The use of acoustic cameras to monitor spawning endangered Sakhalin taimen <i>Parahucho perryi</i>

<p> Abstract: The rivers that the endangered Sakhalin taimen, Parahucho perryi, currently inhabits, or inhabited historically, and catch records therein are relatively well known. However, there are no data on long-term population-size estimates of this species in Japan, making it difficult to assess their extinction risk and to formulate and implement effective conservation measures. In the spring of 2023, a 23-day fish-monitoring program using an underwater acoustic camera in the upper reach of the Karibetsu River, a tributary of the Sarufutsu River in the Soya Hills, Hokkaido, Japan, detected 315 fish during their spawning migration upstream. Based on footage from an optical video camera installed on the riverbank, used as training data, the detected fish were predicted to comprise 139 Sakhalin taimen; 23 masu salmon, Oncorhynchus masou; and 153 Tribolodon species. The estimated number of spawning Sakhalin taimen was roughly 30–40% of estimates obtained in 2013–2015, implying a significant population decline over the past decade. Die-off of Sakhalin taimen resulting from the record-breaking heatwave that hit this region during the summer of 2021 is suspected to be the direct cause of the drastic population decline. The southern part of the Soya Hills, including the Sarufutsu River Basin, is undergoing rapid wind-power development, necessitating urgent establishment of monitoring protocols to assess the impact of large-scale developmental projects on Sakhalin taimen populations. The use of acoustic cameras, which enable quantitative, efficient estimation of fish populations and non-invasive long-term monitoring, should contribute to effective conservation of this endangered freshwater fish.</p>