An Acoustic-Trawl Survey of Middle St. Lawrence Estuary Demersal Fishes to Investigate the Effects of Dredged Sediment Disposal on Atlantic Sturgeon and Lake Sturgeon Distribution
Ian H. McQuinn and Pierre Nellis
Abstract.—Atlantic sturgeon Acipenser oxyrinchus and lake sturgeon A. fulvescens in the middle St. Lawrence estuary are under consideration for designation as “endangered” or “vulnerable” species. A potential threat to these species is the disposal of dredged sediments in an area where young-of-the-year and juvenile Atlantic sturgeon concentrate. The objectives of this study were (1) to study the short-term and cumulative impacts, if any, of the dumped sediments on the abundance, distribution, and movements of the two sturgeon species downstream of the disposal area, and (2) to investigate the usefulness of acoustic surveys for assessing sturgeon density and distribution. A two-phase, combined acoustic-trawl survey was conducted to describe sturgeon distributions and produce concurrent acoustic and trawl estimates of the relative abundance of demersal fish within the area (lake sturgeon and Atlantic sturgeon were presumed to dominate the acoustic echoes). This series of surveys was the first known attempt to quantify sturgeon abundance and distribution with a vertically deployed echo sounder. The results showed good correspondence between the two methods. Replicated sampling also showed the repeatability of the acoustic technique and stability in demersal fish distribution on a scale of days. The acoustic surveys indicated that the spatial distribution of demersal fishes was dependent on substrate, as these fishes avoided areas of dredged sediment dumping and associated sand dunes. Our study showed that under certain conditions, acoustic-trawl surveys can reliably assess sturgeon density and distribution.