Species-specific patterns in spatio-temporal dynamics of juvenile chum salmon and their zooplankton prey in Otsuchi Bay, Japan, revealed by simultaneous eDNA quantification of diverse taxa from the same water samples

To understand the spatio-temporal dynamics of juvenile chum salmon Oncorhynchus keta and their zooplankton prey, the neritic Pseudocalanus newmani, the oceanic Eucalanus bungii, and Themisto japonica, we developed species-specific environmental DNA (eDNA) quantification methods. First, we confirmed that juvenile chum salmon collected in Otsuchi Bay preyed on the target zooplankton by the stomach contents analyses using the developed assays. Size-fraction analyses of the water collected in the bay showed that P. newmani and E. bungii DNA were derived mostly from nauplii and early copepodids. We then investigated the distribution and abundance of zooplankton and juvenile chum salmon in Otsuchi Bay using eDNA analyses of the same water samples collected from January to June in 2018 and 2019. Chum salmon DNA was found in the bay from the end of January to mid-June. P. newmani DNA appeared across the bay throughout the season and were most abundant from February to May, while the other two species DNA were only found in spots, in lower quantities, from March to May. The timings that the zooplankton DNA quantities became greater were corresponded well to that of the Oyashio water intrusion into the bay. In addition, the environmental data indicates that P. newmani was possibly transported also by the Coastal Oyashio water in 2019. The present study successfully revealed species-specific patterns in the dynamics of zooplankton and chum salmon, reflecting their ecological characteristics and oceanographic conditions, and showed the effectiveness of simultaneous eDNA quantifications for diverse taxa from the same water samples.