Anglers as citizen scientists: a case study of resource monitoring in stream salmonids

In the face of global warming and frequent catastrophe, monitoring wild populations of stream salmonids is increasingly important. However, with declining human birthrates and aging populations predicted for developed countries, conventional monitoring surveys conducted solely by fisheries researchers may not be sustainable owing to labor shortages. Local anglers sometimes form communities, such as fishing clubs or cooperatives. Community-based fishing reports can ensure monitoring accuracy similar to conventional population estimates. In this study, a mark-recapture survey using the Petersen method was performed on white-spotted charr Salvelinus leucomaenis and red-spotted masu salmon Oncorhynchus masou ishikawae in a mountain stream of the Fuji River basin, central Japan, from 2018 to 2023. The marking survey was conducted for age-1 and older fish by electrofishing with the participation of local anglers. For the recapture surveys, anglers fished and reported on the number of total and marked individuals in their catch for each species. The total catch by anglers was 1.5-2.9 times higher than the number of fish captured for marking by electrofishing. The reporting method of fishing results varied widely: direct reporting to the lodge staff including the third author, social networking sites, e-mail, telephone, and fax. The estimated population density of both species combined was at least 0.18 individuals m-2, indicating a high abundance in the stream when compared with literature reports for similar mountain streams. Significant positive correlation was shown between the population densities and the number of fish caught per angler per day, suggesting that the estimation accuracy was reasonably robust. The results of the mark-recapture survey were fed back to the anglers every October.