Compared to low-relief soft-sediment environments that dominate the continental shelf of the northwestern Gulf of Mexico, hard banks support diverse fish and coral communities that represent important naturally occurring aggregation areas for exploited populations. Most of these banks are unmonitored and their importance to critical life stages of fish resources has not been quantified. The goal of this project is to evaluate the importance of these banks as habitat for corals and fish populations. As part of this project, we have completed an extensive side scan sonar surveys of Sonnier and McGrail Banks in order to characterize and map habitats. In addition, SCUBA and ROV surveys of the benthos and fish communities at Sonnier Bank were conducted. Results to date indicate the presence and abundance of important fishery resources of snappers and groupers. On both banks, a broad size range and diversity of groupers were observed, and our observations suggest that Sonnier Bank may function as a nursery for scamp, yellowmouth, and Warsaw grouper, and specific areas of each bank may represent spawning sites for scamp and yellowmouth groupers. Sonnier Bank was clearly a more important habitat for snappers than McGrail Bank. In addition, previous analysis of otolith microstructure on a limited number of samples, suggested that age-structure for snappers and groupers at Sonnier Bank are more diverse than what has been reported for other areas in the Gulf and northwestern Atlantic, and samples collected in 2005 have helped to better define the age structure of snapper at Sonnier Bank. Finally, we employed photographic methods to characterize benthic habitats with SCUBA, and computerized image analysis was used to quantify coverage of benthic organisms. This has allowed us to non-destructively quantify and characterize the benthic habitats at the two main peaks of Sonnier Bank. This baseline information has become immediately important due to the potentially catastrophic effects of Hurricane Rita, which passed almost directly across Sonnier Bank as a category-4 hurricane. In response, we will be able to replicate this survey in 2006 and quantify the disturbances caused by this event in terms of the composition, diversity and abundances in both the benthic and fish communities. Finally, in this effort we have confirmed the presence of a non-native coral species at Sonnier Bank. This species is the orange-cup coral (Tubastrea coccinea), which has recently invaded the Gulf of Mexico and presents a conservation concern because it may displace native species.

Below: Bathymetry of Sonnier Bank and McGrail Bank. Detail of areas that were surveyed with SCUBA and ROV are presented as inset side scan sonar images that are labeled alphabetically.

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Many exploited fish species (snappers, grunts, and groupers) settle in near-shore nursery grounds such as seagrass beds or among mangrove prop roots. It is presumed that these habitats provide shelter and food as well as a reduced predation pressure than areas on the coral reef. Coastal development leading to the fragmentation or complete removal of nursery habitats could have a negative impact on adult populations and their associated fisheries. Studies have shown that Caribbean embayments that have seagrass beds and mangroves have a higher density of juvenile fishes and that the presence of adjacent nursery-type habitat significantly increases the number of adults on the reef. Ontogenetic connectivity between biotopes has been inferred by examining spatial and temporal patterns in size distributions between juvenile and adult habitat. Tagging studies have demonstrated connectivity between back-reef habitats on a small spatial scale; however large scale mark-recapture methods are necessary to determine whether juveniles of a species move successfully from putative nursery habitats to adult habitats. Presently, evidence supporting successful movement is largely indirect and the causal agent for this movement has been only briefly addressed (ontogenetic diet shift, morphology changes). We are currently using a suite of natural tracers (stable isotopes in tissue, isotopes/trace elements in otoliths) and tags (coded wire, acoustic) to examine direct and indirect linkages amoung juvenile and asult habitats. In addition, the site- and habitat-specific demographic (size-, age-, sex-based) and biological (growth, reproductive histology) data of exploited species are being collected.

Further Reading
Rooker JR, Secor DH (eds.) (2005) Connectivity in the life cycles of fishes and invertebrates that use estuaries. Estuarine, Coastal and Shelf Science 64: 1-148

Rooker JR, Dennis GD (1991) Diel, lunar, and seasonal changes in a mangrove fish assemblage off southwestern Puerto Rico. Bulletin of Marine Science 49: 684-698

In addition to natural coral reef and hard bank habitats, the abundant onshore petroleum production platforms in the Gulf (ca. 3800) serve as habitat for reef fish assemblages. These structures provide hard substrata which may increase the supply of prey, shelter, and settlement habitat for recruits. Apart from generalized biological descriptions, very little is understood about these artificial reef habitats, particularly with regard to the ecological role of onshore platforms. Production platforms located near the continental shelf margin of the Gulf of Mexico provide a unique opportunity to expand marine research on tropical fishes associated with offshore artificial and natural habitats. Of particular value is Mobil Production Platform High Island (HI) A-389A which is located within the boundaries of the Flower Garden Banks National Marine Sanctuary (FGBNMS). The sanctuary encompasses the northernmost thriving tropical coral reefs on the continental shelf of North America. As a result, fish assemblages located in this area are unique, combining tropical reef-dependent and offshore pelagic fish species. The purpose of this research was to determine spatial and temporal patterns in habitat utilization by juvenile (0-age recruits) and adult fishes occupying an offshore production platform. Emphasis was placed on determining vertical distribution, diel periodicity, and seasonal variation in abundance of reef inhabitants. Microhabitat selection was also investigated. Quantitative surveys from the East and West Flower Garden Banks (EFG and WFG) were conducted for comparisons of natural and artificial reef fish assemblages.

Visual censusing was used to characterize fish assemblages on artificial and natural reefs located within the boundaries of the Flower Garden Banks National Marine Sanctuary (FGBNMS) in the northwestern Gulf of Mexico. Emphasis was placed on determining spatial and temporal patterns in habitat utilization by fishes on an offshore artificial reef (Mobil Platform HI-A389A). Overall, 43 species were observed during diurnal surveys in the upper 24 m of the artificial reef. Midwater pelagic fishes (i.e., carangids and scombrids) accounted for over 50% of all taxa enumerated on the artificial reef; however, these taxa were transient members of the assemblage and were observed infrequently. Labrids, pomacentrids, and serranids were the dominant reef-dependent taxa. Distinct trends in vertical, diel, and seasonal abundances were observed for juvenile and adult fishes. Of the three designated depth zones (upper 1.5-9.0, middle 9.0-16.5; lower 16.5-24.0rm), abundance and species diversity were lowest in the upper zone. Nocturnal counts were characterized by a marked reduction or complete absence of most species, due in part to twilight cover-seeking and movement activities. Seasonal variation in community composition and species abundance (May versus September) was primarily due to recruitment of juveniles (0-age fishes) to the artificial reef in late summer. Increases in total fish abundance (all taxa combined) coincided with both increasing habitat rugosity and degree of fouling. Species richness on natural coral reefs in the FGBNMS was higher than on the artificial reef. Unlike the artificial reef, fish assemblages on the natural reefs were dominated by a single family (Pomacentridae) which accounted for over 50% of all individuals observed.

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