technical report |
Garcia-Sais JR, Williams SM, Esteves RF, Sabater J, Carlo MA
This research provides mapping and a physical description of the mesophotic benthic habitats of Tourmaline Reef, along with a quantitative and qualitative characterization of the sessile-benthic, fish, and shellfish (queen conch, spiny lobster) populations associated with each of the main benthic habitats present within a 30 – 50 m depth range. Density estimates of large commercially important fish and shellfish populations were produced to contribute fishery independent data for assessment of fishery stocks within mesophotic habitats/sites of the Puertorrican EEZ. This project forms part of an on-going research initiative directed towards the characterization of essential fish habitats (EFH) and associated reef communities from Puerto Rico and the U. S. Virgin Islands sponsored by NMFS thru the Caribbean Fishery Management Council (CFMC). This research complements ongoing programs of coral reef community characterizations and monitoring sponsored by NOAA thru the Department of Natural and Environmental Resources of Puerto Rico (DNER) and the U. S. V. I., Division of Coastal Zone Management. Five main benthic habitat types were recognized within the 30 – 50 m depth range. These included a mostly unconsolidated and abiotic 1) sandy substrate; 2) scattered patch reefs surrounded by sand; 3) colonized pavement; 4) algal rhodolith reef deposits; and 5) a slope wall rocky habitat. Sand was the main substrate type in terms of areal cover with approx. 6.7 km2 , or 48.1 % of the total study area, yet mostly uncolonized (abiotic), with the sporadic occurrence of interspersed gorgonians and occasional sightings of milk and/or queen conch. Rhodolith reef deposits were the most prominent benthic habitat present along the western section of the mesophotic outer shelf, and represented the dominant biotic habitat in terms of areal cover with 5.19 km2 , or 37.5 % of the total study area within the 30 – 50 m depth range. Live coral reef habitats within the mesophotic 30 – 50 m depth range were very scarce at Tourmaline Reef and only associated with a small yellow-pencil (Madracis auretenra) biotope growing as a patch within the rhodolith reef. The virtual absence of live coral from mesophotic benthic habitats at Tourmaline appears to be related both to the high areal cover by sand and by its potential abrasive effect on adjacent hard ground attachment substrates. The community structure of sessile-benthic biota evidenced a pattern of higher affinities within habitat types than within depths. Distinct patterns of community structure dissimilarities were detected between habitat types. Sessile-benthic community structure at the wall differed significantly from all other benthic habitat types. Also, statistically significant differences in the taxonomic composition and rank order of sessile-benthic substrate categories were observed between the rhodolith and colonized pavement habitats. Higher percent cover by sponges, and octocorals, and lower percent of substrate cover by abiotic categories were consistently measured from the slope wall habitat, as compared to other benthic habitat types. Also, the density of scleractinian coral colonies (mostly orange cup coral, Tubastraea coccinea) at the slope wall was higher than at any other habitat type surveyed. Colonized pavement and scattered patch reef habitats did not differ significantly in terms of community structure from each other. Both habitats exhibited high abiotic and benthic algae cover, low cyanobacteria cover and high species richness of sponges. Despite what appeared to be adequate hard bottom conditions for attachment and good light penetration, live scleractinian coral cover was very low on both of these habitats probably due to the intense abrasion associated with sand flux. A total of 78 fish and three shellfish species were observed within mesophotic habitats during diver surveys at Tourmaline Reef. The taxonomic composition of reef fishes and their rank order abundance conferred higher affinities within habitat types than within depths, a pattern that is consistent with the sessile-benthic community characterization for this site. Fish community structure at the slope wall (W) differed significantly from all other benthic habitats. Statistically significant differences in the taxonomic composition and rank order abundance of fishes were observed between the rhodolith and scattered patch reef habitats. The fish assemblage at the slope wall differed from all other benthic habitats mostly due to the prominent abundance of blackfin snapper (Lutjanus buccanella) relative to other habitats surveyed. Other fish species were observed only, or in higher abundance at the wall relative to other benthic habitats. These include the blue and sunshine chromis (Chromis cyanea, C. insolata), fairy basslet (Gramma loreto), blackjack and blue runner (Caranx lugubris, C. crysos), French angelfish (Holacanthus ciliaris), and large adult dog and cubera snappers (Lutjanus jocu, L. cyanopterus). The slope wall appears to function as a recruitment habitat for blue and sunshine chromis, and also as a reproductive and foraging site for large demersal and pelagic reef fishes. Differences between fish assemblages at the colonized pavement and scattered patch reef habitats were mostly related to the higher relative abundance of bicolor damselfish at the colonized pavement, and an overall higher number of species at the former. Fish assemblages at the scattered patch reef habitat were unique in that the most abundant species within transects surveyed was the squirrelfish (Holocentrus rufus). In addition to the squirrelfish, the scattered patch reef habitat exhibited higher relative abundance of yellowtail snapper (Ocyurus chrysurus), reef butterfly fish (Chaetodon sedentarius), long jaw squirrelfish (H. adcensionis), orangeback basslet (Serranus annularis), doctorfish (Acanthurus chirurgus) and redspotted hawkfish (Amblycirrhitus pinos) than the colonized pavement habitat and the rhodolith reef habitat. The rhodolith reef exhibited the typical fish community structure that has been previously reported for this type of mesophotic benthic system. Numerically dominant species of the fish assemblage include the bicolor damselfish (Stegastes partitus), cherubfish (Centropyge argi), chalkbass (Serranus tortugarum), yellow-head and blue-head wrasses (Halichoeres garnoti, Thalassoma bifasciatum), and the greenblotch parrotfish (Sparisoma atomarium). A total of 318 queen conch individuals were observed within belt-transects during fishery independent surveys at Tourmaline Reef. Queen conch were present in all four benthic habitats surveyed, but were more abundant at the rhodolith reef (7.4 Ind/1000 m2 ) and colonized pavement habitats (7.0 Ind/1000 m2 ) within a depth range of 30 – 40 m. The mean density of queen conch from all mesophotic habitats surveyed at Tourmaline Reef was similar to that found at neighbor mesophotic habitats of Abrir la Sierra. The maximum size (length) however, as well as the proportion of larger individuals within the population appeared to be higher at Abrir la Sierra. Spiny lobsters were observed from mesophotic habitats at Tourmaline Reef, with higher densities at the colonized pavement and lowest (none) at the rhodolith reef habitat. The size distribution indicates that both juvenile and adult spiny lobsters are utilizing mesophotic habitats from Tourmaline Reef. Mutton, blackfin, dog and cubera snappers, red hinds, lionfish, hogfish and queen triggerfishes were the most abundant of the large demersal commercially important fishes present within the mesophotic habitats of Tourmaline Reef. Mean density of queen conch, hogfish, mutton, dog and cubera snappers were much higher at Tourmaline and Abrir La Sierra than at oceanic mesophotic systems previously studied. It is here suggested that such higher abundance is related to the stronger physical connectivity of mesophotic habitats at Tourmaline and Abrir la Sierra with recruitment habitats of the shallow neritic shelf as compared to oceanic sites (Desecheo and Bajo de Sico) that are separated from the insular shelf by oceanic depths.