Sharks swimming round the shallows of Porirua and Pauatahanui inlets are in the sights of scientists setting overnight traps for them.
National Institute of Water and Atmospheric Research scientists have been catching juvenile rig sharks in the past two weeks in preparation for a nationwide survey.
Commonly referred to as lemon fish, rig sharks are best known for often turning up as the fish in fish and chips.
Niwa fisheries principal scientist Malcolm Francis says researchers are setting out each day on an inflatable boat and catching the fish in the large shallow basin of each inlet.
"We want to know why they like the areas they inhabit, and what risks they face from fishing, heavy metal pollution, dredging, and sedimentation."
Rig are known to be abundant in the shallow basins of both inlets, and after being born in springtime the sharks remain in the estuary till late autumn.
Dr Francis says the sharks are about 30 centimetres long when they are born. They feed mainly on animals that burrow in the seafloor, especially crabs.
"They have flattened teeth arranged like paving stones, to form grinding plates."
The researchers set about a dozen traps and one set net each night, comparing the catch from each.
They caught "about a dozen" juveniles, and were testing the effectiveness of the traps in comparison to the set nets, in preparation for the planned nationwide survey, as traps posed lower risk to other sealife.
ONCE caught, the rig will be measured and sexed, and notes taken of any distinctive umbilical marking scars that help confirm they are newborn.
Scientists are also experimenting with different bait to catch the rig, trying paddle crabs, crabs cut in half, and minced paddle crabs. Dr Francis says the crabs are an important part of the diet of older rig.
They detect crabs and worms by swimming close to the sea bottom, using their nostrils and electromagnetic sensors on the underside of their snouts.
In the nationwide survey, estuaries and harbours will be ranked in order of importance based on rig population density estimates and an assessment of the amount of available rig habitat within each estuary.
Rig are commercially fished all around New Zealand.
Dr Francis hopes the survey will help improve knowledge of the links between juveniles in estuaries and the adult stocks found in more open coastal waters, and contribute to the sustainable management of the fisheries.
- The Dominion Post
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