Common New Zealand Fishing Methods
Trawling involves towing a specialised net. Steel paravanes (trawl doors) are adjusted to "fly" through the water in opposing directions and hold the mouth of the net open. The trawl doors are attached to winches on the boat by heavy steel cables (trawl warps).
A trawl net towed across the fishing grounds catches marine life that cannot out-swim it. The net is set up to herd marine life into its mouth. Eventually, fish become worn out from swimming and end up in the narrow tail of the net (cod end). The size of net mesh controls what size fish can escape from the trawl.In New Zealand, most trawling is carried out near the bottom, and in water depths ranging from around ten metres to more than a kilometre.
Set netting involves setting a net, either in mid-water, or on or near the bottom. Set nets are made from fine nylon, so fish can't see them. They may be up to 10 metres high and several hundred metres long.
These nets catch marine life that swims into them and gets tangled. Fish bigger than the net's mesh size get tangled in the net by their gills or fins; smaller fish swim through the net.
Set netting usually occurs in shallow waters, within a few miles of the coast.
Longlining involves setting a single line with many baited hooks on it. Commercial longlines vary in length from tens of metres to several kilometres in length and can have tens to thousands of hooks on a single line. These lines catch marine predators with a mouth large enough to take in the bait and hook.
Surface longlines are usually set in the open ocean, at depths of 50-200 metres or more below the surface. Conventional bottom longlines are set along the sea floor, in water depths ranging from around 10 metres down to around two kilometres. Bottom "droplines" are set vertically, and have only a few hooks on each line.
Dredging involves towing a steel net (dredge) across the bottom. Most dredges in New Zealand are around 2-4 metres across the mouth. They catch anything that lives on the bottom and can't swim away.
Dredging usually occurs in shallow waters, between 10 and 100 metres deep.
Seining involves setting a net to surround and trap a school of fish. The net is then hauled in, and the fish captured.
Purse seining is used to catch surface-schooling fish. A small boat is used to lay a specialised net around the school to surround them. A drawstring running around the bottom of the net is then pulled tight, preventing escape out the bottom. The net is then drawn into a smaller and smaller circle. This eventually traps the fish in a tiny area, and they can be scooped out with smaller nets.
Danish seining is used to catch schools of fish near the bottom, particularly in inshore waters where trawling is not practical (eg places that are surrounded by rocks and reefs). Danish seines are like trawl nets, but with long "wings" and weighted ropes either side of the net opening. The net and ropes are set to surround the schools of fish. As the weighted ropes are drawn in, they drag across the bottom, scaring the fish towards the net. The net is then hauled up and onto the boat.
Potting involves setting a baited trap on the sea floor. These traps (pots) have one or more entrances through which animals can enter easily, but find it difficult to get back through. They also have "escape" holes that let undersized animals get out easily.
Some pots are made of nylon mesh; others are made from steel and wire.