NIWA goes shark hunting

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Scientists are playing catch with rig sharks this week, in Porirua and Pauatahanui Inlets. They hope to trap and catch upwards of thirty sharks. This fisheries assessment work, funded by the Ministry of Fisheries, will help develop the best methods for a planned nationwide rig nursery-ground survey.

Rig, also called lemon fish, are better known as the fish in ‘fish ‘n’ chips’.

The scientists set sail in a small inflatable boat. They will be comparing catches in traps and set nets. The research work will occur in the large shallow basin part of each inlet.  These basins are about two metres deep and are known for their abundance of rig sharks, which are born in spring at a length of about 30 cm, and then remain in the estuary until late autumn.

NIWA’s fisheries principal scientist Dr Malcolm Francis says, “about a dozen traps and one set net will be laid each night. The set net will be used as a 'control' against which the catches in traps can be compared. If traps prove effective, produce catch rates comparable to set nets, and do not have too many zero catches, we will use traps for the planned survey.”

Set nets work well but the scientists would prefer to use a method which poses lower risk to other sea life. “There are some areas that we want to survey, and we can’t use the nets. Lyttelton Harbour, for example, is closed to nets, because it is an area that endangered Hector’s dolphins frequent,” says Dr Francis.

The rig will be caught, measured, sexed, and the scientists will also make a note of any distinctive umbilical scars which help confirm that they are new-born. Live rig will be released.

Rig are found all around New Zealand. There is a commercial fishery for them. They are bronze coloured with many small white spots above, and are white on their belly. They live near the sea floor in coastal waters and the continental shelf. They have flattened teeth arranged like paving stones, to form grinding plates. Rig feed mainly on animals that burrow in the sea floor, especially crabs.

“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. We are going to experiment with different sorts of bait: paddle crabs, crabs cut-in-half, and minced paddle crabs; we know older rig like them and they are an important part of their diet,” says Dr Francis.

Environmental data will be measured at each site, including water column characteristics of temperature, salinity, dissolved oxygen, and turbidity.

“We want to know why they like the areas that they inhabit, and what risks they face from fishing, heavy metal pollution, dredging, and sedimentation,” says Dr Francis.

Rig make seasonal migrations into coastal waters in spring and summer. If the scientists can identify the preferred habitats of newborn rig, then they might be able to predict and map other areas of suitable habitat.

Dr Francis will be analysing all available information on the geographic location of juvenile rig to develop a working hypothesis of which estuaries, harbours, and in-shore regions are most important as rig nurseries. Geographical gaps in that distribution will be examined with the aim of determining whether any nurseries occur in these areas. This overview will then be used to design a nationwide rig nursery survey.

In that nationwide survey, estuaries and harbours will be ranked in order of importance based on the rig population density estimates, and a basic assessment of the amount of available rig habitat within each estuary. There are five managed stocks of rig around New Zealand, but the relationship between the adult populations and their nurseries are poorly understood. It is hoped that this survey will improve our knowledge of the links between the juveniles in estuaries and the adult stocks found in more open coastal waters, which will contribute to the sustainable management of the fisheries.
 
On this survey, Christy Getzlaff from Massey University will also be collecting rig specimens for her study on the food and feeding of newborn rig.

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Principal Scientist - Fisheries
Rig, also called lemon fish, are better known as the fish in ‘fish ‘n’ chips’.