That tag has a much greater diameter than any normal tag used in derbies, etc. A radio transmitter makes much more sense based on shape alone, although finding one on a stocked RT is surprising, given the existing general knowledge of their movement in Pa. One the other hand, it may be a university research project, including one by a grad student working on a thesis seeking more water specific info regarding movement, or an environmental consulting firm study dealing with a proposed or planned project in a particular waterway that could affect fish or fisheries. Some tags do more than just provide locational info. Some have other sensors built in, such as those for temperature. If curious, I’d contact the local WCO and local AFM first to seem what they might know re odd looking tags or radio telemetry research projects by universities or consultants in the area.