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Different techniques have been employed for fish surveys in freshwater impoundments, however, despite the increasing applicability of underwater video systems for fish sampling in different aquatic ecosystems, such technology has not been tested in reservoirs. This study compares estimates of fish species richness, community composition, and size structure derived from visual census (VC) and underwater video (UWV) in two Neotropical reservoirs. The total number of species recorded by both sampling methods was similar, even though UWV recorded higher mean number of species than VC, in both reservoirs. In general, VC was more efficient in recording cryptic and resident species, while UWV, was better for highly mobile species. Considering size classes, VC was more efficient than UWV in detecting smaller individuals (i.e., 31–79 mm total length). Although observational sampling methods depend on water transparency, UWV can be considered a complementary and promising technique for fish surveys in shallow areas of clear reservoirs.

A visual census technique is described in which the results of three separate enumerations of fish at a site are combined to produce a ‘best estimate’ of the fish fauna present. Its precision and accuracy are examined, and compared to those of censuses obtained by modifications of the technique. Visual censuses can display high repeatability, but they seldom (if ever) completely sample the fish present at a site. Accuracy varies with technique used. In our tests, the preferred method yielded 82% of species and 75% of individuals known to be present and potentially censurable at the time the observations were made. Visual censuses are of comparable accuracy to ichthyocide collections of unenclosed sites, but the two methods sample different components of the total fish fauna. It is important when using visual censuses to remember that their accuracy is not 100%.

Relative abundances for 80 species in a fish assemblage were estimated using three visual assessment methods. Results of two species-time methods, the Rapid Visual Census (RVC) of Jones & Thompson (1978) and a new method, the Visual Fast Count (VFC), are compared to the traditional transect method. The VFC is similar to the RVC but it is an attempt to improve: on the quantitative nature of the results. Results from each method were significantly correlated, but important differences between methods were found. With an equal number of replicates both RVC and VFC methods yielded significantly more species than the transect method. Relative abundances estimated by the RVC were significantly different from those determined by either VFC or transect methods, but VFC and transect results were not different. A qualitative similarity index using presence-absence data was calculated and compared among methods. Values ranged from 84.3 to 85.9%, illustrating a consistent level of similarity for the faunas censused regardless of census method. Quantitative similarities (percent similarity), however, ranged from 45.8 to 85.7% and suggest a closer agreement between the transect and the VFC methods than between these methods and the RVC. If transect results are assumed to be the most accurate, the comparison suggests that the VFC method yields more accurate relative abundances than does the RVC. Ranges in percent similarity values calculated within each method (8 replicate samples) were used to compare precision among methods. While transect and RVC scores are slightly more precise (ranges of 3.10% and 2.20% respectively) than the VFC method (range = 4.89%), no method was statistically more precise than another (P