2.1 | Study species
White-spotted charr (Figure 1) is a common salmonid fish inhabiting mountain streams in the Japanese archipelago (Hosoya, 2013). Like many other salmonids, they have two types of life history in Hokkaido Island: some individuals remain and reproduce in their natal river throughout their lives (i.e. stream residents), whereas others migrate to the sea or lakes and later come back to the natal rivers for reproduction (i.e. migrants) (Morita, 2001; Morita et al., 2009). Above natural waterfalls or man-made dams, most individuals mature as residents (Morita et al., 2009).
In our study systems, white-spotted charr have frequent infections by parasitic copepods in their mouth cavities (Hasegawa & Koizumi, 2021). These copepods were identified as S. cf. markewitschibased on morphological observations and molecular analysis in a previous study (Figure 1; Hasegawa et al., 2022b).