New system for remotely monitoring the three-dimensional movement of acoustically tagged fish

A passive acoustic tag system was developed to monitor the three-dimensional movements of migrating fish with submeter resolution. The acoustic tag receiver monitors an array consisting of up to 16 omnidirectional hydrophones, with received signals synchronized to determine the arrival times for each pulse transmitted by the acoustic tag. Arrival times are then used to calculate the three-dimensional position of a tagged fish as it moves through the array. Algorithms were developed to precisely calculate the three-dimensional positions of the hydrophones, and of each acoustic tag. Over the last 4 years, this system was used at several dams in the United States. Most studies to date monitored downstream migrating juvenile salmonids as they approached and passed turbine intakes, spillways, and juvenile bypass systems at hydroelectric dams. Fish movement patterns were tracked in three dimensions over time, typically with submeter resolution. Tagged fish were 160–240 mm long. Acoustic tags were approximately 7 mm in diameter by 23 mm long, weighted 2 g, and transmitted at 307 kHz. Tag codes (up to 500), pulse width (typically 1–5 ms), and ping rate (typically 0.3–3 pings/s) were field programmable. Current tags incorporate signal encoding for an improved signal-to-noise ratio, and weigh as little as 1 g.