Variation in population assemblage of Acanthuridae and Scaridae across human concentration levels during a bleaching event affecting the coral reefs of Guna Yala, Panama
Authors: Heather Jackson 
Published: May 2024

Abstract
Coral reefs are the ocean's most productive ecosystem, supporting a huge amount of biodiversity, and are relied upon by millions of people worldwide. As we progress into the Anthropocene, coral reef ecosystems are increasingly threatened by a swath of pressures. Of the most impactful are El-Niño Southern Oscillation-related heat pulses, which result in the bleaching and potential mortality of coral reefs. Bleaching events, in synergy with other threats, are increasing algal-dominated reefs, which impact the productiveness of coral reef ecosystems. Herbivorous fish are the prominent algal consumers and are essential in controlling algal blooms. Species from the family Acanthuridae and Scaridae are the most conspicuous of herbivorous fish on coral reefs, and through a diverse array of physiological differences compose several unique functional groups. The population distribution of Acanthuridae and Scaridae are directly correlated with coral reef characteristics and composition and can suffer from bleaching events as well. Despite their importance in maintaining reef health and improving resilience to disturbances, there is a very scarce collection of literature that describes the population assemblage of Acanthuridae and Scaridae spp. during bleaching events. This study aims to contribute to this underreported phenomenon. Visual surveys of Acanthuridae and Scaridae species were conducted to measure the average density, mean size, and biomass in terms of coral reef characteristics across a human concentration gradient in Guna Yala, Panama. Results suggest a similar overall abundance of herbivorous fish between the two reef locations, with significantly larger sizes and biomass of A. coeruelus, S. aurofrenatum, S. viride, and S. iseri in reefs further away from human concentration. This correlates with significantly higher rugosity on these reefs. The mean size of each respective species appears to be lower than previously reported across the Caribbean. Additionally, the biomass distribution was strikingly skewed by S. viride which corresponds to almost ¾ of the total biomass. This indicates that the current bleaching event is having drastic impacts on the distribution of herbivorous fish populations, which is exacerbated by proximity to human concentration. This can have very negative consequences for the recovery of these reefs following the bleaching event. Of specific concern are the coral reefs closer to humans. Data from benthic composition surveys show that macroalgae cover and overall coral bleaching are significantly higher compared to reefs far from humans. This study provides vital information that may be useful for future studies to identify coral reef resilience to bleaching events through the scope of Acanthuridae and Scaridae population distribution.
Lay Summary
The ocean is a crucial and largely unexplored part of our planet. One area that has begun to receive sufficient scientific attention is coral reefs. This is because, despite only covering less than 1% of the ocean’s surface, coral reefs support a quarter of all marine life. This rich environment provides more than a billion people with their primary source of protein and offers coastal protection to hundreds of millions of people. However, the future of coral reefs is under threat. Due to climate change and human pressures, coral reefs are facing mortality on a worldwide scale. Of the most lethal are bleaching events, when the water temperature rises past the threshold of coral, and they turn white and die. At the same time, corals face a daily threat from increasing algae coverage, which can smother and kill the coral. These threats have intensified in the recent few decades and may permanently alter the biodiversity and productivity of coral reefs. Understanding these changes is of utmost importance, and one way to do so is to study the animals that eat algae and give coral a chance to thrive—herbivorous fish. This paper analyzes the population distribution of two herbivorous fish families: Acanthuridae (surgeonfish) and Scaridae (parrotfish) during a bleaching event. Learning how a bleachingevent can impact the presence of these essential species among coral reefs has rarely been accomplished in scientific literature, so this paper offers valuable contributions to a more comprehensive understanding of reef health during bleaching events and our changing world.