Sound sees more: A comparison of imaging sonars and optical cameras for estimating fish densities at artificial reefs

Imaging sonars are increasingly being utilised in fish surveys in conjunction with or as substitutes for optical instruments. To justify the use of imaging sonars, we must first describe their application, limitations, and efficacy compared to optics. This study compared quantitative data of fish assemblages obtained using imaging sonars operating at four frequencies (0.75, 1.2, 2.1, and 3 MHz) with simultaneous optical camera footage at two artificial reefs. Fish densities were on average three times higher for sonar than optics. Greater detection by the sonar was attributed to site-attached fishes that were camouflaged against the artificial reefs or the adjacent seabed, which could be discriminated by imaging sonar, but not using optics. This suggests that differences in habitat and fish assemblage composition could influence the relative performance and density estimates of imaging sonar versus optics. Several limitations of imaging sonar were identified that need to be accounted for in future survey designs, including: discriminating fishes from benthic growth; an inability to detect fishes within complex habitat structures; and seabed and side-lobe interferences that truncate survey volume. Overall, this study demonstrates the value of imaging sonar for quantifying fish communities and describes limitations and recommendations for their deployment in future surveys.