Tuna - sampling methods

Electric fishing is a method which uses electricity to temporarily stun fish, so that they rise to the surface for removal by the researcher. The fish are then placed in a bucket of freshwater, where they can be anaesthetised, identified and measured, revived, and returned to the waterway when finished.

This method is typically restricted to use in non-saline, relatively shallow waters. Electric fishing is effective in streams and along shallow shorelines but not in deep lakes and rivers.

There are a number of types of electric fishing machines currently in use in New Zealand, including:

• battery- or generator-powered backpacks - the EFM300 model is made in New Zealand
• electric fishing boats. These can be assembled from imported and/or local products. One unit, owned by the University of Waikato, is available for hire
• shore-based, mains electric fishing sets.

More information about the EFM300

A combination of single-pass (once over) and multiple-pass (exhaustive, quantitative) electric fishing sampling can be used. Electric fishing may underestimate the occurrence of some species, which may be startled and swim out of the range of the electric fishing machine (e.g. smelt).

A large variety of nets and traps are available to capture fish.

Fyke nets

Fyke nets are recognised to be size selective and therefore this technique is only suitable for collecting data from eels of a size that can be retained within the net.

Small eels escape through the mesh of standard commercial fyke nets (12 mm) and may avoid nets in which large eels are already present. Even fine-meshed fykes (4 mm) do not capture small eel size classes in expected proportions. Large fish can feed on other fish that are captured (be aware of this when doing feeding or diet studies) or prevent smaller fish from entering the net. Baited nets tend to be more effective than un-baited nets as they will attract fish from a wider area, especially in flowing water.

Typically, a series of nets or traps (e.g. 10–20, set at either a pre-determined distance apart and/or in a variety of representative habitats) are used to sample stretches of the main stem rivers and streams (where water is deep enough). Netting should be concentrated over as few nights as possible to diminish the likelihood of environmental parameters affecting catches unduly.

One way of estimating the eel population of an area is by using a depletion fyke netting method. In this method, fyke nets are set to fish overnight in a particular area for a period of three nights, although the nets are emptied each day and the catch is retained in a keep bag, to permit the numbers/species of eels to be recorded. Once the day's information has been logged, the eels are released at a location at least 5 km upstream of the nets to ensure that the same eels are not recaptured in the nets on subsequent nights. Over the course of the netting period, the population of eels in the area is expected to decrease, as eels are captured. The input of survey data to a population estimator can then be used to provide an estimate of the capture efficiency and hence population size.

Mark and recapture methods are also used around the world to estimate fish populations.

Gee-minnow traps, scoop and whitebait nets

Gee-minnow traps, hand-held scoop nets, whitebait nets, and seine and gill nets are also methods that can be used to sample freshwater fish, depending on the habitat and species of interest. Hand-held scoop nets and whitebait nets can be used to sample glass eels entering freshwater from the sea between August and December.

Night time observations of fish can be made using spot lighting equipment, where the water is clear and shallow enough to see clearly. Although this is relatively cheap and easy to undertake, there are a number of drawbacks to this technique. For example, if you wanted to quantitatively characterise the tuna population, sometimes it is difficult to see clearly, difficult to identify shortfin from longfin tuna, and some tuna may be covered by in stream debris or living under the banks.