Cumulative impact from disturbances drives hard coral loss and Chaetodon butterflyfish decline in Nha Trang Bay Marine Protected Area, Vietnam.

Abstract

Loss of live hard coral cover often drives declines in reef fish abundance, particularly among obligate corallivores such as butterflyfishes (Family Chaetodontidae). In this study, we investigated the variations in hard coral cover and Chaetodonbutterflyfish densities within the Nha Trang Bay Marine Protected Area (MPA), Vietnam, spanning nearly two decades (2005–2023). We monitored hard coral cover and butterflyfish densities at 5-year intervals from 2005 to 2015, then increased the monitoring frequency thereafter, and found that live hard coral cover plummeted by 25.8% across the entire study period. This habitat degradation directly mirrored changes in the butterflyfish assemblages. Between 2005 and 2018, both post-recruit (1.1–5.0 individuals/100 m²) and recruit (0.1–1.6 individuals/100 m²) densities remained stable. However, both cohorts suffered sharp declines through 2023. We attribute this collapse to cumulative acute disturbances, including two major typhoons (Damrey in November 2017 and Rai in December 2021), a mass coral bleaching event, and a crown-of-thorns starfish outbreak in 2019. Crucially, we detected no signs of coral or fish recovery following these impacts, even within the strictly protected core zones (NWMU and SWMU) of Mun Island.