Relationships among side-scan sonar classified substrates and fish-catch rates at multiple spatial scales

Physical habitat is crucial for structuring local fish assemblages. Understanding habitat structure is important for fish management and conservation. Herein, we assessed whether sonar-derived substrate data could explain spatial variation in species-catch rates. Using a side-scan sonar, we mapped the substrates of two non-wadeable rivers in Illinois, USA and conducted standardized fish surveys at 40 sites over a 3-year period. We used four fish species from lentic and lotic guilds, with each guild represented by a large piscivore and small insectivore. For each of the 40 sites, we characterized substrate composition at five spatial scales (0.1, 0.5, 1, 2, and 5 km) and used linear regression to explain site variations in species abundance or biomass. We hypothesized that larger spatial scales would better explain the catch rates of large species, and that biomass would be better explained than abundance. The proportion of variance in fish-catch rates, explained by substrate composition, varied greatly (0.02 <= adjR(2) <= 0.74, mean adjR(2)= 0.38) with respect to species, spatial scales, and predictors used. However, we did not observe a consistent relationship between body size and the most relevant scale. Species biomass was more closely related to substrate composition than was species abundance and the best models selected based on AICc reached an average adjR(2)of 0.49 (0.25-0.74) across the four species, compared with that of 0.43 (0.20-0.64) obtained via the best abundance-based models. We conclude that substrate data obtained using side-scan sonar are useful for improving our understanding of river fish ecology.