The second paper of three, included here (Sanderson et al., 2023b) examines how reliably a tagged fish can be detected as it passes by an array of receivers in Minas Passage.

When combined with detection efficiency estimates, this information can help determine probability of fish-turbine encounter at the FORCE site.

Prior tag detection efficiency estimates (Sanderson et al. 2023a) relied on tags and receivers deployed at fixed locations on the seafloor and revealed that signal paths are sometimes blocked by variable bathymetry.

However, many fish swim well clear of the seafloor in Minas Passage, and their movements are similar to those of drifters that move passively with tidal currents (Sanderson et al. 2021). As such, deploying acoustic tags on GPS-tracked drifters permits verification of prior detection efficiency estimates using tags positioned higher in the water column, and also enables demonstration of a method for obtaining probability of detecting a tagged fish as it passes by a receiver array. Sanderson et al. (2023b) suspended multiple 170 kHz HR acoustic tags and a 69 kHz PPM at varying depths (as surrogates for fish swimming at varying depths) below GPS-tracked drifters and released them in Minas Passage for varying lengths of time.

Results confirm the findings of (Sanderson et al. 2023a)that 69 kHz PPM signals are poorly detected by acoustic receivers when tidal current speeds are high (> 3.5 m/sec), and is due to the extended period of time required to transmit the entire PPM signal. However, 170 kHz HR signals were usually detected by the receiver array even in fast currents (>3.5 m/sec) during spring tides, and short signal transmission intervals (≤ 2 seconds) are needed to ensure detection across tidal current speeds. Indeed, there is a high likelihood (≥0.90) that frequently transmitted HR signals will be detected across current speeds ≥ 3.5 m/sec at the FORCE tidal demonstration site if acoustic receivers in an array are spaced ≤ 150 m apart.

Interestingly, drifters that were deployed for extended periods of time become ‘caught’ in quasi-steady trajectories by tidal currents and typically pass through the center of Minas Passage to the south of the FORCE tidal demonstration site. These results are consistent with Sanderson et al. (2021), and given that the movements of drifters and some fish species are similar, it is reasonable to expect that the majority of individual fish tracks also pass through the center of Minas Passage to the south of the FORCE tidal demonstration site.