Fish Avoidance of Hydroacoustic Research Vessels in Lake Erie

Fish avoidance of large research vessels via vertical and horizontal movement is a well-documented phenomenon in marine environments. We assessed fish avoidance of three vessels (55 m, 21 m, and 7.6 m) simultaneously collecting hydroacoustic data using 120-kHz BioSonics transducers and DT-X echosounders along three parallel transects spaced 0.5 to 1 km apart in central Lake Erie on three consecutive nights in July 2014. Data collection followed Great Lakes Standard Operating Procedures (GLSOP). We performed two sets of analyses: one following GLSOP for echo integration, and a second with SD of beam angles increased to 3.0 (standard specifies 0.6). Values for ABC, mean number of single targets, mean target strength, and mean density were very similar for the two larger vessels regardless of SD of beam angles. For the smallest vessel, large targets were not captured at deeper depths when SD=0.6 was used for single target identification. We speculate vessel and transducer movement given vessel speed, water depth, and pulse duration resulted in high variation that caused restrictive ST detection parameters to fail to detect STs at depth. We conclude little evidence of differential vessel avoidance, but that standard analysis methods must accommodate vessel-induced restrictions on detecting single targets.