Habitat characteristics of the Sakhalin taimen Parahucho perryi in the Butokamabetsu River, northern Hokkaido, Japan

Abstract

The Sakhalin taimen Parahucho perryi, one of the largest freshwater fish in Japan, is listed as Critically Endangered (CR) on the IUCN Red List. It occurs in rivers that contain marshes and wetlands with little land development. Therefore, floodplains are considered essential habitat for this species. However, most of our knowledge of Sakhalin taimen ecology concerns adults, and there is limited scientific knowledge of juveniles. In this study, we sought to identify the habitat characteristics of juvenile and adult Sakhalin taimen and compare them with those of sympatric fish species. The study took place in the Butokamabetsu River, which flows through the Uryu Experimental Forest of Hokkaido University and has a floodplain and well-developed anastomosing channels. Two areas were surveyed: the secondary side-channels, via electrofishing, and the main channel, via snorkelling. We also examined the physical environment of the study sites and the stomach contents of captured fishes. Thirty side-channel sites were surveyed, and all captured Sakhalin taimen were juveniles with fork lengths of 69–137 mm. Principal component analysis of the physical environment showed that juvenile Sakhalin taimen tended to be more abundant in lentic environments with low flow velocities and high turbidity. The stomachs of juvenile Sakhalin taimen contained fish, amphibians, and zooplankton (Cladocera), and a smaller proportion of terrestrial insects compared to the sympatric white-spotted charr Salvelinus leucomaenis and masu salmon Oncorhynchus masou. Twenty-one main-channel sites were surveyed, and all observed Sakhalin taimen were subadults or adults with fork lengths of 300–800 mm. Principal component analysis of the physical environment showed that subadult/adult Sakhalin taimen tended to be more abundant in deeper pools with more cover structure, such as woody debris. These results indicate that juvenile Sakhalin taimen select side channels with very slow currents, whereas subadult/adult Sakhalin taimen use deep pools in the main channel with sufficient cover. Conservation of floodplains and anastomosing channels is vital for the persistence of Sakhalin taimen in the wild.