We are currently in the process of evaluating the mercury (Hg) concentration in the tissue of pelagic fishes from the NW Gulf of Mexico, examining spatial variation in concentration of Hg in the tissue of pelagic fishes and developing predictive models relating fish size and feeding histories (based on natural dietary markers-fatty acids) to Hg profile in pelagic fishes. Current work centers on relating patterns of movement and site fidelity (pop-up satellite tags) to fish tissue Hg, and developing a predictive model to evaluate risk factors associated with consumption of pelagic fishes with varying levels of fish tissue Hg.

Recently, mercury (Hg) concentration was determined in the tissues of ten pelagic fishes, with a special emphasis on apex predators (Table 1). Highest Hg levels were observed in blue marlin, carcharhinid sharks (Genus Carcharhinus) and little tunny, ranging from ~1.0 to 10.5 ppm. Moderate to low concentrations (<1.0 ppm) were observed in blackfin tuna, cobia, dolphin, greater amberjack, king mackerel, wahoo, and yellowfin tuna. For the majority of species examined, Hg concentrations did not vary significantly between location (Texas vs. Louisiana) or collection period. Relationship between Hg concentration and fish size/weight were also explored for six taxa (blackfin tuna, carcharhinid sharks, dolphin, king mackerel, wahoo, yellowfin tuna), with each showing a significant positive relationships between Hg concentration and body size and/or weight. Natural dietary tracers, stable isotopes (δ15N, δ13C) and fatty acids, were used to evaluate the relationship between Hg and trophic position and the relationship between Hg and dietary history. Stable nitrogen isotope analysis showed that Hg concentrations in fish tissues were positively associated with trophic position. Based on the δ13C and δ15N values of pelagic fishes, three natural associations were identified and groupings were identical to those detected with fatty acid signatures. No connection between the three natural trophic associations and Hg was observed, and thus the bioaccumulation of Hg in these pelagic taxa was not related directly to prey selection patterns.

Further Reading
Cai Y , Rooker JR, Gill G, Turner JP (2007) Bioaccumulation of mercury in pelagic fishes from the Gulf of Mexico: evaluating links to dietary history and trophic position using stable isotopes and fatty acids. Canadian Journal of Fisheries and Aquatic Sciences 64: 458-469

Cai Y, Rooker JR, Gill G (2006) Bioaccumulation of mercury in pelagic fishes in the NW Gulf of Mexico and its relationship with location, length, collection year and trophic level. Proceeding of the Gulf and Caribbean Fisheries Institute 57: 317-326

The aim of our research on red snapper (Lutjanus campechanus) was to determine patterns of habitat use by newly settled individuals, and assess the quality of different banks and habitats used during early life. Trawl surveys were conducted to measure patterns of habitat use by newly settled red snapper (Lutjanus campechanus) at three natural banks on the inner continental shelf of Texas. Digital side-scan sonar and multibeam bathymetric data were used to define inshore (mud), ridge (shell), and offshore (mud) habitats for Freeport Rocks, Heald Bank, and Sabine Bank. Otter trawls were conducted July through September in 2003 (Heald Bank, Sabine Bank) and in 2004 (Freeport Rocks) during the settlement period for red snapper. Freeport Rocks had markedly higher densities of red snapper (91 ha−1) in 2004 than Heald Bank (6 ha−1) or Sabine Bank (<1 ha−1) in 2003. A significant habitat effect was observed at Heald Bank and densities were higher at offshore mud habitats; no habitat effect was detected for Freeport Rocks or Sabine Bank. Growth rates varied from 0.86 mm⋅d−1 at Sabine Bank up to 1.12 mm⋅d−1 at Freeport Rocks, and rates were higher on inshore and offshore mud than ridge habitats. Otolith based estimates of age indicated that settlers were first detected at 22–28 d and the majority of individuals were 30–60 d. Hatch dates peaked from early June to early July in both 2003 and 2004. Results from this study indicate that both shell and inshore and offshore mud habitats associated with these natural banks serve as settlement habitat of red snapper, and all three habitats have the potential to function as nursery areas of this species.

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Density of newly settled red snapper on Freeport rocks, red indicates high density. Plots from top left (clockwise) early July, late July, early August, and late August.

Further Reading
Geary BW , Mikulas JJ , Rooker JR, Landry AM (2007). Patterns of habitat use by newly settled red snapper in the northwestern Gulf of Mexico. Pages 37 - 53 In W. F. Patterson et al. eds. Red Snapper Ecology and Fisheries in the U.S. Gulf of Mexico. American Fisheries Society, Symposium 60, Bethesda, Maryland.

Rooker JR, Landry AM, Geary BW, Harper JA (2004) Assessment of a bathymetric high and associated habitats as nursery grounds of postsettlement red snapper. Estuarine, Coastal and Shelf Science 59: 653-661

A broad goal of oceanographers and marine ecologists has been to understand trophic relationships in shelf and open ocean communities. To date, considerable work has focused upon characterizing benthic ecosystems, and these efforts have led to valuable information on energy flow and food web structure. Unfortunately, information on the structure and dynamics of pelagic ecosystems has not received the same attention, despite the fact that these systems contribute substantially to total primary production, biogeochemical cycling and global fishery yields (Pauly & Christensen 1995). The lack of information on pelagic food webs is of particular concern since large predatory fishes within these ecosystems have experienced substantial declines over the past few decades, thus altering food web structure and the relative impact of top–down controls (Jackson et al. 2001, Watson & Pauly 2001, Meyers & Worm 2003). As a result, there is a clear need to investigate pelagic food webs and this information is prerequisite for maintaining biodiversity and fishery yields.

Apart from floating Sargassum spp. (hereafter ‘Sargassum’), the pelagic zone of the Atlantic Ocean and the Gulf of Mexico is characterized by lack of structure and low levels of primary production. Sargassum, a brown macroalgae (Phaeophyceae) comprised of 2 species (S. natans and S. fluitans), is a ubiquitous surface feature in this region. Sargassum often accumulate in large mats or windrows, thereby forming a structured habitat for pelagic fauna, and survey work indicates that these floating mats represent a critical habitat for several members of the pelagic community, including a variety of invertebrates, fishes and sea turtles. Moreover, several recreationally and commercially important finfish use Sargassum mats as refuge during early life, and it is likely that the structural complexity afforded by floating Sargassum reduces predation-mediated mortality. If this assumption is valid, survival and recruitment success of certain pelagic fishes will be linked to the distribution and abundance of Sargassum.

Understanding the trophic structure of Sargassum communities requires a detailed understanding of the feeding histories of associated fauna. In recent years, stable isotopes and fatty acids have been used extensively to investigate marine food web structure, since consumer tissues reflect the isotopic and fatty acid composition of prey in a predictable. These natural biomarkers provide time integrated or long term measures of diet, and both approaches afford information on source(s) of organic matter supporting local food webs as well as trophic relationships of associated consumers. In a recent study, we used both stable isotope and fatty acid signatures to identify the source(s) of organic matter supporting pelagic fishes in mid-shelf waters of the Gulf of Mexico. In addition, these natural dietary markers were used to delineate pathways of energy flow through the Sargassum complex from autotrophs to apex predators, with a species emphasis on fishes and their presumed prey (e.g. crabs, shrimps). The goals of this work were to enhance our understanding of food web dynamics within this prominent yet poorly understood component of the mid-shelf shelf ecosystem, and to determine whether Sargassum is an important source of energy for pelagic fishes.

Synopsis of results to date: Stable carbon isotope ratios (d13C) of four autotrophs (Cladophora sp., phytoplankton [based on POM], Sargassum fluitans, Sargassum natans) were distinct (range:-16.3 to -21.0 ‰), with S. fluitans and S. natans enriched by 2-5 ‰ relative to Cladophora and POM. Stable nitrogen isotope ratios (d15N) of both S. fluitans and S. natans were depleted by 5 –7 ‰ compared to Cladophora and POM. The majority of d13C values of consumers were between -16 and -18 ‰, and d13C values were most depleted for juvenile shrimps, juvenile crabs, and certain juvenile fishes. Stable carbon isotope ratios of adult fishes varied from -16.1 to -17.5 ‰. Enrichment of δ15N occurred with increasing trophic position and the lowest values were observed for juvenile crustaceans, which ranged from 6.0 to 8.7 ‰. The majority of juvenile fishes were secondary heterotrophs (δ15N values ca. 8.0 to 11.0 ‰), while most adult fishes were tertiary consumers with δ15N values ranging from 11.9 to 14.3 ‰. Carbon source estimates from the two-source mixing model indicated that ~ 80 % of organic matter supplied to consumers (pooled across taxa) in the Sargassum complex was derived from POM. Fatty acid signatures of the primary producers were significantly different and used to further evaluate organic matter contribution to Sargassum-associated consumers. C22 polyunsaturated fatty acids (PUFAs) (22:6n-3, 22:5n-3) were most abundant in POM, while high levels of C18 and C20 PUFAs were observed for Cladophora and Sargassum (18:2n-6 and 20:4n-6, respectively). Consumer signatures were dominated by 22:6n-3.

Stable carbon and nitrogen isotope ratios (‰ mean ±1 SE) of producers and consumers associated with the Sargassum complex in the northwestern Gulf of Mexico Boxes with dashed lines are used to represent the stable isotope ratios of producers and arrows denote the expected trajectory of enrichment with increasing trophic position. Lower and upper case letters were used to denote invertebrates and fishes, respectively.

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Percent composition of fatty acids within autotrophs and trophic levels (TL) 1.0-4.0. A suite of invertebrates and fishes were included in each trophic level category: TL 1.0 (Balistes capriscus, Latreutes fucorum, Leander tenuicornis), TL 2.0 (Acanthocybium solandri, Caranx crysos, Histrio histrio, Monocanthus hispidus, Portunus sayi), TL 3.0 (Coryphaena hippurus, Kyphosus saxatilis, Makaira nigricans, Seriola dumerili, Thunnus atlanticus, Thunnus albacares), TL 4.0 (Euthynnus alletteratus, Scomberomorus cavalla). Mean values are reported for grouped saturated, monounsaturated, 5 abundant polyunsaturated fatty acids (PUFAs) 18:2n-6, 20:4n-6, 20:5n-3, 22:5n-3, 22:6n-3, and all other PUFAs. Sample size (N) denoted for each category.

Further Reading
Turner JP, Rooker JR (2006) Fatty acid composition of flora and fauna associated with Sargassum mats in the Gulf of Mexico. Marine Biology 149: 1025-1036

Rooker JR, Turner JP, Holt SA (2006) Trophic ecology of Sargassum-associated fishes in the Gulf of Mexico determined from stable isotope and fatty acid analysis. Marine Ecology Progress Series 313: 249-259

Wells RJD, Rooker JR (2004) Distribution, age, and growth of young-of-the year greater amberjack, Seriola dumerili, associated with pelagic Sargassum. Fishery Bulletin 102:545-554

Wells RJD, Rooker JR (2004) Spatial and temporal habitat use by fishes associated with Sargassum mats in the NW Gulf of Mexico. Bulletin of Marine Science 74: 81-99