, squid and crabs, that bury themselves in the sand "The electromagnetic pores under the head area help it detect food," says Dr Francis.
Hammerheads are olive grey to dark grey above, and white on the belly. They grow to four metres in length. The median fins are darker edged and the pectoral fins may sometimes be 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 bears 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 50 – 60cm 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," says Dr Francis.
Juvenile hammerhead sharks, up to 1.5 metres long, have been caught around the entire northern North Island, with most captures in the Firth of Thames, Hauraki Gulf and the eastern Bay of Plenty. Sharks were mostly caught in shallow coastal waters, at depths of 9 –110 metres. This suggests they use our warm coastal waters as a nursery ground.
Sub-adults are common off the North Island's west coast beaches, and have been seen in large numbers during aerial surveys.
Large mature adults are uncommon in coastal waters, suggesting that they may inhabit regions to the north of New Zealand.
Important areas for juveniles
The Hauraki Gulf, Firth of Thames, eastern Bay of Plenty and 90-Mile Beach are important for juveniles, but they are widely distributed north of New Plymouth and Napier. Other areas are probably also important, (e.g., Kaipara and Manukau Harbours) but data to confirm this are lacking.
About NIWA principal scientist Malcolm Francis
Malcolm Francis, PhD, worked for the Ministry of Agriculture and Fisheries from 1981 and joined NIWA in 1995. He is currently researching the behaviour and distribution of the great white shark and rig (spotted dogfish).