scientific article | Proc 12th Int Coral Reef Symp | open access
Kosaki RK, Kane C, Pyle RL
Mesophotic coral ecosystems, also known as the coralreef “twilight zone”, are receiving increased attention from coral-reef ecologists and managers because of their potential contributions to tropical biodiversity, and their potential to serve as refugia for fishes that are depleted due to anthropogenic activities on shallow reefs. However, the composition and trophic structure of these fish communities are poorly characterized. We present the results of the first quantitative mesophotic coral reef fish assemblage characterizations from the Northwestern Hawaiian Islands (NWHI). Each fish species recorded was assigned to one of six feeding guilds. When compared to shallow reefs (<30 m) of the NWHI, the mesophotic reefs exhibited a near absence of herbivores, and an abundance of planktivores. These deep reef fish assemblages (50-80 m) were also found to have the highest recorded levels of endemism of any marine ecosystem, with endemic species comprising >90% of the total assemblage at Midway and Kure Atolls. High endemism was driven by the numerical dominance of small-bodied, endemic planktivorous fishes. This planktivore community is supported by seasonal increases in planktonic productivity from the high-chlorphyll waters of the Transition Zone Chlorophyll Front (TCZF). The TCZF shifts far enough south in the winter that the northern atolls of the NWHI lie within its productive waters. Small increases in sea surface temperatures may drive the TZCF slightly north, depriving these planktivore-dominated, high-endemism fish assemblages of this enhanced planktonic primary productivity. Climate change may thus threaten the trophic structure of these ecosystems and their extraordinary levels of endemism.