Nationwide rig shark survey completed

Scientists from NIWA and Massey University have surveyed rig shark (aka lemon fish) nurseries in 13 estuaries nationwide. They also recorded a number of environmental parameters, which will help scientists understand more about the preferred habitats of rig sharks.

Rig is an important commercial fish. Currently, we don’t know whether the species inhabits entire estuaries, or just portions of estuaries. There are five managed stocks of rig around New Zealand, but the relationship between the adult populations and their nurseries is poorly understood. It’s 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.

Ways of catching the sharks

Last December, NIWA principal fisheries scientist Dr Malcolm Francis tested two methods of catching juvenile rig in Porirua and Pauatahanui Inlets. He and colleagues caught a small number of juveniles (and many adults) in set nets, but no juvenile rig in fish pots, despite considerable fishing effort.

They concluded that set nets rather than fish pots were the only effective method of catching juvenile rig. Unfortunately, this means that Lyttelton and Akaroa Harbours, where set nets are banned to protect Hector’s dolphins, cannot be surveyed for juvenile rig.

Survey locations

The following thirteen estuaries and harbours around New Zealand were surveyed:

Kaipara Harbour (northeastern arms)
Manukau Harbour
Upper Waitemata Harbour
Tamaki Estuary
Raglan Harbour
Tauranga Harbour
Porirua/Pauatahanui Inlets
Whanganui (Westhaven), northwest Nelson
Farewell Spit/Golden Bay
Waimea/Mapua, Nelson
Pelorus and Kenepuru Sounds
Blueskin Bay, Otago
Otago Harbour

The Ministry of Fisheries and NIWA identified these estuaries and harbours as being potentially important rig nurseries – either because they have known populations of rig or contain possible rig habitat, based on earlier surveys. This survey included some estuaries that haven't been sampled before, carried out more days of fishing, and set more fishing gear each day than previous surveys. The aim was to obtain a more representative and comprehensive assessment of the use of estuaries as rig nurseries.

Measuring environmental data

As well as measuring, sexing, and recording any newborn rig caught, environmental data were measured at each site. These included water column characteristics of temperature, salinity, acidity (pH), 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.

Early findings

Scientists found that rig sharks were most common in Kaipara and Upper Waitemata Harbours, and the Tamaki Estuary. Numbers were lowest in the South Island sites surveyed.

Analysis of data is underway, and a full report is due later in 2011. The work was funded by the Ministry of Fisheries.

Contact: Dr Malcolm Francis

Watch a video of a juvenile rig shark being caught in Porirua Harbour, and read the media release, including background information on rig sharks.