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Permanent URI for this collectionhttps://hdl.handle.net/1969.6/86488
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Browsing Publications by Author "Ajemian, Matthew J."
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Item An Analysis of Artificial Reef Fish Community Structure along the Northwestern Gulf of Mexico Shelf: Potential Impacts of “Rigs-to-Reefs” Programs(PLoS ONE, 2015-05-08) Ajemian, Matthew J.; Wetz, Jennifer J.; Shipley-Lozano, Brooke; Shively, J. Dale; Stunz, Gregory W.Artificial structures are the dominant complex marine habitat type along the northwestern Gulf of Mexico (GOM) shelf. These habitats can consist of a variety of materials, but in this region are primarily comprised of active and reefed oil and gas platforms. Despite being established for several decades, the fish communities inhabiting these structures remain poorly investigated. Between 2012 and 2013 we assessed fish communities at 15 sites using remotely operated vehicles (ROVs). Fish assemblages were quantified from standing platforms and an array of artificial reef types (Liberty Ships and partially removed or toppled platforms) distributed over the Texas continental shelf. The depth gradient covered by the surveys (30–84 m) and variability in structure density and relief also permitted analyses of the effects of these characteristics on fish richness, diversity, and assemblage composition. ROVs captured a variety of species inhabiting these reefs from large transient piscivores to small herbivorous reef fishes. While structure type and relief were shown to influence species richness and community structure, major trends in species composition were largely explained by the bottom depth where these structures occurred. We observed a shift in fish communities and relatively high diversity at approximately 60 m bottom depth, confirming trends observed in previous studies of standing platforms. This depth was also correlated with some of the largest Red Snapper captured on supplementary vertical longline surveys. Our work indicates that managers of artificial reefing programs (e.g., Rigs-to-Reefs) in the GOM should carefully consider the ambient environmental conditions when designing reef sites. For the Texas continental shelf, reefing materials at a 50–60 m bottom depth can serve a dual purpose of enhancing diving experiences and providing the best potential habitat for relatively large Red Snapper.Item A Comparison of Fish Community Structure at Mesophotic Artificial Reefs and Natural Banks in the Western Gulf of Mexico(Taylor & Francis, 2017-04-11) Streich, Matthew K.; Ajemian, Matthew J.; Wetz, Jennifer J.; Stunz, Gregory W.Oil and gas platforms along the northwestern Gulf of Mexico (GOM) shelf have served as artificial reefs since oil and gas exploration intensified in the 1950s. As these structures are decommissioned, they must be removed; however, some are converted to permanent artificial reefs. Despite the potential effects these artificial habitats may have on marine fisheries, investigations that assess the fish communities inhabiting these structures relative to natural habitats are rare. During fall 2012, we used remotely operated vehicle surveys to compare fish communities between artificial reefs (i.e., reefed platforms; n = 5) and adjacent natural banks (n = 5) in the western GOM. Our surveys successfully documented 79 species representing 28 families. Multivariate analyses suggested that fish communities at artificial reefs were distinct from those at natural banks. Post hoc analyses indicated that the differences were driven by high abundances of transient, midwater pelagics and other gregarious species at artificial reefs. Many fisheries species, like the Red Snapper Lutjanus campechanus, were found in both habitat types, with density at artificial reefs estimated to be nearly eight times greater than at natural banks. Despite lower densities at natural banks, the disproportionately large areas of these habitats resulted in relatively high total abundance estimates—approximately 5% of the 2012 GOM Red Snapper annual catch limit (3.67 million kg [8.08 million lb])—a finding that has significant implications for Red Snapper and artificial reef management in the GOM. Our study suggests that although fish community structure may differ between these two habitats, artificial reefs serve as important habitat for species like Red Snapper by potentially diverting fishing pressure from natural habitats; however, future studies that address species-specific life history traits will be needed to better understand the function and performance of artificial reefs in supporting fisheries productivity.Item Effects of a New Artificial Reef Complex on Red Snapper and the Associated Fish Community: an Evaluation Using a Before–After Control–Impact Approach(Taylor & Francis, 2017-09-21) Streich, Matthew K.; Ajemian, Matthew J.; Wetz, Jennifer J.; Shively, J. Dale; Shipley, J. Brooke; Stunz, Gregory W.Artificial reefs are commonly created with the goal of enhancing fish populations. However, many studies evaluating their effects on these populations have been hindered by a lack of preconstruction data from existing natural habitats and temporal comparisons with control areas. Here, we present findings from a before–after control–impact study designed to assess the effects of a new artificial reef on fish populations in the western Gulf of Mexico. Vertical line and fish traps were used to sample the reef site and a paired control site with soft bottom substrates for 1 year before and 2 years after reef construction. Prior to reef construction in October 2013, and over bare substrates in general, infrequent catches of sea catfishes and small coastal sharks were observed. With the exception of rare occurrences of juvenile Gray Triggerfish Balistes capriscus and Red Snapper Lutjanus campechanus, which were observed only during the summer recruitment season, the control site exhibited a lack of reef species. In contrast, we documented dramatic increases in the frequency of occurrence and abundance of multiple reef species at the reef site following the addition of structured habitat. Distinct cohorts of Red Snapper could be followed through time suggesting site fidelity, and few fish greater than age 2 years were captured indicating limited migration of older fish from other areas. Given that the reef supported high densities of juvenile Red Snapper that were in good condition, growing quickly, and protected from potential shrimp trawl mortality, export of juveniles (i.e., production) to the adult population was evident and likely greater on a per-unit-area basis than for the control site. Our study highlights the potential benefits of artificial reefs to species like Red Snapper; however, future studies should investigate the relative roles of emigration and fishing mortality to better understand the effects of these structures on reef fish population dynamics.Item Habitat-Specific Reproductive Potential of Red Snapper: A Comparison of Artificial and Natural Reefs in the Western Gulf of Mexico(American Fisheries Society, 2018-07-16) Downey, Charles H.; Streich, Matthew K.; Brewton, Rachel A.; Ajemian, Matthew J.; Wetz, Jennifer J.; Stunz, Gregory W.Energy exploration in the Gulf of Mexico (hereafter, Gulf) has resulted in the addition of numerous oil and gas pro-duction platforms that create structurally complex habitat in an area otherwise dominated by barren mud/sand bottom. How these artificial structures affect fish populations is largely unknown, and there is ongoing debate regarding their value as surrogate habitats for ecologically and economically important reef fish species. Thus, the purpose of this study was to characterize trends in Red Snapper Lutjanus campechanus reproductive potential in the western Gulf at oil and gas platform reefs relative to reproductive potential at natural banks. Red Snapper (n=1,585) were collected during 2013–2015 from standing platforms, decommissioned platform artificial reefs, and natural banks by using standardized vertical line gear. Comparisons of gonadosomatic index, male : female ratios, batch fecundity, annual fecundity, spawning frequency, and number of spawning-capable individuals indicated that Red Snapper reproductive biology was similar among natural bank, standing platform, and artificial reef habitats. These results suggest that in terms of reproductive output, fish inhabiting artificial reefs are functionally similar to similar-aged fish on natural banks. This work can be used to make informed management decisions and suggests that there are benefits to converting decommissioned platforms into designated artificial reefs. Future studies should consider site-specific characteristics, such as depth, vertical relief, and proximity to other structures, to elucidate how habitat characteristics may influence reproduction, ultimately improving future artificial reef deployments for fisheries enhancement in the western Gulf.Item Movement patterns and habitat use of tiger sharks (Galeocerdo cuvier) across ontogeny in the Gulf of Mexico(PlosONE, 2020-07) Ajemian, Matthew J.; Drymon, J. Marcus; Hammerschlag, Neil; Wells, R. J. David; Street, Garrett; Falterman, Brett; McKinney, Jennifer A.; Driggers III, William B.; Hoffmayer, Eric R.; Fischer, Christopher; Stunz, Gregory W.The tiger shark (Galeocerdo cuvier) is globally distributed with established coastal and open-ocean movement patterns in many portions of its range. While all life stages of tiger sharks are known to occur in the Gulf of Mexico (GoM), variability in habitat use and movement patterns over ontogeny have never been quantified in this large marine ecosystem. To address this data gap we fitted 56 tiger sharks with Smart Position and Temperature transmitting tags between 2010 and 2018 and examined seasonal and spatial distribution patterns across the GoM. Additionally, we analyzed overlap of core habitats (i.e., 50% kernel density estimates) among individuals relative to large benthic features (oil and gas platforms, natural banks, bathymetric breaks). Our analyses revealed significant ontogenetic and seasonal differences in distribution patterns as well as across-shelf (i.e., regional) and sex-linked variability in movement rates. Presumably sub-adult and adult sharks achieved significantly higher movement rates and used off-shelf deeper habitats at greater proportions than juvenile sharks, particularly during the fall and winter seasons. Further, female maximum rate of movement was higher than males when accounting for size. Additionally, we found evidence of core regions encompassing the National Oceanographic and Atmospheric Administration designated Habitat Areas of Particular Concern (i.e., shelf-edge banks) during cooler months, particularly by females, as well as 2,504 oil and gas platforms. These data provide a baseline for future assessments of environmental impacts, such as climate variability or oil spills, on tiger shark movements and distribution in the region. Future research may benefit from combining alternative tracking tools, such as acoustic telemetry and genetic approaches, which can facilitate long-term assessment of the species’ movement dynamics and better elucidate the ecological significance of the core habitats identified here.Item Movement, Behavior, and Habitat Use of a Marine Apex Predator, the Scalloped Hammerhead(Frontiers in Marine Science, 2018-09-10) Wells, R. J. David; TinHan, Thomas C.; Dance, Michael A.; Drymon, J. Marcus; Falterman, Brett; Ajemian, Matthew J.; Stunz, Gregory W.; Mohan, John A.; Hoffmayer, Eric R.; Driggers III, William B.; McKinney, Jennifer A.Conservation and management efforts of marine apex predators are more reliable when information on movement and habitat use patterns are known. The scalloped hammerhead (Sphyrna lewini) was the first shark species to be protected under the U.S. Endangered Species Act and has life history characteristics that make this species particularly at risk for local depletion. Consequently, the goal of this study was to better understand the movement dynamics of this species in the Gulf of Mexico (GOM) where discards through the longline fishery can be substantial. A total of 33 scalloped hammerheads were tagged with fin mounted satellite tags and tracked for an average of 146 days (ranging from 5 to 479 days) to examine horizontal movements and quantify space use. Scalloped hammerheads showed a wide range of movements throughout the GOM continental shelf with limited long-distance dispersal and females displayed a shelf-edge association relative to more mid-shelf use by males. A generalized additive model was developed to identify habitat suitability for scalloped hammerheads in the GOM, while state-space modeling was used to examine movement behaviors. Model results highlighted the use of continental shelf waters with high occurrence at close proximities to both artificial and hard-bottom habitat combined with low chlorophyll a concentrations (∼0–4 mg m-3) and moderate salinities (33–35.5). Habitat suitability for scalloped hammerheads was predicted to be high on the mid to outer continental shelf inside the 200 m isobath and state-space model results suggest area-restricted behavior was most common relative to transient behavior. Findings from this study provide important information on movement of this species in the GOM and highlight their restricted use of continental shelf habitat and resident behavior that will need to be incorporated in future stock assessments and extinction risk analyses.Item Population connectivity of pelagic megafauna in the Cuba-Mexico-United States triangle(Scientific Reports, 2019-02-07) Rooker, Jay R.; Dance, Michael A.; Wells, R. J. David; Ajemian, Matthew J.; Block, Barbara A.; Castleton, Michael R.; Drymon, J. Marcus; Falterman, Brett J.; Franks, James S.; Hammerschlag, Neil; Hendon, Jill M.; Hoffmayer, Eric R.; Kraus, Richard T.; McKinney, Jennifer A.; Secor, David H.; Stunz, Gregory W.; Walter, John F.The timing and extent of international crossings by billfishes, tunas, and sharks in the Cuba-Mexico-United States (U.S.) triangle was investigated using electronic tagging data from eight species that resulted in >22,000 tracking days. Transnational movements of these highly mobile marine predators were pronounced with varying levels of bi- or tri-national population connectivity displayed by each species. Billfishes and tunas moved throughout the Gulf of Mexico and all species investigated (blue marlin, white marlin, Atlantic bluefin tuna, yellowfin tuna) frequently crossed international boundaries and entered the territorial waters of Cuba and/or Mexico. Certain sharks (tiger shark, scalloped hammerhead) displayed prolonged periods of residency in U.S. waters with more limited displacements, while whale sharks and to a lesser degree shortfin mako moved through multiple jurisdictions. The spatial extent of associated movements was generally associated with their differential use of coastal and open ocean pelagic ecosystems. Species with the majority of daily positions in oceanic waters off the continental shelf showed the greatest tendency for transnational movements and typically traveled farther from initial tagging locations. Several species converged on a common seasonal movement pattern between territorial waters of the U.S. (summer) and Mexico (winter).Item Was Everything Bigger in Texas? Characterization and Trends of a Land‐Based Recreational Shark Fishery(American Fisheries Society, 2016-11-10) Ajemian, Matthew J.; Jose, Philip D.; Froeschke, John T.; Wildhaber, Mark L.; Stunz, Gregory W.Although current assessments of shark population trends involve both fishery‐independent and fishery‐dependent data, the latter are generally limited to commercial landings that may neglect nearshore coastal habitats. Texas has supported the longest organized land‐based recreational shark fishery in the United States, yet no studies have used this “non‐traditional” data source to characterize the catch composition or trends in this multidecadal fishery. We analyzed catch records from two distinct periods straddling heavy commercial exploitation of sharks in the Gulf of Mexico (historical period = 1973–1986; modern period = 2008–2015) to highlight and make available the current status and historical trends in Texas’ land‐based shark fishery. Catch records describing large coastal species (>1,800 mm stretched total length [STL]) were examined using multivariate techniques to assess catch seasonality and potential temporal shifts in species composition. These fishery‐dependent data revealed consistent seasonality that was independent of the data set examined, although distinct shark assemblages were evident between the two periods. Similarity percentage analysis suggested decreased contributions of Lemon Shark Negaprion brevirostris over time and a general shift toward the dominance of Bull Shark Carcharhinus leucas and Blacktip Shark C. limbatus. Comparisons of mean STL for species captured in historical and modern periods further identified significant decreases for both Bull Sharks and Lemon Sharks. Size structure analysis showed a distinct paucity of landed individuals over 2,000 mm STL in recent years. Although inherent biases in reporting and potential gear‐related inconsistencies undoubtedly influenced this fishery‐dependent data set, the patterns in our findings documented potential declines in the size and occurrence of select large coastal shark species off Texas, consistent with declines reported in the Gulf of Mexico. Future management efforts should consider the use of non‐traditional fishery‐dependent data sources, such as land‐based records, as data streams in stock assessments.