Fish collection

We sampled 17 species of coral reef fishes (Acanthurus nigricans, Cephalopholis argus, Cephalopholis urodeta, Chromis iomelas, Chromis margaritifer, Ctenochaetus marginatus, Ctenochaetus striatus, Paracirrhites arcatus, Plectroglyphidodon dickii, Pseudanthias bartlettorum, Pseudanthias dispar, Pseudanthias mooreanus, Pseudanthias olivaceous, Pseudianthias pascalus, Pseudanthias spp., Stegastes aureus, Stegastes fasciolatus) across three archipelagos and 19 islands (Jarvis, Kingman, Kiritimati, Palmyra, Tabuaeran, and Teraina in the Northern Line Islands; Flint, Malden, Millennium, Starbuck, and Vostok in the Southern Line Islands; Huahine, Moorea, Raiatea, Rangiroa, Tahiti, Takapoto, Tetiaroa, and Tikehau in French Polynesia), for a total of 5,251 fish. These 17 broadly distributed host species represent a large spectrum of body sizes, including multiple taxonomic and trophic groups. Due to the limitations on the natural geographic ranges of some species, not all species could be sampled on all islands; when we found that a target species was not present on a given island, we sampled a close congener instead (e.g., Stegastes fasciolatus instead of Stegastes aureus). In general, fish greater than 10 cm in total length were collected using three-pronged spears, while hand nets were used to capture fish smaller than 10 cm in length. Most fish were sampled from depths of 8 to 18 m from the leeward forereef of each island. After collection, fish were humanely euthanized using protocols in UC San Diego IACUC protocol #S09392, which provided ethical approval for this study. We then recorded morphometric data, including the total length of each fish (mm), before freezing fish immediately after collection. Fish were transported frozen back to the laboratory and were kept frozen until they were thawed for parasitological examination.

Parasite abundance assessment

We performed a comprehensive examination of each fish, designed to detect most metazoan parasites. We did not count mobile skin parasites or micropredators, as these are easily lost when the host is captured, and we did not search for myxozoan parasites, but all other metazoans should have been detected with our protocol. We adapted the dissection protocol to the morphology of each fish species (keeping the protocol consistent within species); both the generalized dissection protocol and departures from it are detailed elsewhere [see appendix E in Wood, Sandin, et al. (2014)]. Briefly, we examined the following organs individually under a stereomicroscope: fins, gills, eyes, heart, liver, spleen, gonad, gills, muscle, skin, and intestines [after Wood, Sandin, et al. (2014)]. When an organ was missing or damaged (e.g., part of fin was missing), we recorded “n/a,” not “0,”for any parasite species typically found in that organ (e.g., fin metacercariae). Photographs of each parasite species (along with detailed images of diagnostic morphological features) and voucher specimens were archived and are available for examination by request to the corresponding author. We identified parasites to the lowest possible taxonomic level using published keys [see appendix F in Wood, Sandin, et al. (2014)]: Wood CL, Sandin S, Zgliczynski B, Guerra AS, and Micheli F. 2014. Fishing drives declines in fish parasite diversity and has variable effects on parasite abundance. Ecology 95: 1929-46.