2.4.1 | Calculating variables
The infection level (prevalence, intensity, and mean intensity) was calculated following Bush et al.’s (1997) method. The estimated charr abundance in each section was calculated using the removal method implemented in the program CAPTURE (White et al., 1978; available at http://www.mbr-pwrc.usgs.gov/software/index.html). The host density in each section was calculated from estimated charr abundance, and each section area (m2) calculated from mean stream width and section length (i.e. 25 m or 11 m). For the accurate evaluation of the host body condition, we used the residual index (Jakob et al., 1996): we calculated residual distances of individual points from the regression of ln (BW) with ln (FL). As fish body condition is assumed to be different among seasons in salmonids (e.g. Morita et al., 2011), we calculated residuals in each capture session (i.e. June 2020, July 2020, October 2020, and July 2021). We also calculated the growth rate (mm / day) for each host individual for each capture-recapture interval.