You may be surprised to know that, beneath the calm waters of Pāuatahanui Inlet, there is lurking a species of shark that can attain a length of 1.5 meters when fully grown. It uses the shallow, sheltered waters of our harbour as a nursery, gathering throughout the months of October to March to mate and give birth. It is not one of those large, dangerous, species that are the subject of books and movies but, instead, a commercially very important fish that most of us will have made part of our diet at some time in the past.

Profile

The rig is a shallow water shark of the smooth-hound genus, Mustelus, of which our species – Mustelus lenticulatus is otherwise known as spotted estuary smooth-hound.

This shark is only found in New Zealand but is very common throughout our coastal waters, particularly in shallow bays and estuaries. Generally bronze in colour on the upper surface, with numerous blue and white spots, it is characterised by having two dorsal fins and an anal fin. The bottom surface is white.  The teeth are flattened and arranged like paving stones to form grinding plates designed for crushing.  The rig feeds mainly on animals that burrow in the sea floor, especially Crustacea such as crabs.

The main diet is the species Hemiplax hirtipes, (stalk-eyed mud crab), constituting up to 95% of the total taken, with Austrohelice crasa (burrowing mud crab) making up most of the rest. Feeding is accomplished by sucking up mouthfuls of sea floor sediment containing the burrowed animals.  The shark separates out the prey by ejecting the mud and sand through its gills, leaving the animals behind to be crushed and swallowed.

This feeding habit also enables a rig to maintain respiration without the need to keep moving, as is the case with many sharks.  Unlike its relatives of the open sea, the rig can keep the flow of water moving over its gills while remaining relatively still in the shallow waters of the estuaries.

Sharks can sense the presence of prey using the sensitive Ampulae of Lorezini, a collection of tubules that open to the sea by small pores located around the snout. This organ is electrically sensitive to the muscular activity of the small Crustacea helping the rig to locate a buried population of invertebrates with little difficulty.

During the much of the year most adult rig spend their lives in open waters, hugging the seabed, but during spring and summer, mature females, 4-7yrs old, make inshore migrations, congregating in sheltered, shallow, harbours and estuaries to spawn and mate. Males are seldom found in these areas.

Unlike many shark, rig are viviparous, that is they give birth to live young with the eggs maturing within the body of the female over a period of 11 months after fertilisation. The young are born at a size of 20 to 30cm, with each female producing an average of 11 offspring. This occurs during the months of October to December with the new-born fish staying in the Inlet, possibly through to April or June. The young grow rapidly, reaching maturity in 5-8 years, with a natural lifespan of over 15.

After spawning, adults remain in the harbour to mate during the remainder of the summer months and it is known that several males can mate with one female, i.e. they are polygamous. By the end of February most adults have returned to the open sea. During the next spring males tend to return to the same estuary and, therefore, are local to the Inlet, while females can come from a wider area of the coast.

Rig are caught in commercial quantities all around New Zealand. Although they are landed throughout the year, the largest catches are made between October and March.

Rig breed successfully only in harbours and sheltered inlets of the North Isalnd. Porirua Harbour is one of these. While not as important as Kaipara and Raglan harbours, Porirua is now known to be of high value as a nursery for first and second year fish.

Pāuatahanui Inlet is one of the most important spawning grounds in central New Zealand and the overall population of rig is estimated to be between 4 and 6 thousand individuals over the course of a year, peaking during the summer months.

Genetic studies tell us that all rig belong to one common stock and that females do not return to their own place of birth to deliver their young. 

Thus rig found in the South Island must have migrated there from the north during their early years.

Commercial Importance and Research

As part of New Zealand’s coastal ecology, rig shark is a highly important inshore commercial fish species, less prone to overfishing than most shark. You will probably know it as ‘lemon fish’ and, under this name, have eaten it as the ubiquitous serving of ‘fish and chips’.  ‘Spotted dogfish’ is another common, but false, identifier as true dogfish are a different order of shark. Rig are taken mainly by set net and bottom trawl and there are six managed stocks of rig around New Zealand shores with three within 40km of Porirua harbour. This makes Pāuatahanui Inlet a highly important spawning ground for rig. Yet, despite its importance to the fishing industry, migratory movements of this shark have been poorly understood in the past and it has been difficult for the fisheries to be managed optimally.

To this end, Warrick Lyon (Institute of Marine Science, Auckland University, and Research Technician, NIWA) has been conducting research into population patterns over several years, throughout the spawning season. A technique using ‘mark-and-recapture’, with GPS technology as the primary tracking method, has been developed by Warrick and Peter de Joux of NIWA with the intention of monitoring rig in real-time with fine-scale accuracy. This is a world first as previous research has used acoustic receivers only and these are not as accurate in providing locational data. For the new research technique a GPS receiver is attached to one dorsal fin of a shark via a 7m monofilament so that it floats on the surface of the water and continuously records position. The information is then relayed to a number of receivers placed around the Inlet that pass the data to a land-based computer. At this hub the information is collated and analysed. Much of the information gleaned by this research has gone into the above description of the rig’s lifestyle, an achievement that has been made possible with financial support given by NZ Marine Research Foundation.