A multi-frequency acoustic method to estimate mean standard length of juvenile salmon in the Discovery Islands, British Columbia

Abstract

A method to estimate the mean standard length (L¯) of juvenile salmon in aggregations using multi-frequency (67 and 125 kHz) acoustic analysis was developed. An autonomous, single beam echosounder (Acoustic Zooplankton and Fish Profiler (AZFP), ASL Environmental Sciences) was deployed on the seafloor in Okisollo Channel, British Columbia during peak juvenile salmon migration season in 2015 and 2016. A positive correlation was found between the difference in the mean volume backscattering strength at 67 and 125 kHz (ΔMVBS67-125kHz) of juvenile salmon aggregations and the mean standard length of juvenile salmon collected with a purse seiner. A relationship of the form ΔMVBS67-125kHz=Alog10L¯+B was established. High variability was observed due to differences in salmon species composition and lower sampling sizes at the tail end of the migration periods, but a consistent relationship with comparable parameters was found. We suggest that this method is valuable particularly in the case of long-term monitoring of species that are known to aggregate in schools of individuals with similar size, as it can be used to estimate mean size without the use of invasive capture techniques.