Coal mining in the Elk River Valley (British Columbia) has been ongoing since the 1880s, elevating trace-element concentrations in surface and ground waters across the Elk River basin. Of particular concern is selenium (Se), which is transported downstream to the transboundary waters of Lake Koocanusa (Canada-USA). Se is an essential micronutrient that bioaccumulates in aquatic food webs and can cause toxicity at slightly elevated concentrations in egg-laying animals. Current management guidance assumes Se is transferred from the liver to the ovary during vitellogenesis, therefore, ovary samples collected early in development may underestimate ovary Se concentrations at spawning. We evaluated this assumption by quantifying how ovary Se changes with oocyte maturation, comparing species that develop eggs at the same time (synchronous spawner; northern pikeminnow, peamouth) versus species that develop eggs in multiple batches over time (asynchronous spawner; redside shiner) in Lake Koocanusa. Across the three fish species, we paired ovary Se with gonado-somatic index (GSI) and histological staging to test whether GSI reflects gonadal maturation and whether ovary Se differs among maturation stages and spawning status. By clarifying how ovarian development affects ovary Se variability, results will inform the timing of monitoring Se in reproductive tissues.