NIWA Ocean Survey 20/20

Coastal and Marine Data Portal

Fisheries Acoustics Transects

Methods used to collect data

Beam trawl

A beam trawl is a low net with a wide mouth held open by a beam, dragged along the bottom. It cannot be used in areas which are as rocky as dredges and sleds can. Like all trawl nets, the catch washes down the net to be retained in a bag at the end of the net, the "cod end". The mesh size used determines what sorts of organisms will be retained, if the mesh is too coarse, it will let specimens of interest escape, but if it is too fine, then it is likely to clog up and even tear the net from the frame.

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NIWA’s rock dredge is a strong toothed structure used to sample reef areas where the epibenthic seamount sled could not be used because the seafloor was too rough or too hard. This dredge was towed at 1.2–1.5 kn. over a distance of about 0.5 nautical mile.

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Core Sampling

A core sample is a cylindrical section of a naturally occurring medium consistent enough to hold a layered structure, so rocks and sand can be samples, but some mud and other very soft sediments can be too fluid to retain their shape in a corer. Most cores are obtained by drilling into the medium, for example sediment or rock, with a hollow steel tube called a corer. A variety of corers exist to sample different types of substrate under different conditions.

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Shipek (Bottom) Grab

The shipek grab is used to sample sediment and benthic organisms that live at, or immediately below, the water/seafloor interface. When it reaches the seafloor a trigger system is activated and the grab sampling compartment encloses a sample of seafloor. Thus, the sample is recovered on the vessel in an undisturbed and unwashed state. This grab was used as an alternative to the multi-corer at sandy stations.

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Brenke sled

A brenke sled is used to sample free-swimming macrofauna such as amphipods, isopods, and other small crustaceans. Although it can be used at any depth, the brenke sled can only be used on even seafloor.  It has 2 fine-mesh (500 μm) nets with rigid cod-end containers, arranged one above the other. A mechanical lever on the sled bottom opens the net mouths when the sled touches the seafloor and closes them when the sled is hauled to retain the samples.

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Epibenthic seamount sled

This small sled was primarily developed to sample large epibenthic fauna on areas of rough terrain.  The mouth of the sled used on RV Tangaroa is 1m wide and 0.5m high and the sled has a short net of 25mm mesh inside a chafing of 100 mm mesh. The strong structure allows sampling on a wide range of substrate types at any depth. In this survey it was towed at 1.2-1.5kn. over a distance of about 0.5 nautical mile at gravely and rocky sampling stations.

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DTIS is NIWA's 'Deep Towed Imaging System'. It is a camera system designed and built by NIWA staff in Wellington that is being used to take pictures and videos of deep sea biodiversity and seafloor habitats. It has high definition still and video cameras, and lights, strobes, laser pointers and batteries mounted in a rectangular frame. It is attached to the ship via a conducting wire.

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