Relative importance of host‐dependent versus physical environmental characteristics affecting the distribution of an ectoparasitic copepod infecting the mouth cavity of stream salmonid

Abstract

Understanding parasite distributional patterns is fundamental for elucidating host-parasite relationships. The genus Salmincola is an ectoparasitic copepod group specifically infecting freshwater salmonids. Considering their strong association with their hosts, we can predict that the distribution and prevalence (analogs to abundance) of Salmincola reflect host salmonids. An alternative hypothesis is that their distribution will be strongly affected by environmental factors like stream drift because they have a free-living stage with low swimming ability. If this is the case, we predict a longitudinal gradient with higher occurrence or infection levels in downstream areas. To estimate the relative strength among factors affecting infection levels, we investigated the distribution pattern of Salmincola sp. on wild white-spotted charr Salvelinus leucomaenis in a southern Hokkaido river system. Based on data from 19 sites across three seasons, we found that host density and flow velocity affected the prevalence of Salmincola. On the other hand, no longitudinal gradient was observed and the prevalence was extremely low in some fragmented habitats (i.e., above dams and waterfalls). This indicates some compensation mechanisms against unidirectional downstream dispersal. We found that parasite prevalence and intensity were much higher in large migratory (anadromous) fish and, therefore, hypothesize that long-distance upstream migration helps the redistribution and population persistence of parasites in upstream areas.

Community ecology in a bottle: Leveraging eDNA metabarcoding data to predict occupancy of co‐occurring species

“…edwardsii may be dependent on host density, as has been noted for other Salvelinus fishes (Hasegawa & Koizumi, 2021). We anticipated gill lice occupancy would be negatively associated with brown trout biomass given that S. edwardsii is not known to parasitize brown trout (Mitro, 2016), but this effect was of similar magnitude to the effect of brook trout biomass.…”

“…Although we did not detect gill lice DNA in these disjunct streams during our study, a brook trout from this stream cluster captured via electrofishing was reported to have a single gill louse the year after our study (Katz et al., 2023). Future work considering how temporal and environmental variation affect trout and gill lice occupancy could help clarify distributional differences among these species and potentially aid in detecting early gill lice infestations (Hasegawa & Koizumi, 2021; Wilcox et al., 2018). For example, sampling across multiple seasons may reveal differing relationships for DNA occupancy of focal taxa and prey diversity.…”

“…For example, sampling across multiple seasons may reveal differing relationships for DNA occupancy of focal taxa and prey diversity. Although major movements of brook and brown trout primarily occur during spawning (Hoxmeier & Dieterman, 2013), trout diets (Petty et al., 2014; Sweka & Hartman, 2008) and gill lice prevalence (Hasegawa & Koizumi, 2021; Mitro, 2016) can vary seasonally. Additionally, dynamic occupancy models can be used to incorporate eDNA detection data from seasonal sampling across multiple years to track so‐called “colonization” and “extinction” rates (Martel et al., 2021).…”

“…Historically, inferring distribution of salmonids and Salmincola ectoparasites has been predominantly accomplished with electrofishing surveys (Hasegawa & Koizumi, 2021; Mitro, 2016; Mitro & Griffin, 2018; White et al., 2020). However, this can unnecessarily harm fish if simply detecting presence is desired, and inferring gill lice distribution from electrofishing surveys may be biased as this relies on capturing infected hosts (i.e., free‐swimming stages likely go undetected).…”

“…Although major movements of brook and brown trout primarily occur during spawning (Hoxmeier & Dieterman, 2013), trout diets (Petty et al, 2014;Sweka & Hartman, 2008) and gill lice prevalence (Hasegawa & Koizumi, 2021;Mitro, 2016) can vary seasonally.…”