Abstract
Mesophotic coral ecosystems (MCEs; reefs 30-150m depth) are of increased research interest because of their potential role as depth refuges from many shallow reef threats. Yet few studies have identified patterns in fish species composition and trophic group structure between MCEs and their shallow counterparts. Here we explore reef fish species and biomass distributions across shallow to upper-MCE Caribbean reef gradients (5-40m) around Utila, Honduras, using a diver-operated stereo-video system. Broadly, we found reef fish species richness, abundance and biomass declining with depth. At the trophic group level we identified declines in herbivores (both total and relative community biomass) with depth, mostly driven by declines in parrotfish (Scaridae). Piscivores increased as a proportion of the community with increased depth while, in contrast to previous studies, we found no change in relative planktivorous reef fish biomass across the depth gradient. In addition, we also found evidence of ontogenetic migrations in the blue tang (Acanthurus coeruleus), striped parrotfish (Scarus iserti), blue chromis (Chromis cyanea), creole wrasse (Clepticus parrae), bluehead wrasse (Thalassoma bifasciatum) and yellowtail snapper (Ocyurus chrysurus), with a higher proportion of larger individuals at mesophotic and near-mesophotic depths than on shallow reefs. Our results highlight the importance of using biomass measures when considering fish community changes across depth gradients, with biomass generating different results to simple abundance counts.
Highlights
Mesophotic coral ecosystems (MCEs; zooxanthellate coral reefs from 30m to approximately 150m depth) are understudied [1], yet may have a linear extent as great as shallow coral ecosystems [2]
Fish species richness gradually declined with depth from an average per transect of 12.7 ± 1.0 species at 5m to 4.0 ± 0.9 species at 40m (Fig 2A), whereas abundance and biomass rapidly declined between 5m and 15m and did not change with further increases in depth (Fig 2B & 2C)
Hard substratum effects varied with shore, with higher hard substrate at MCEs at the Maze on the north shore (59.8 ± 14.19%) compared to Spotted Bay and the south shore sites (13.8 ± 2.0%)
Summary
Surveys were conducted at seven fringing coral reef sites around Utila, Bay Islands, Honduras (see S1 Table for GPS locations). Utila is located at the southern end of the Mesoamerican Barrier Reef. Two sites (The Maze and Spotted Bay) were located on the island’s exposed north shore, and five were located on the more sheltered south shore (Stingray Point, Little Bight, Black Coral Wall, Coral View and Lighthouse Reef) (Fig 1). On the south shore the reef slope reaches a maximum depth of 40-60m where the seabed levels off stretching to mainland Honduras. On the north shore of Utila, the reef slope continues to >100m. Shallow reefs at south shore sites consist of a spur and groove system, whereas shallow reefs on the north shore are on steeper reef walls. Field Permits were issued by the Instituto de Conservacion Forestal (ICF), Honduras
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