1. Fish sampling and parasite identification
We used unbaited minnow traps to collect adult threespine stickleback from lake, river, and estuary sites on Vancouver Island in late May and early June 2009. Collections were approved by the University of Texas IACUC (07-032201) and a Scientific Fish Collection Permit from the Ministry of the Environment of British Columbia (NA07-32612). Fish were euthanized in MS-222 and then preserved in formalin, after the caudal fin of each individual fish was removed and preserved in pure ethanol for genotyping. Details are provided in Bolnick & Ballare (2020). These samples were used to examine within- and between-population variation in diet (Bolnick & Ballare, 2020), parasite community composition (Bolnick, Resetarits, Ballare, Stuart, & Stutz, 2019), and parasite species richness (Bolnick, Resetarits, Ballare, Stuart, & Stutz, 2020). The parasite infection data for this study are archived at Dryad Digital Repository (Bolnick and Ballare, 2020). Here, we focus on a random subset of 26 sampling sites (N = 1437 stickleback) for which we also genotyped individuals for MHC IIβ. For each stickleback individual, we recorded sex and body length and then dissected each fish to count and identify macroparasites as described in (Stutz & Bolnick, 2017).