A novel spawning survey: identifying spawning times and locations of jack mackerel in a semi-closed bay using environmental DNA analysis

Abstract

AbstractUnderstanding spawning ecology is vital for species/population management and conservation. Conventional surveys for this, however, are generally time- and labour-demanding and invasive. To overcome these challenges, environmental DNA (eDNA) analysis has emerged as a promising spawning survey method. This study aimed to identify the spawning season, times, and locations of Japanese jack mackerel (Trachurus japonicus) through eDNA analysis and to investigate the utility of eDNA analysis in spawning surveys. We expected that eDNA concentration and nuclear/mitochondrial DNA ratio should change overnight due to sperm-derived eDNA as this species is a nocturnal spawner. The eDNA concentrations at sunset and sunrise at three sites in Maizuru Bay were compared on a monthly for one year. Our results showed significant increases in eDNA concentrations and ratios at all sites in July, suggesting potential spawning occurrences. At the site with a particularly large increase in concentration, temporal monitoring showed a diurnal eDNA peak (21:00–24:00), likely reflecting the jack mackerel spawning time window. Subsequently, our eDNA-based estimations were supported by the successful capture of their eggs using a plankton net. This is the first report providing evidence of saltwater fish spawning using eDNA analysis, underscoring the usefulness of this approach for spawning detection.