Reducing Uncertainty in Pelagic Fish Assessments: Analysis of Hydroacoustic- and Trawl-Derived Data

To better understand, and subsequently reduce, uncertainty in pelagic fish population data, analyses were conducted on simultaneously-collected trawl and hydroacoustic data from over 200 sampling events in Lake Champlain, a mesotrophic freshwater lake located between New York (USA), Vermont (USA), and Quebec (Canada). A model set, which used a number of factors to explore sources of variance, was analyzed. Factors explored were acoustic density estimation method, sampling time, sampling season, water temperature, light, invertebrate presence, fish size, fish species, and collection depth. Trawl avoidance was higher during the day, and varied with trawl type. Sources of variance in hydroacoustic estimates were the size and depth of analysis regions, presence of invertebrates, and fish shoaling behavior. All fish species could not be differentiated using acoustic target strength, thus necessitating trawl collections to estimate species-specific abundance. Trawl collection showed vertical separation of adult pelagic species in Lake Champlain, and suggested side- or up-looking hydroacoustic and surface gillnet surveys may be necessary to reduce uncertainty in abundance estimates. Surveys using multiple sampling methods likely result in abundance estimates closest to reality.