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The following techniques are used with fish: gill nets
Gill nets Description: comprise a wall of netting (usually nylon) suspended by floats and weighted on the bottom, and anchored to a fixed object. There is a range of mesh sizes that dictates the size of fish caught. Nets need to be continuously monitored otherwise a large proportion of the fish may die. Seine nets Description: similar to a gill net (nylon filaments with floats and weights) but is hauled or dragged through the water to encircle an area containing fish. Mesh size dictates the size of fish caught. Larger mesh sizes are easier to handle as there is less drag through the water. Seine nets are difficult to use in rough bottom environments and areas with dense vegetation. Fish traps Description: usually square or round box of mesh with a funnel entrance at one or both ends. Placed on bottom and fish swim into them but are unable to exit. Bait attracts different types of fish. Electrofishing Description: a device passes an electric current through the water to either stun the fish (AC operation) or attract the fish towards a net (DC operation). The stunning operation requires good visibility and quick response in order to collect fish before they sink or revive and disappear. Electrofishing requires waterbodies with high conductivity to be effective (many Top End water bodies have low conductivity for much of the year). Pop-net traps Description: a square enclosure of netting with a float frame at the top and weighted frame at the bottom. Top and bottom are bound together by a trigger-strap. The net is placed on the bottom, in a shallow area with dense vegetation. An operator waits by the net until a sample of fish settle over the set trap, and then the net is triggered by pulling a rope. The captured fish are then removed by hand net. Visual census Description: where water clarity is good, visual counting can be done from below or above the water. A prior knowledge of species identification is necessary. Differences between observers may create problems in quantifying data. Video Description: underwater video towed behind a small boat. More appropriate for use in ocean environments but may be suitable for large water bodies with high clarity. Processing of data post-sampling may be time consuming. Sonar Description: used mainly in large open water bodies. Requires detailed calibration to be able to identify individual species. Poisons Description: natural plant toxins are widely used by Aboriginal people. Rotenone, a derivative from derris roots, is widely used for fish sampling. A big disadvantage is that it kills everything in the area being sampled.
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