Hiodon tergisus (Mooneye) is a member of the big river, mid-water fish assemblage occurring in the Oswegatchie River, a medium-sized river of northern New York. It is classified as a threatened species by the New York Department Environmental Conservation, and agency staff planners need details of its life history for its conservation and recovery. Since there were 8 recent records for Mooneye in the river segment both upstream and downstream of a dam at Heuvelton, we chose this segment of the Oswegatchie River to characterize the river's population of Mooneye. Mooneye records also came from the nearby tributary Indian River, and spawners could travel to the rapids at Rossie upstream of Black Lake without any barriers. We caught 68 Mooneye in April–September 2021 in this study reach, using floating gillnets and boat electrofishing. Catch rates using either method varied widely among seasons. Intentions to find spawning aggregations were finally successful in late April 2023 in 2 of the previously studied locations and 1 new site. On 25 and 28 April, we caught spawners downstream of barriers, 2–3 m in height, to movement on the Oswegatchie River at Heuvelton, Coopers Falls, and at Rossie on the Indian River.