2.3 | Mark-recapture survey
During the study period of 2 and 6 June 2020 (hereafter the period
called “June 2020”), we captured white-spotted charr by two-pass
electrofishing using a backpack Electrofisher unit (300 V DC, Model
12-B, Smith-Root Inc., Vancouver, WA, USA) and a dip net (2-mm mesh) at
each section to estimate the charr abundance by the removal method (e.g.
Riley & Fausch, 1992). Block nets were set at the start and end points
of each section to prevent fish from entering or leaving during
electrofishing. Captured fish were anesthetised using FA100 (DS Pharma
Animal Health Co., Ltd.), and body length (fork length; FL) and body
weight (BW) were measured to the nearest 1 mm and 0.1 g, respectively.
We checked the presence and number of copepods by observing the fins,
body surface, and mouth cavity of each fish. Fluorescent elastomer tags
(North-west Marine Technology Inc., Shaw Island, WA, U.S.A.) were
injected with a unique combination of six colours at four landmarks in
each fish (four landmarks at the head and posterior points to each eye).
Photographs of all individuals were taken on the left side with a
digital camera (TG4, Olympus, Tokyo, Japan) to double-check the
individual identification based on the variation of the white-spot
pattern (Watz et al., 2019). After fish recovery, we gently
released the fish into the middle of each section from which they were
captured. As age-0 fish were rarely infected by the copepod in previous
studies (Hasegawa & Koizumi, 2021), we only captured and marked age-1
and older fish individuals.
Recapture sessions were conducted three times: July 2020 (4–9 July
2020), October 2020 (31 September–3 October 2020), and July 2021 (5–9
July). Fish were recaptured and treated in the same manner as the
marking session (i.e. June 2020), except for the July 2021 survey, in
which we captured marked fish by angling and two-pass electrofishing
(Hasegawa & Koizumi under review 2). For individuals with partly fading
elastomer colours or exhibiting body length shrinkage, we confirmed and
identified them by checking the photographs. A few individuals were
initially captured but eventually excluded from the analysis due to
undetermined identity. During the recapture session, we also captured
unmarked fish for host abundance estimation and parasite prevalence
calculation, but we did not mark the fish. Given the relatively long
interval between October 2020 and July 2021 (i.e. 9 months), we did not
use the data of July 2021 in the estimation of causality by SEM;
however, they were used for the survival rate estimation (see Section
2.4.3). All capture histories are represented in Figure 2 and Supplement
1.