An underwater top-pivoting antenna assembly (or array thereof) suitable for subsurface RFID tag interrogation in flowing water such as a river. In preferred embodiments, the antenna interrogates RFID tags implanted in aquatic species. The antenna resides in an elongate antenna housing whose cross-sectional shape is preferably a symmetric hydrodynamic teardrop shape. When the assembly is deployed in water with an upper end thereof suspended above a lower end, the upper end of the housing is linked to a pivot/swivel mechanism such that when the pivot/swivel mechanism is held substantially stationary with respect to the water flow, the lower end of the housing is free to rotate generally about the first end, including in a substantially vertical plane parallel to the water flow direction. The length of the antenna housing is advantageously selected to enable the antenna to monitor for signals across substantially the entire water depth.