A volunteer-populated online database provides evidence for a geographic pattern in symptoms of black spot infections

Infections of parasitic digenean trematode metacercariae may lead to a visually observable syndrome in fish commonly called black spot disease. While black spot has been noted from various locations throughout North America, patterns in prevalence across the continent remain unknown. Funding to investigate continental-wide prevalence of low-mortality parasitic infections represents a barrier to such studies. I utilize iNaturalist.org's photograph database to examine fish for signs of black spot infections across North America. Fish targeted include blacknose dace, creek chub, chubs (Nocomis spp.), and stonerollers (Campostoma spp.). Photos were visually examined for symptomatic black spots indicative of infection by trematode species linked to black spot disease. Regardless of fish species group, symptoms of black spot pathogens were highly prevalent (27.1% of 314 fish) in watersheds of southern Ontario Canada, whereas mean prevalence was comparatively low elsewhere (7.8%). In one instance, a user uploaded a higher number of photos, with a higher percentage exhibiting signs of infection than other users in the watershed. However, it is difficult to tease apart if that user fished in waterbodies with high infection rates, uploaded more photos of symptomatic fishes, or some other explanation for the differences in user-reported fish with symptoms. Beyond this exception, geographic patterns in the frequency of black spot symptoms do not appear to be related to solely the users, suggesting the observed pattern is biological or ecological. While causative explanations remain conjectures, the data reported herein provides evidence that across four groups of fish, signs of black spot infections are more common in southern Ontario than other areas studied in North America. This work also represents an initial and unexpected utility of volunteer-population databases such as iNaturalist. Further data contributions could lead to better understanding of the causative agents to variation in black spot pathogens' occurrences.