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nets at both the upstream and downstream boundaries to isolate the sampling reach is not required to collect a representative sample of fish. If blocking nets are used, they must be used in the same manner during repeated sampling of the reach for assessment of temporal trends.

A different electrofishing operating technique is required in areas of habitat complexity. Fish congregating near habitat features are generally dispersed by continuous application of electricity or they are difficult to remove once stunned by exposure to the electrical field. An effective technique for capturing fish associated with habitat features is to approach the habitat feature with the electrical current off. The anode is thrust close to the habitat feature, the electrical current is generated, and the anode is withdrawn in a sweeping motion away from the habitat feature. The fish response to this procedure is galvanotaxis, creating the effect of "pulling" the fish away from the habitat feature and facilitating their capture.

Sampling in riffle areas requires the operator to sweep the anode across the riffle from upstream to downstream while walking in an upstream direction. Crew members with dip nets should be positioned downstream of the operator to allow the flow to carry stunned fish into the net. This minimizes escape and avoidance of the electrical field by fish species, such as darters and sculpins, that commonly inhabit riffles.

All captured fish are placed immediately in either a holding box or live well for later processing. A holding box is usually a net box (generally 1.2 m x 1.2 m x 1.2 m) used to hold fish. Because fish placed in a net holding box are held in ambient water, mechanical aeration is not necessary. However, in streams with high water temperatures and low dissolved- oxygen levels, fish placed in a holding box may become stressed and die as a result of crowding under poor water-quality conditions. Under such conditions a live well is preferable, but mechanical aeration and frequent changes of water are required. If a holding box is used, it should not be placed within the sampling reach where the fish being held could potentially be re-exposed to the electrical field. In either case, all fish are processed immediately following completion of the electrofishing pass and released downstream of the sampling reach to minimize the potential for re-capture.

After the first pass is completed and all fish are processed, a second pass is conducted in the same manner as the first. The second pass is generally, but not always, conducted through the same area as the first pass. For example, in a braided stream, a first pass can be conducted through one channel and the second pass through another channel.

Nonwadeable Streams

Nonwadeable streams are sampled using electrofishing boats. Electrofishing boats vary in design but usually consist of a gasoline-powered generator and an electrical output control mechanism in an aluminum boat. The electrical configuration of the boat also varies. However, generally the boat is configured as the cathode, with anode arrays consisting of single (stainless steel cable), circular (hollow stainless steel ball), or multiple (several stainless steel cables) configurations.