Identifying spawning events in fish by observing a spike in environmental DNA concentration after spawning

Abstract

An understanding of the reproductive biology of aquatic organisms is crucial for the efficient conservation and management of species and/or populations. Nevertheless, conventional spawning surveys such as visual- and capture-based monitoring generally require laborious, time-consuming work and are subject to monitoring biases such as observer bias, as well as miscounts due to false spawning. In addition, direct capture often damages eggs or individuals. Thus, an efficient noninvasive approach for monitoring spawning events on aquatic species would be a valuable tool to understand their reproductive biology and conserving biodiversity. Here, we proposed an environmental DNA (eDNA)-based approach for monitoring and understanding spawning events by observing spikes in eDNA concentration after spawning events. We found in hybridization experiment using two medaka species (Oryzias latipes and Oryzias sakaizumii, 1:1 individual per tank) that a spike in eDNA concentration occurred in male species after spawning. Besides, the magnitude of the spike in eDNA concentration was dependent on the number of spawning activities with egg and sperm release. In the field survey during the reproductive season, eDNA concentrations after spawning were 3–25 times higher than before the expected time for spawning. Additionally, there was no spike in eDNA concentration during the non-reproductive season. Therefore, our results demonstrated that spike in eDNA concentration is mainly caused by the released sperm during spawning events, and it can be used as evidence of spawning in the field survey. The presented approach could be a practical tool for studying reproductive biology and provides an opportunity to design effective conservation and environmental management actions.