For example, in extremely low-conductivity water (less than 20 microsiemens per centimeter), electrofishing is ineffective. In sampling reaches that do not contain riffles but do contain a large number of woody snags, debris, or other obstructions, seining may be ineffective. In these cases, other sampling methods may be necessary. The decision to substitute sampling methods is based on factors such as site conditions, consultation with local fish ecologists, and State regulations regarding the use of other sampling methods. The most commonly used other sampling methods for collecting a representative sample of fish include gill netting and hoop netting.
Gill netting is the capture of fish by entanglement in a fabric mesh that is not actively moved by man or machine. Although gill netting requires little skill or special training, it requires two trips (one to deploy the net and one to collect the fish ) and has a potential for vandalism of the gear. Gill netting also kills many fish caught in the net or injures fish upon removal. For this reason, gill netting must not be conducted in areas where endangered, threatened, or special-concern species may be present.
Gill nets are composed of fabric mesh attached to a lead line and float line. Gill nets vary in material, mesh size, and type, all contributing to the selectivity of the gear. Gill net mesh can be made of cotton, linen, nylon, and monofilament or mult ifilament twine. However, monofilament and multifilament mesh are considered superior to other materials because they are less visible to fish, easier to clean, and more durable. Mesh size is generally expressed as bar measure or stretch measure. Bar measure is the distance between knots. Stretch measure is the length of a single mesh when the net is stretched taut. Mesh sizes vary with the three types of gill nets available--standard, trammel, or experimental.
Standard gill nets are composed of a single panel of mesh of one size. Because standard gill nets are constructed of one mesh size only, they tend to be selective for individuals of the same size or in some cases a single species (Hubert, 1983). Therefore, standard gill nets are generally not effective for collecting a representative sample of the fish community.
Trammel nets are composed of three panels of mesh suspended from a float line to a single lead line. The two outer panels of mesh are constructed of a larger mesh size than the inner panel. Although trammel nets are less size selective than standard gill nets, trammel nets tend to be selective for fish species with rough surfaces and protrusions, such as sturgeon, catfish, or temperate basses (Hubert, 1983).
Experimental gill nets usually have several 4.5- to 15-m panels of various mesh sizes, thus reducing the potential for size selectivity. Unlike trammel nets, experimental gill nets are less selective for any particular groups of species. Experimental gill nets 1.8-m deep with six sections of monofilament mesh 7.6-m long each, using 13-, 25-, 38-, 51-, 76-, and 102-mm mesh size (bar measurements) are used to collect a representative sample for NAWQA (fig. 3).