Reef Fish Community Biomass and Trophic Structure Changes across Shallow to Upper-Mesophotic Reefs in the Mesoamerican Barrier Reef, Caribbean

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.

Fig 1. Map of study sites around Utila, Honduras.

Survey locations are marked, with numbers indicating sites as follows: (1) Spotted Bay, (2) The Maze, (3) Stingray Point, (4) Little Bight, (5) Black Coral Wall, (6) Coral View and (7) Lighthouse Reef. Inset map shows the location of Utila relative to the Caribbean region. Map sourced from GADM database of Global Administrative Areas under a CC BY licence with permission.

Fig 2. Changes in fish communities with depth.

(A) Species richness, (B) abundance, (C) biomass down the depth gradient. Figure shows mean and SE. Letters indicate statistically different groups at the p<0.05 level.

Fig 3. Change in percentage cover of key benthic groups with depth.

Figure shows mean and SE, letters indicate differences (p>0.05 with one-way ANOVA).

Fig 4. Non-metric multidimensional scaling plot of fish biomass weighted communities for each depth and site.

(A) Overall NMDS plot showing community clustering by depth, contour lines indicate percentage cover of hard coral and (B) fish community biomass standardised by percentage hard coral availability at each site.

Fig 5. Change in percentage of different trophic groups with depth.

Fish community was weighed by (A) abundance and (B) biomass. Percentage abundance and biomass of each trophic group was calculated by summing all fish identified on all transects at all sites in a depth band.

Fig 6. Whole fish community length distributions at each depth across all transects and sites.

Number of fish lengths recorded (n), and the separation bandwidths to produce kernel density estimates (h) calculated by Sheather-Jones selection procedure are shown.

Fig 7. Comparison of kernel density estimates for four fish species between shallow and deep reefs.

Comparisons based on all fish individuals measured on shallow (5 and 15m) and deep reefs (25 and 40m) at all sites. Grey shaded regions represent one standard error either side of the null model, n = number of individual fish measured, p indicates whether the length distributions are significantly different based on permutation tests.