Additional methods have been described for collecting fish (Bagenal, 1978; Nielsen and Johnson, 1983; Bryan, 1984) and may be considered as possible substitutes for the methods previously described. For example, snorkeling or trawling may offer the best solution to collecting a representative sample in those areas where previously described methods are ineffective. However, all additional methods should be considered only in areas where the described methods are ineffective, after consultation with local fish ecologists concerning the most appropriate method for collecting a representative sample, and in cooperation with local fish ecologists. Once methods have been chosen for sampling a reach, the same methods must be used for subsequent sampling to assess temporal trends.
Sample processing requires handling fish to collect information on taxonomic identification, length, weight, and the presence of external anomalies. Unfortunately, fish can suffer stress and mortality as a result of handling. Handling stress can be minimized and fish can be handled more easily if they are anesthetized. Quinaldine, tricaine methanesulfonate, and benzocaine are chemicals that are commonly used as fish anesthetics (Stickney, 1983). However, because of the restrictions on use of these chemicals (Schnick and others, 1979), they should not be used for NAWQA sampling. Carbon dioxide, present in carbonated water or generated from a solid tablet, serves as an effective fish anesthetic (Summerfelt and Smith, 1990). No restrictions exist concerning the use of carbon dioxide; therefore, it is the chemical used to anesthetize fish collected for NAWQA.
Fish are typically placed into a holding container filled with ambient water. Carbon dioxide (approximately 350 mL of carbonated water or two tablets containing carbon dioxide per 12 L of ambient water) is added to the water in the container. Only a relatively few fish should be anesthetized at one time to minimize any potential mortality as a result of prolonged sedation. Fish remain in the container until the desired level of sedation is achieved (about 2 to 5 minutes). Once sedated, the fish may be handled as needed but must not be kept out of water any longer than absolutely necessary.
Taxonomic identification is made only by an ichthyologist who is familiar with the taxonomy of fish species commonly found in the study unit. Taxonomic nomenclature follows that established by the American Fisheries Society's Committee on Names of Fishes (Robins and others, 1991). An attempt is made to identify all fish in the field to the species level. Measurements of length and weight and the presence of external anomalies are determined in the field for those fish that can be so identified. Uncertainty regarding identification requires that those specimens be preserved for later identification in the laboratory.
Length measurements are determined using a measuring board consisting of a linear metric scale on a flat wooden or plastic base with a rigid head piece. Fish are measured with