Pushing the Boundaries of Half-Duplex PIT Tag Antenna Design: A Case Study from Two High-Velocity Water Diversions

Most pass-through PIT tag antennas are designed and operated in low- to moderate-velocity streams or rivers and often, can take advantage of restrictions of flow (i.e., through culverts). Custom fabricated large-scale PIT tag readers were used to estimate entrainment in two diversion tunnels (up to 3.8 m x 4.4 m) that experienced discharge as high as 31.1 cubic meters per second and operated in water depths of up to 6.4 m. Project requirements dictated that the antennas could not constrict or interrupt water flow through the diversions. To meet the objectives, the PIT arrays, consisting of three separate antennas stacked one on top of the other, were hand-molded from fiberglass into a hydrodynamic ‘wing’ shape. This design enabled the antennas to withstand high flows and moderate debris loads, while simultaneously detecting tagged fish (12 mm and 23 mm half-duplex) over the entire monitoring area of the diversion tunnels and for the duration of the study. The antennas operated continuously for over a year, and monitored a majority of all diverted flows through the tunnels due to minimal required maintenance. A total of 220 unique detection events representing 58 fish were observed in a variety of hydrologic conditions.