Learning By Doing - Best Laid Plans: Side-Looking Hydroacoustics in a Large Tidal River Prior to Restoration Activities

One hopeful benefit of dam removal is increased native fish abundance. Since hydroacoustics are used to generate such metrics in marine environments, and increasingly in rivers, this seemingly simple and powerful approach was applied downstream of the lowermost dams of the Penobscot River, Maine for 4 years prior to their removal. This enabled an estimated relative abundance of 480-7000 fish d^-1 with peaks in spring and fall, and an indication of inter-annual variability, up to 3-fold year-to-year. Pre-conceived notions about the application of side-looking hydroacoustics in rivers: (1) complete cross-sectional river coverage; (2) acoustic beam fit; and (3) low cost; resulted in estimate uncertainty. The first notion resulted in the use of multiple transducers on opposite sides of the river. This resulted in space-time varying interference in most data files (8 Tb) and required the majority of data to be manually processed. The use of elliptical transducer beams (with large side-lobes, 13 dB down) resulted in such high noise levels that angular data were contaminated, resulting in manual data processing and the inability to examine fish movement direction and target strength. Lessons learned from this approach, including the need for better networking among acousticians and restoration scientists, will be highlighted.