Host density, host body size, and ambient temperature have all been positively associated with increases in parasite infection. However, the relative importance of these factors in shaping long-term parasite population dynamics in wild host populations is unknown due to the absence of long-term studies. Here, we examine long-term drivers of gill lice (Copepoda) infections in Arctic charr (Salmonidae) over 32 years. We predicted that host density and body size and water temperature would all positively affect parasite population size and population growth rate. Our results show that fish size was the main driver of gill lice infections in Arctic charr. In addition, Arctic charr became infected at smaller sizes and with more parasites in years of higher brown trout population size. Negative intraguild interactions between brown trout and Arctic charr appear to drive smaller Arctic charr to seek refuge in deeper areas of the lake, thus increasing infection risk. There was no effect of host density on the force of infection, and the relationship between Arctic charr density and parasite mean abundance was negative, possibly due to an encounter-dilution effect. The population densities of host and parasite fluctuated independently of one another. Water temperature had negligible effects on the temporal dynamics of the gill lice population. Understanding long-term drivers of parasite population dynamics is key for research and management. In fish farms, artificially high densities of hosts lead to vast increases in the transmission of parasitic copepods. However, in wild fish populations fluctuating at natural densities, the surface area available for copepodid attachment might be more important than the density of available hosts.

A total of 6893 Arctic charr were checked macroscopically for S. edwardsii on the gills and branchial cavity shortly after capture, and the total number of adult female copepods were counted. Arctic charr were sampled annually during the ice-free season from 1980 to 2018, except in 1998, but fish sampled from Takvatn prior to 1987 were not checked for S. edwardsii infection. Multimesh gillnets were placed in the littoral (<15 m depth), profundal (25–40 m depth) and pelagic (offshore, >30 m depth) habitats of the lake, and fished overnight for approximately 12 hours. Each fish was wet-weighed (±0.1 g), had its fork length measured (±0.1 cm) and its sex determined.

Any program that can open .csv files, such as Excel or R.