Black Spot Syndrome in ocean surgeonfish: using video-based surveillance to quantify disease severity and test environmental drivers on Curacao reefs

Abstract
Observations of Black Spot Syndrome (BSS), a pigmented dermatopathy in marine fishes, have been increasingly reported in important grazers such as surgeonfish and parrotfish in the Caribbean. This condition has been linked to infection by the trematode parasite, Scaphanocephalus spp., although relatively little is known about the environmental drivers of infection and how they vary spatially. This study introduces a non-invasive, video-based method to survey BSS presence and severity in ocean surgeonfish (Acanthurus tractus). We then apply the approach across 35 coastal sites in Curaçao to evaluate the influence of environmental factors on BSS, including longitude, herbivorous fish density, wave energy, depth, nutrient pollution, and inhabited surface area. Of the 5,123 fish surveyed, 70% exhibited visible signs of BSS, and the average number of spots per fish increased by ~5-fold from eastern to western sites along the leeward coastline. Within a site, estimates of BSS severity were broadly consistent between different divers, reviewers of video footage, and date of sampling, emphasizing robustness of the surveillance approach. Analyses of environmental factors indicated that BSS decreased with wave intensity while increasing in association with higher nutrient runoff and fishing pressure. This study provides insight into environmental correlates of BSS severity while highlighting the use of video-based surveillance as a non-invasive survey method. The precise mechanisms linking environmental factors with BSS remain unknown, emphasizing the need for long-term and experimental studies in this system.