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This is an interesting subject of research. Indeed the authors are assessing spawning activity in aquatic species based on environmental DNA which is a non-invasive methodological approach of research for monitoring spawning activity on aquatic species. They studied on two medaka species (Oryzias latipes and O. sakaizumii, in cross experiments and under laboratory conditions. They proposed an environmental DNA approach for monitoring and understanding spawning activity in fish. Their results are suggestive that an eDNA spike occurred in only male species after spawning activity AND THAT the magnitude of the eDNA spike was dependent on the number of spawning activities. They also demonstrated that spikes in the eDNA concentration are mainly caused by the release of sperm during spawning activity, and it can be used as evidence of spawning in field survey. Although the whole experimental design seems clear and straightforward, I would suggest that care should be taken intro account when interpreting laboratory -based finding into filed results. It would be also interesting to extend the discussion part about whether stress related behavioural responses or eDNA concentrations could be related to daily activities and also does personality may matter at all? In terms of data reproducibility it would be also helpful if the author provide the data accessible on online repository services for further post research examination by other researchers.