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OPTION 3 - PLANIMETRIC MAPPING The mapping method may be used either for individuals or groups of fish, and can be used to study several species at once. Large scale hydraulic contour maps with isolines of equal depth, velocity, substrate, and cover are individually prepared for each sampling or observation reach (Figure 7). These contour maps are prepared for each discharge at which fish were sampled or observed. Sampling techniques amenable to this option include electrofishing, spot concussion, spot poisoning, hook and line, trot line, seining, drifted gill nets, and batteries of strategically placed, baited hoop nets. Stationing gill nets or hoop nets with leads should be avoided because they sample fish in transit, not at rest. Problems associated with baited hoop nets will be discussed in the section entitled Sampling Bias. During each sampling run, or for a sampling period, the locations of individuals or groups of fish at each discharge are marked on blank maps of the study reach drawn to the same scale (Figure 8). The distri- bution and numbers of fish observed or captured at each discharge are then entered onto the appropriate location on the contour maps. Hydraulic parameters for each fish location may then be read directly from the contour maps. SAMPLING BIAS Hunter (1973:p. 2) warns that "... measurements for ... game fish should be selected from an area that presents a variety of hydraulic conditions to the fish. Unless the fish is given a choice of water depths, velocities, and gravel size, the measurements taken will reflect the hydraulic conditions available in that specific stream and not the conditions ... which the fish would really prefer...". In the determination of biological criteria, and particularly probability-type criteria, the minimization and avoidance of sampling bias is critical. The type of bias described by Hunter may be overcome quite simply by sampling areas with a wide variety of conditions avail- able, by sampling several streams, or both. However, sampling bias may also occur as a result of gear selec- tivity, sampling technique, or observation technique. Several examples are given below to warn of potential bias problems with different col- lection and observation techniques. 25