Ecological Drivers of Invasive Lionfish (Pterois volitans and Pterois miles) Distribution Across Mesophotic Reefs in Bermuda

Invasive lionfish (Pterois volitans and P. miles) are now ubiquitous throughout the Caribbean and Western Atlantic on shallow and deep reefs. Recent surveys in Bermuda have revealed dense aggregations of lionfish on mesophotic reefs (60 m depth), yet these densities are not pervasive across reefs at this depth. Using diver-led visual surveys of mesophotic reef sites, this study examines how variations in potential ecological drivers may affect lionfish distribution. Significant correlations of lionfish densities were found with prey fish density and prey fish biomass, where sites with higher abundances of prey fishes have greater densities of lionfish. Furthermore, higher densities of lionfish also correlated significantly with higher juvenile Paranthias furcifer biomass, a preferred prey type for lionfish. Prey fish diversity, on the other hand, was not related to lionfish density, nor did prey fish community composition differ in a way that reflected lionfish distributions. The influence of seawater temperature was found to have the strongest effect on lionfish distribution, where higher lionfish densities were found at sites with lower bottom temperature. However, temperature co-varied with prey fish density, prey fish biomass, and P. furcifer biomass, implying that physical parameters of the environment (i.e., temperature) likely influence ecological parameters (i.e., prey fish abundance), contributing to the structuring of lionfish distributions. We suggest, therefore, that cold-water upwelling currents may be fueling the food chain in certain locations, resulting in high abundances of prey fishes and thus lionfish. Understanding the factors that influence lionfish distributions will ultimately increase the efficacy of management strategies, which, as the data presented here suggest, must incorporate mesophotic lionfish populations

Population Structure of Montastraea cavernosa on Shallow versus Mesophotic Reefs in Bermuda.

Mesophotic coral reef ecosystems remain largely unexplored with only limited information available on taxonomic composition, abundance and distribution. Yet, mesophotic reefs may serve as potential refugia for shallow-water species and thus understanding biodiversity, ecology and connectivity of deep reef communities is integral for resource management and conservation. The Caribbean coral, Montastraea cavernosa, is considered a depth generalist and is commonly found at mesophotic depths. We surveyed abundance and size-frequency of M. cavernosa populations at six shallow (10m) and six upper mesophotic (45m) sites in Bermuda and found population structure was depth dependent. The mean surface area of colonies at mesophotic sites was significantly smaller than at shallow sites, suggesting that growth rates and maximum colony surface area are limited on mesophotic reefs. Colony density was significantly higher at mesophotic sites, however, resulting in equal contributions to overall percent cover. Size-frequency distributions between shallow and mesophotic sites were also significantly different with populations at mesophotic reefs skewed towards smaller individuals. Overall, the results of this study provide valuable baseline data on population structure, which indicate that the mesophotic reefs of Bermuda support an established population of M. cavernosa

Mean Size-Frequency by Depth.

<p>Size-frequency distributions of <i>M</i>. <i>cavernosa</i> on a logarithmic scale represented as the mean proportion of individuals (± SE) within each log transformed size class for measured colonies from all shallow (10m; gray bars) and all mesophotic (45m; black bars) survey locations (Rita/XL, Coopers, Tuckers, Spittal, Devonshire, Hungry Bay).</p

PCO of Population Structure by Site.

<p>Principal coordinates analysis (PCO) of <i>M</i>. <i>cavernosa</i> size-frequency distributions for each survey location (Rita/XL, Coopers, Tuckers, Spittal, Devonshire, Hungry Bay) at 10m and 45m depths. PCO1 and PCO2 axes together capture 94.7% of the total variation in size-frequency distribution.</p

Survey Locations.

<p>Details of site locations surveyed including site map label (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0142427#pone.0142427.g001" target="_blank">Fig 1</a>), corresponding site name, depth (m), date surveyed, and GPS location (latitude and longitude).</p

Survey Map.

<p>Survey locations on the south shore of Bermuda at shallow (10m; gray markers) and mesophotic (45m; black markers) sites. S1: Rita, 10m; D1: XL, 45m; S2: Coopers, 10m; D2: Coopers, 45m; S3: Tuckers, 10m; D3: Tuckers, 45m; S4: Spittal, 10m; D4: Spittal, 45m; S5: Devonshire, 10m; D5: Devonshire, 45m; S6: Hungry Bay, 10m; D6: Hungry Bay, 45m.</p

Distribution Parameters.

<p><i>M</i>. <i>cavernosa</i> population distribution parameters including site name, depth (m), geometric mean surface area (μ; cm²), skewness (g₁), kurtosis (g₂), standard deviation (SD), maximum colony surface area (95%; cm²), the probability that the populations is from a normal distribution (Pnorm), and the sample size (n) for each site surveyed and for all shallow sites and all mesophotic sites combined.</p

Distribution Parameters by Depth.

<p>Mean standard deviation, skewness, and kurtosis (± SE) of <i>M</i>. <i>cavernosa</i> population size-frequency distributions from measured colonies at shallow (10m; gray squares) and mesophotic (45m; black triangles) sites (Rita/XL, Coopers, Tuckers, Spittal, Devonshire, Hungry Bay).</p

Colony Abundance, Surface Area, and Percent Cover by Depth.

<p>(a) mean number of <i>M</i>. <i>cavernosa</i> colonies per 60m² ± SE at shallow (10m; gray bars) versus mesophotic (45m; black bars) sites (Rita/XL, Coopers, Tuckers, Spittal, Devonshire, Hungry Bay); (b) mean <i>M</i>. <i>cavernosa</i> colony surface area (cm²) ± SE at shallow (10m; gray bars) versus mesophotic (45m; black bars) sites; (c) mean percent cover ± SE of <i>M</i>. <i>cavernosa</i> at shallow (10m; gray bars) versus mesophotic (45m; black bars) sites (n = 6 per depth).</p

Nutrient Concentration and Temperature by Depth.

<p>(a) mean (±SD) concentration (μM) of nitrate (NO₃) + nitrite (NO₂) and silicate (SiO₄^-2) on shallow (10m; n = 4 per site) versus mesophotic sites (45m; n = 4 per site) from water samples collected September 5, 2015 (NO₃ + NO₂, p<0.0001, Tuckers, p = 0.019, Spittal; SiO₄^-2, p = 0.001, Tuckers, p = 0.058, Spittal; Student’s <i>t</i>-tests); (b) box blot of seawater temperature at shallow (10m) and mesophotic (45m) sites showing median values (solid horizontal line), 25^th and 75^th percentile values (box outline), and minimum and maximum values (whiskers) recorded between July 2014 and January 2015 from 6 paired shallow (10m) and mesophotic (45m) survey sites (2 dives per site); Rita/XL, Coopers, Tuckers, Spittal, Devonshire, and Hungry Bay (p<0.0001, Students <i>t</i>-test, n = 6).</p