Decadal changes in parrotfish assemblages around reefs of Guam, Micronesia

Abstract

Modern coral reefs face substantial threats that endanger their capacity to function and provide resources for human populations. Chronic human pressure through exploitation and land-based pollution has led to reef degradation and lower productivity. More recently, this pressure is compounded with successive episodes of coral bleaching across the tropical oceans. However, response trajectories of reef resources to exploitation, habitat degradation, and local management may not be straightforward, as these ecosystems are incredibly complex. We employed a comprehensive and standardized survey of parrotfish assemblages within reef systems in Guam to examine trajectories of biomass and function across a decade of change. Parrotfishes represent one of the most commercially and ecologically important coastal resources throughout Micronesia. We found that parrotfish biomass declined by nearly 30% over the course of ten years, with concomitant decreases in grazing and bioerosion rates at a similar magnitude. There was evidence of fishery exploitation playing a role, but overall patterns were not always aligned with the hypothesis of fishery-induced decline. For example, two-thirds of marine-protected area sites declined in biomass and mean body length was stable for several highly targeted species. Further, most biomass decline stemmed from small species with higher resilience to fishery exploitation, whereby three of the most vulnerable fishery target species either maintained or increased total biomass. The ultimate drivers of parrotfish assemblage trajectories in Guam are likely a complex mixture of exploitation, habitat change from multiple stressors, and responses to management measures. However, the potential future decline in fishery production and ecological function is substantial and merits continuous monitoring and proactive management.

Herbivory through the lens of ecological processes across Pacific coral reefs

“…Across the Pacific, we detected regional variance in the ecological processes of herbivorous fishes. Overall, we observed nonlinear, positive relationships between fish biomass and rates of herbivory, consistent with parrotfish scraping and erosion in the Maldives and Chagos Archipelago (Lange et al, 2020; Taylor et al, 2022). For a given level of biomass, the corresponding rates of herbivory varied among regions, and we observed instances where islands with the greatest biomass did not also have the highest rates of herbivory.…”

Branching Lithophyllum Coralline Algae: Dominant Reef Builders on Herbivory-Depressed Tropical Reefs after High Coral Mortality

“…Historically, fishing pressure on Guam's reefs has been high and the island's reef fish populations are in a near-collapsed state [5]. This downward trend of Guam's reef fish stocks has not been halted, and a 30% reduction in parrotfish biomass was reported for the period between 2011 and 2021 [6]. Reefs with depleted herbivorous fish stocks are more prone to elevated coral mortality and negative coral recovery trajectories following heat stress events [7].…”