Highly Threatened Status for the Relict Populations of Ectoparasitic Copepod <i>Salmincola californiensis</i> in Japan

ABSTRACT Many species have been threatened over the past century because of anthropogenic disturbances. Parasites are among the most vulnerable groups because they rely on host organisms, many of which are now endangered. While many studies have argued and evaluated the risk of parasite extinction, empirical evidence is still lacking, especially from aquatic ecosystems. Here, we show the highly threatened status of relict populations of the ectoparasitic copepod Salmincola californiensis in Japan. S. californiensis attaches to the branchial cavities of freshwater salmonids of the genus Oncorhynchus spp., and only four local populations have been reported from disparate regions of Japan, isolated probably due to range contractions after glacial periods. Through citizen‐led field surveys, we found no copepod infections in half of the S. californiensis populations previously reported, suggesting that local extinction has occurred within the last 50–60 years. The upstream reaches of the Kiso River and the Naka River harboured the only sustained populations, though the Kiso population may also have experienced population decline. Our results indicate that parasites can quickly decline over a large geographic scale, especially at range margins. When focal parasites are visible, citizen science is an effective approach for identifying the distributional range of rare parasites and aiding their conservation.