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Garcia-Sais et al. 2007


technical report | Acta Ichthyol Piscat

Characterization of benthic habitats and associated reef communities at Bajo de Sico Seamount, Mona Passage, Puerto Rico

Garcia-Sais JR, Castro R, Sabater-Clavell J, Carlo M, Esteves R


Abstract

Bajo de Sico (BDS) is a seamount that rises from a deep platform (177 m) of the insular slope in the west coast of Puerto Rico. Reef bathymetry is characterized by a ridge of rock promontories aligned southeast – northwest which rise from a platform at 45 m to a reef top at 25 m, and an extensive, mostly flat, homogeneous and gradually sloping shelf that ends as a vertical (shelf-edge) wall at depths between 90 – 100 m reaching down to depths of 200 – 300 m. Salient oceanographic features of the water column influencing the reef system include a warm mixed surface water mass with a summer thermocline at a depth of 45 – 50 m, strong, persistent northwesterly surface currents, and high water transparency with 1% light penetration reaching depths of almost 80 m. Benthic habitats that were identified and field verified to a maximum depth of 50 m at BDS include: a reef top and a vertical reef wall associated with rock promontories, colonized pavement and sand channels at the base of promontories, uncolonized gravel and rhodoliths at the reef slope, and a colonized rhodolith reef habitat surrounding the rock promontories at least to a depth of 50 m. Benthic habitats beyond 50 m have not been field verified, but several video images produced by the R/V Nancy Foster detected coral growth down to a maximum depth of 90 m along the deep shelf platform at BDS. From the multi-beam bathymetry survey of the reef produced aboard the R/V Nancy Foster, the total extension of BDS includes a surface area of approximately 11.1, km 2 of which only 3.6 % is associated with rock promontories (0.4 km 2 ) and more than 88 % corresponds to the deep shelf platform at depths below 50 m. The sessile-benthic community at the reef top was characterized by a highly diverse assemblage comprised by benthic algae (52%), sponges (26%), scleractinian corals (8%), octocorals (5%) and hydrozoans (3%), with an abiotic cover of less than 1.5%. Scleractinian corals were represented at the reef top by 13 species within transects surveyed, with a mean substrate cover of 8.0% and a mean density of almost 20 colonies/m2 . Growth of scleractinian corals at the reef top was characterized by a species rich and numerous assemblage of small, isolated encrusting colonies that contributed minimal topographic relief. Lettuce corals, mostly Agaricia lamarki and A. grahame were the dominant assemblage in terms of reef substrate cover and density of colonies. Tubastrea coccinea, Porites astreoides and Montastraea cavernosa were also v common at the reef top. Sponges, represented within transects by at least 12 species were the dominant sessile-benthic invertebrate in terms of reef substrate cover (mean: 26%) at the reef top. Due to their large size and abundance, sponges contributed substantially to the reef topographic relief and served as an important habitat for fishes and invertebrates. The reef wall habitat was characterized by irregular formations that appear to have been influenced by erosional processes, with deep crevices, undercuts, gaps, ledges and other substrate irregularities. The sessile-benthos of the reef wall habitat resembled the reef top in that it was also highly diverse and taxonomically complex, comprised by sponges (43%), benthic algae (26%), octocorals (14%), scleractinian corals (5.5%), antipatharians (3%) and hydrozoans (2%). Abiotic cover was approximately 4%. Sponges were the most prominent component of the sessile-benthos at the reef wall, with at least 11 species present within transects surveyed and the prevalence of large erect and branching growth forms providing substantial topographic relief and reef substrate complexity. Octocorals (gorgonians), particularly the deep sea fan, Iciligorgia schrammi combined with black corals (Antipatharians), mostly the Caribbean bushy coral, Antipathes caribbeana to contribute an average reef substrate cover of 17%, adding to the benthic substrate heterogeneity and providing protective habitat for fishes at the reef wall. As in the reef top, scleractinian corals were present as a species rich assemblage of numerous, but small isolated colonies growing encrusted to the hard ground substrate and contributing minimally to the reef topographic relief. The deep platform rhodolith reef, at least down to the maximum surveyed depth of 50 m, appears to be a vast deposit of crustose algal nodules or rhodoliths overgrown by a dense macroalgal carpet, mostly the encrusting fan-leaf alga, Lobophora variegata. The sessile-benthic invertebrate community was characterized by relatively low taxonomic diversity, with virtual absence of gorgonians and antipatharians, low substrate cover and species composition by scleractinian corals and a marked decline of cover and species composition by sponges, relative to the reef top and wall habitats. With few exceptions, scleractinian corals and sponges grow attached to rhodoliths, and are therefore not fixed to the bottom. Lettuce corals, Agaricia spp. were the dominant scleractinian taxa in terms of reef substrate cover. vi Reef fishes associated with BDS were comprised by a combination of the typical shallow water reef species, a small assemblage of deep reef species, large demersal predators (snappers and groupers), and the group of pelagic highly migratory oceanic predators. Zooplanktivorous schooling fish populations are abundant at BDS and appear to serve as the main forage for large pelagic and demersal piscivorous fishes of the reef. Both fish abundance and species richness declined markedly with increasing depth at the benthic habitats studied. Variations of fish taxonomic composition and abundance between habitats appear to be influenced by the availability only of microhabitats at the deep rhodolith reef and the limited assemblage of species adapted for the vertically oriented habitat of the reef wall. Reef promontories at BDS represent an important residential and foraging habitat for a group of large, commercially important species of snappers (Lutjanus cyanopterus, L. jocu) and groupers (Epinephelus striatus, Mycteroperca bonaci, M. venenosa, M. interstitialis) that have virtually disappeared from most reef systems in Puerto Rico. It also represents a spawning aggregation site for red hind (Epinephelus guttatus), and possibly other groupers within Mona Passage. The deep rhodolith reef appears to be the residential habitat for the red hind and for an assemblage of fishes that are typical of deep reefs, including some of which are highly valuable for the aquarium trade industry. The reef system at BDS serves as an important foraging and residential habitat for the endangered hawksbill turtle (Eretmochelys imbricata). Its population in the reef promontories is impressive because of the large size and high abundance of individuals. The seamount also functions as a foraging area for large migratory pelagic fishes, including the wahoo (Acanthocibium solanderi), mahi-mahi (Coryphaena hippurus), tunas (Thunnus spp.) and marlins (mostly Makaira nigricans). From the field survey at BDS, the following recommendations are included: a. A year-round permanent closure for fishing of demersal fish species at the entire reef platform is recommended for the protection of what could be one of the few remaining actively reproducing populations of black, yellowmouth, yellowfin and Nassau groupers in Puerto Rico. vii b. Characterization of benthic habitats and associated communities of the deep shelf platform below depths of 50 m using autonomous underwater vehicles (AUV) or similar systems that could provide high resolution images of the reef substrate. c. The hermatypic and ahermatypic coral assemblage at BDS consists of species that have not been previously reported for coral reef systems in PR, and a more comprehensive characterization will require further exploration and research d. The Caribbean Fishery Management Council (CFMC) should convey further research attention to the large grouper-snapper populations at BDS, particularly aspects of their reproductive biology, trophic interactions and larval dispersal and recruitment dynamics

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