Using a baited imaging sonar (BISON) to quantify the density, size, and detection range of fishes in a shallow, nearshore habitat

Methods that supplement optical instruments with bait, such as baited remote underwater video (BRUV), are used worldwide to detect and quantify marine life. Optical instruments only detect targets within visible range, such that BRUVs may underestimate fishes in light-limited habitats, especially fishes that respond to the bait at ranges beyond visibility. Alternatively, light-independent instruments (e.g., imaging sonars) can detect and quantify fishes regardless of visibility. This study presents the first application of a baited imaging sonar (BISON), deployed to survey fishes around a small, shallow artificial habitat in a turbid embayment in southern Florida. To establish the influence of bait on fish detection, BISON trials were alternately conducted alongside deployments of an unbaited control, with a high-definition camera integrated to ascertain visibility and inform species composition. For fishes of two size classes, larger (> 30 cm) and smaller (10–30 cm), maximum density (MaxD) and range of detection were quantified. Although the densities of larger and smaller fishes quantified by the BISON and unbaited control did not differ, over 55% of larger fishes were detected at ranges beyond maximum visibility, with asymptotes in fish density on the BISON identified at 15–20 min and 5–10 min for larger and smaller fishes, respectively. Overall, this study demonstrates the potential of BISONs as both a complementary and alternative method to BRUVs for quantifying fishes, especially in habitats of limited visibility. Future applications of BISONs in other habitats will further demonstrate its value as a tool to detect and enumerate aquatic assemblages.