Reducing bias in Coregonus artedi abundance estimates using stationary up-looking acoustics

Abstract

Mobile hydroacoustic surveys using ship-based down-looking transducers are widely used to estimate densities for ecologically and economically important pelagic fishes. However, this method likely underestimates densities of some surface-oriented species due to biases associated with the acoustic surface exclusion zone and ship avoidance behaviours. We compared cisco (Coregonus artedi) density estimates from a stationary up-looking platform survey to a standard down-looking acoustic survey. Both systems were deployed during the fall cisco spawn in Thunder Bay, Lake Superior 2020–2022. Cisco density estimates from the stationary up-looking platform were on average 6.7 times higher in the upper water column (∼1–10 m) and 2 times higher over the entire water column (∼1–45 m) than those from standard mobile surveys. Ship avoidance behaviour associated with mobile surveys was apparent in the upper water column; median cisco densities observed by the platform fell from ∼36 to ∼9 fish/ha when the ship passed near the platform. Abundance estimates from the platform when not influenced by ship avoidance provided higher quota estimates than the standard survey in 2020 and 2022, but were similar in 2021. A multi-day deployment of the platform tracked a progressive daily increase in fish densities, highlighting the sensitivity of mobile survey results to the day they are conducted, often dictated by environmental conditions. Our results show promise in applying stationary acoustic deployments in fisheries surveys, with improved accuracy and reduced effort compared to mobile acoustic surveys in the management and monitoring of pelagic fishes in the Great Lakes.