Swimming with sharks

By Guest Author 28/01/2011

The latest in a summer series from NIWA…

The rig shark can be found all around New Zealand at this time of year, from spring to summer. It is found in coastal waters, estuaries and inlets, down to a depth of 200 metres.

Rig is most often served as the fish in ‘fish ‘n’ chips’. So if there’s a shark beside you in the sea, it’s probably in more danger than you are!

NIWA fisheries principal scientist, Dr Malcolm Francis, says some important areas that rig are found in over summer are:

• Kaipara, Manukau, and Waitemata Harbours

• Firth of Thames

• Golden and Tasman Bays.

They might be seen pretty much anywhere on the east and west coasts of both North and South Islands.

Rig underwater credit    M Francis/NIWA
Rig underwater credit M Francis/NIWA

Rig are a social fish and in spring and summer they form small groups. So you could find yourself surrounded by a group of friendly sharks, so friendly that if you put your hand in their mouth they don’t bite back. People are most likely to encounter them in estuaries and shallow harbours where the water is so murky that they probably wouldn’t even know the sharks were there.

These friendly sharks are also known as lemon fish, spotted dogfish, and smooth-hound. They grow to about 1.5 metres in length. Rig are born in October to December, at about 25—30 cm in length. They reach about 45—55 cm by April to May, before departing from shallow coastal waters out to sea. Juvenile rig live in estuaries and shallow harbours, although researchers also find them in coastal waters during trawl surveys.

Like most sharks, they have two dorsal fins. They are bronze coloured with many small distinctive white spots above, and are white on their belly. They live near the sea floor.

Rig have flattened teeth arranged like paving stones to form grinding plates. They feed mainly on animals that burrow into the sea floor, especially crabs.

Rig have an incredibly good sensory system. ’They have both nostrils and electromagnetic sensors on the underside of the snout — so they swim along with their snout close to the sea bottom detecting worms or crabs,’ says Dr Francis.

If you rub your hand over their body from head to tail, they feel smooth and leathery, but if you rub your hand towards the head, they feel rougher because they have little embedded ‘scales’ which point backwards, directing the water towards the tail and reducing drag as they swim.

Rig underwater Photo credit    M Francis/NIWA
Rig underwater Photo credit M Francis/NIWA

Rig can see well with their largish green eyes. They can probably see to the limit of underwater visibility, which may be more than 20 metres in places, but is usually only a few metres in their preferred murky habitat.

Rig are good swimmers,, and can travel incredibly long distances. One tagged rig travelled 1159 kilometres — almost the length of New Zealand – from the Snares Islands, south of Stewart Island, to Golden Bay near Nelson.

The commercial fishery catches rig in bottom set nets, and in trawl nets, with the largest catches made between October and March. They reach maturity at between five and eight years, and some live to at least 15 years old.

The Ministry of Fisheries has funded a nationwide survey of estuaries in 2011 to identify important nursery areas, including some estuaries that haven’t been sampled before. In this way, it is hoped that a more representative and comprehensive assessment can be made of the rig’s use of estuaries as nursery grounds.

Species Fact file: Rig

Māori name: Pioke, Makoo
Scientific name: Mustelus lenticulatus
Other names: Lemonfish, spotted dogfish
Size: 151 cm
Lifespan: 15 years
Diet: Crabs and other small invertebrates
Reproduction: They give birth to between 2 and 37 live young. They average 11 young per litter each year.

It has moderately fast growth and reproductive rates that make it less prone to overfishing than other shark species.

Things you need to know: They are never aggressive towards humans and are totally harmless.
Something strange: They can’t swim backwards, but they can swim upside down!

About Malcolm Francis:

Malcolm Francis, PhD, worked for the Ministry of Agriculture and Fisheries from 1981 and joined NIWA in 1995. He is currently working on the behaviour and distribution of the great white shark, Porbeagle shark, and rig (spotted dogfish).

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