Environmental drivers of non-commercial reef fisheries in Guam

Small-scale artisanal reef fisheries in the Pacific islands are prevalent and essential for society and culture, yet they remain poorly understood compared to commercial fisheries. General catch-and-effort trends have been characterized, but few studies have coupled landings data with fisher interviews to determine how both catch and effort may be related to environmental factors. This study examined several metrics of catch and effort with respect to fishing locations, fishing methods, moon phases, and seasonal cycles that accounted for differences in windspeed and rainfall in Guam. Creel surveys were conducted over a one-year period from 2021 to 2022, sampling 1836 kg of landed fish. Hierarchical regression trees for environmental factors showed season was the most important predictor of average daily landings, whereby six times more landings were predicted during the calm season compared to the windy season due to higher catch success on exposed windward reefs that became accessible during calm months. Moon phase was a secondary predictor of average daily landings whereby four times more landings were predicted during full moon for spearfishing, while double the landings were predicted during new and mid-moons for bottom fishing. Catch composition also differed across seasons and geography, with a greater presence of large parrotfishes and surgeonfishes, caught on the exposed windward reefs as their habitats became more accessible during calm months. While most target species were abundant on both leeward and windward reefs, several species appeared to have habitat preferences and were better represented on either windward or leeward reefs regardless of season. Moon phase was the top predictor of size within the spearfishing sector, whereas season best predicted size for bottom fishing. Thus, both phenology and fishing access appeared to drive fish sizes. The smaller sizes and lower catch biomasses on the accessible leeward reefs are a concern for Guam fishers that are subjected to trade wind seasons limiting access to productive fishing grounds for half of the year. Cumulatively, the results provide the first predictive framework of landings, composition, and size for Guam’s coral reef fisheries across all seasons to guide and improve ongoing fisheries management planning.