Papers by Matthew Ajemian
Artificial structures are the dominant complex marine habitat type along the northwestern Gulf of... more 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.
Remotely operated vehicles (ROVs) provide a non-extractive approach to characterizing fish commun... more Remotely operated vehicles (ROVs) provide a non-extractive approach to characterizing fish communities in complex habitats. Despite the demonstrated effectiveness of ROVs in studying reef fishes over natural hard-bottom and small artificial reefs, there has been little application of this technology to larger artificialstructures (10s of m tall and wide). We explored the utility of ROVs in rapidly characterizing an assemblageof fishes associated with an artificial reef complex in the western Gulf of Mexico (26.9–28.2◦N; 95.5–97.0◦W) dominated by partially removed and toppled oil and gas platforms. This study reports on an efficient method to sample these structures, where we integrated depth-interval transect (DIT) and continuous roving transect (CRT) protocols to document fish distribution and community structure on 14 artificialreef sites. Consistent with previous hydroacoustic studies, south Texas artificial reefs exhibited a vertically heterogeneous distribution of fishes that varied with structure orientation. These reefs were dominated by economically important lutjanids and carangids, both of which presented sampling challenges dueto their patchy distribution around these vast structures. The non-uniform distribution and mobility of these dominant taxa highlight the utility of adopting roving approaches to assess fish communities on these complex structures. We conclude our study with a discussion of important logistical challenges associated with micro-ROV surveys in deepwater habitats, and potential complementary approaches to assist documentation of demersal fishes inhabiting a persistently turbid bottom layer.
Satellite-tracked manatees routinely lose satellite tags or tag functionality, resulting in the l... more Satellite-tracked manatees routinely lose satellite tags or tag functionality, resulting in the loss of valuable data on migration and habitat use patterns. Fortunately, some movement data from these animals remain salvageable because manatees typically retain a peduncle belt containing an acoustic transmitter that can be detected with a submersible hydrophone. We deployed an array of moored datalogging hydrophones at key locations in our study area to detect manatee belt-embedded acoustic transmitters, a technique not typically used to track manatees. Our array was successful in detecting five tagged manatees, and concurrently detected compatible acoustic tags of other estuarine fauna (e.g. Bull Sharks) tagged by local researchers. Moored datalogging hydrophones, therefore, provided a method to mitigate the loss of satellite tags from estuarine megafauna, and enhanced collaborative opportunities with researchers who tagged other species using compatible equipment.
Coastal shark abundance and community structure
was quantified across 10 geographic areas in the
... more Coastal shark abundance and community structure
was quantified across 10 geographic areas in the
northeastern Gulf of Mexico using fishery-independent
gillnet data from 2003 to 2011. A total of 3,205 sets were
made in which 14,244 carcharhiniform sharks, primarily
juveniles, were caught comprising 11 species from three
families. The three most abundant species, Atlantic
sharpnose Rhizoprionodon terraenovae, bonnethead
Sphyrna tiburo and blacktip sharks Carcharhinus
limbatus, were consistently captured over all sampling
sites regardless of environmental conditions; however,
some species (e.g., bull C. leucas, blacknose
C. acrontous, finetooth C. isodon, and sandbar sharks
C. plumbeus) were restricted to a specific area or a range
of areas. Two-way crossed analysis of similarity
(ANOSIM) found geographic area to significantly influence
shark species-life stage assemblages while season did
not. Resemblance matrices between environmental data
and shark community assemblage found the two were
weakly but significantly correlated, with the combination
of salinity and water clarity producing the highest
Spearman rank correlation value. Species diversity varied
by geographic area, but was generally highest in areas with
the greatest amount of fresh and saltwater fluctuations. Our
results suggest that estuarine conditions adjacent to river
mouths may affect juvenile shark assemblages across similar
latitudes and some areas of the northeastern Gulf of
Mexico may be considered important nursery areas for
select shark species. This study provides important insight
into the habitat use of a variety of coastal shark species and
can be used to better manage these species through the
determination of critical habitat.
Batoids are important mesopredators whose high mobility and extensive migrations can link seeming... more Batoids are important mesopredators whose high mobility and extensive migrations can link seemingly distant food webs in coastal ecosystems. Despite this recognition, our knowledge of the movement patterns of many species is limited due to the logistical challenge of tracking these animals on multiple scales. Smart Positioning or Temperature (SPOT) satellite-linked transmitters allow for precise, multi-scale tracking of species that regularly use surface waters. To date, SPOTs have been predominantly used on sharks, with only a single application to a batoid. Given the epipelagic nature of myliobatid stingrays, we examined the potential for towed-float SPOT transmitters to monitor large-scale movements of two representative species: the Cownose Ray (Rhinoptera bonasus; n = 15) and Spotted Eagle Ray (Aetobatus narinari; n = 9). Tracking data identified several consistent outmigration patterns of Cownose Rays along the Mississippi-Alabama shelf and seasonal variation in movement rates along barrier island habitats. We also documented sex-related differences in movement rates and habitat use of Spotted Eagle Rays along the Bermuda platform, where males exhibited significantly higher movement rates than females and more transient behavior between inshore lagoons and outer coral reefs. Both Cownose and Spotted Eagle Rays were shown to exhibit connectivity among several habitat types along continental shelves in their respective locales, demonstrating future challenges to the management of these species over large spatial scales. While reductions in tag size and improved tethering techniques would undoubtedly broaden the applicability of towed-float satellite telemetry to other species and sizes, our work highlights the strong potential for this technology to provide insights into the spatial ecology and habitat use of myliobatid rays.
Understanding how animals alter habitat use in response to changing abiotic conditions is importa... more Understanding how animals alter habitat use in response to changing abiotic conditions is important for effective conservation management. For bull sharks (Carcharhinus leucas), habitat use has been widely examined in the eastern and western Gulf of Mexico; however, knowledge of their movements and the factors influencing them is lacking for populations in the more temperate north-central Gulf of Mexico. To examine how changes in hydrographic conditions affected the presence of young bull sharks in Mobile Bay, Alabama, thirty-five sharks were fitted with internal acoustic transmitters and monitored with an acoustic monitoring array consisting of thirty-three receivers between June 2009 and December 2010. Tagged sharks ranged in size from 60 to 114 cm fork length and were detected between the upper and lower portions of Mobile Bay. Despite a variety of freshwater sources associated with this highly productive estuary, sharks were most consistently detected at the largest input to the system -the Mobile and Tensaw Rivers. Our findings suggest a combination of hydrographic factors interact to influence the distribution of juvenile bull sharks in Mobile Bay. The factors affecting the probability of detecting at least one bull shark varied both temporally (2009 vs 2010) and spatially (upper vs lower bay). Electivity analysis demonstrated that bull sharks showed highest affinity for warm water (29-32uC), moderate salinities (10-11 psu) and normoxic waters (5-7 mg/l), although these patterns were not consistent between regions or across years. We suggest future studies coupling telemetry and hydrographic variables should, when possible, consider the interactions of multiple environmental parameters when defining the dynamic factors explaining the spatial distribution of coastal sharks.
Human-induced processes like eutrophication are increasing water turbidity and altering vegetated... more Human-induced processes like eutrophication are increasing water turbidity and altering vegetated habitats in the Baltic Sea. Unfortunately, the influence of these environmental changes on predator–prey interactions remains poorly studied in mobile taxa of this region. We used three-spined stickleback (Gasterosteus aculeatus) as a model species to study the combined effects of turbidity and shoot density on habitat choice (shelter vs. open) in the presence and absence of piscivorous perch (Perca fluviatilis). Habitat choice of sticklebacks was video-monitored and compared between two paired observation periods: “control” (no predator) and “risk” (predator present). Though sticklebacks exhibited a general preference for sheltered habitat across treatments, repeated measures ANOVA found that sticklebacks responded to predator presence by significantly increasing use of the sheltered habitat. However, shoot density of the shelter interacted with observation period, where risk-induced shifts of sticklebacks into sheltered habitat were most apparent and sustained at higher shoot densities. Stickleback activity level was generally reduced with predator presence at higher turbidity and shoot density levels, suggesting a possible adaptation to reduce predator encounter rates in visually deteriorated conditions. Overall, our study demonstrates that relatively minute increases (1’s – 10’s of NTU) in algal turbidity intensify three-spined stickleback sheltered habitat use, but vegetation density may still play a larger role in avoiding predators at these turbidity levels. When compared with a similar study on 0+ perch, our work suggests that eutrophication (i.e., increased turbidity, loss of habitat complexity) may have variable, taxon- and/or habitat-specific effects on predator–prey interactions of the Baltic Sea.
Large mobile predators are hypothesized to fulfill integral roles in structuring marine foodwebs ... more Large mobile predators are hypothesized to fulfill integral roles in structuring marine foodwebs via predation, yet few investigations have actually examined the foraging behavior and impact of these species on benthic prey. Limited studies from the Cape Lookout system implicate large schooling cownose rays (Rhinoptera bonasus) in the devastation of patches of commercially harvested bay scallop via strong density-dependent foraging behavior during migrations through this estuary. However, despite the extensive Atlantic range of R. bonasus, the pervasiveness of their patch-depleting foraging behavior and thus impact on shellfisheries remains unknown outside of North Carolina waters. To further understand the potential impacts of cownose rays on benthic prey and the role of bivalve density in eliciting these impacts, we conducted exclusion and manipulation experiments at two sites in the northern Gulf of Mexico frequented by rays during spring migrations. Despite a correlation in ray abundance with haustorid amphipod (primary natural prey) density at our study sites, we were unable to detect any effect of rays on amphipod densities. In addition, through manipulation of predator access, we determined the main cause of mortality to manipulated patches of hard clams was predation by smaller predators such as Callinectes sapidus and not cownose rays. While cownose rays consume hard clam in other parts of their range, we suggest rays along northern Gulf of Mexico barrier islands may prefer foraging on smaller and thinner-shelled bivalves (e.g., Donax sp.), as well as more abundant amphipod crustaceans. We caution that these preferences may have reduced our ability to detect effects of rays on manipulated prey, and thus future impact experiments should strongly consider the local diet of these predators and explore novel techniques to estimate effects on small crustaceans. Further synchronized experimentation along basin-wide scales may elucidate the environmental factors that determine the severity of cownose ray foraging impacts across their range.► We quantify the foraging effects of cownose rays on natural and manipulated prey. ► Field experiments on sandflats indicate cownose rays have no detectable impacts. ► Smaller predators appear to more negatively impact shellfish along barrier islands. ► We suggest potential latitudinal variation in the foraging impacts of cownose rays.
Environmental Biology of Fishes, Jan 1, 2011
Past studies have suggested that increases in cownose ray (Rhinoptera bonasus) abundance may pose... more Past studies have suggested that increases in cownose ray (Rhinoptera bonasus) abundance may pose problems for fisheries management due to their specialized diet of exploitable mollusks. However, more recent work has identified cownose rays as opportunistic generalists, consuming mainly noncommercial prey (e.g. soft-bodied invertebrates) most abundant in their locale. To better assess the generalist vs. specialist foraging behaviors of cownose rays, including their impact to commercial shellfish of the north-central Gulf of Mexico, we conducted gut content analysis on 201 individuals from coastal Alabama. Prey items were analyzed for frequency of occurrence and percent composition by weight, which were used to develop an index of importance. Our diet analysis indicated a minimal impact of cownose rays to exploitable shellfish species of coastal Alabama. Further, we observed important spatial and ontogenetic diet variability: adult diets were dominated by crustaceans along barrier islands and open waters of the Gulf of Mexico, whereas juvenile and young-of-the-year individuals almost exclusively consumed bivalves in riverine and estuarine areas. Individual cownose ray diets were primarily monotypic (54.3%) and appeared selective for amphipods (i.e. Haustorius sp.) when hyperabundant along gulf barrier islands and bivalves at estuarine sites regardless of ambient densities. While this study highlights the importance of locality in determining dietary composition of cownose rays, we do not suggest this species is opportunistic or completely molluskspecialized. Alternatively, we propose cownose rays exhibit a continuum of foraging behaviors between specialization and generalization, depending on locale and prey availability.
PloS one, Jan 1, 2012
Declines of large sharks and subsequent release of elasmobranch mesopredators (smaller sharks and... more Declines of large sharks and subsequent release of elasmobranch mesopredators (smaller sharks and rays) may pose problems for marine fisheries management as some mesopredators consume exploitable shellfish species. The spotted eagle ray (Aetobatus narinari) is the most abundant inshore elasmobranch in subtropical Bermuda, but its predatory role remains unexamined despite suspected abundance increases and its hypothesized specialization for mollusks. We utilized a combination of acoustic telemetry, benthic invertebrate sampling, gut content analysis and manipulative experiments to assess the impact of spotted eagle rays on Bermudian shellfish resources. Residency and distribution of adult spotted eagle rays was monitored over two consecutive summers in Harrington Sound (HS), an enclosed inshore lagoon that has historically supported multiple recreational and commercial shellfish species. Telemetered rays exhibited variable fidelity (depending on sex) to HS, though generally selected regions that supported relatively high densities of potential mollusk prey. Gut content analysis from rays collected in HS revealed a diet of mainly bivalves and a few gastropods, with calico clam (Macrocallista maculata) representing the most important prey item. Manipulative field and mesocosm experiments with calico clams suggested that rays selected prey patches based on density, though there was no evidence of rays depleting clam patches to extirpation. Overall, spotted eagle rays had modest impacts on local shellfish populations at current population levels, suggesting a reduced role in transmitting cascading effects from apex predator loss. However, due to the strong degree of coupling between rays and multiple protected mollusks in HS, ecosystem-based management that accounts for ray predation should be adopted.
Waterbirds, Jan 1, 2011
A Great Blue Heron (Ardea herodias) foraging on an Atlantic Stingray (Dasyatis sabina) was observ... more A Great Blue Heron (Ardea herodias) foraging on an Atlantic Stingray (Dasyatis sabina) was observed off coastal Mississippi. Although elasmobranchs have been previously reported in the diets of birds, this observation was the first documenting the attack and consumption behavior by a bird on an elasmobranch. The behaviors in the event mirror previous observations of A. herodias attacking large and unwieldy prey items. Consumption of a high trophic-level fish by A. herodias further emphasizes its role as a top predator in shallow marine food webs. Re-
Journal of the Marine Biological …, Jan 1, 2007
The chain catshark (Scyliorhinus retifer) is a member of one of the most well studied groups of e... more The chain catshark (Scyliorhinus retifer) is a member of one of the most well studied groups of elasmobranchs (Order: Carcharhiniformes), yet its feeding strategy is unknown due to the inaccessible deep water environment in which it lives (greater than 200 m). To investigate the feeding biology of captive S. retifer, high-speed videography was used to document the kinematic events during capture of two differently sized fish pieces, 'large' (scaled to the mouth width), and 'small' (scaled to one-half the mouth width). Despite a significant delay in the timing of select kinematic variables for large food captures, a multivariate analysis of the feeding behaviour indicated that overall, food capture kinematics and strike behaviour in S. retifer were not statistically different between large and small items (MANOVA, Wilks' Lambda P =0.598, df=4,24). Food capture in S. retifer was suction-dominant, with items being drawn into the mouth about twice the distance that the predator moved. While suction-dominant food capture appears to be an advantage to benthic predators, it contrasts sharply with the phylogenetic position of S. retifer among several ram-feeding carcharinoid sharks including Cephaloscyllium ventriosum, a member of the same family (Scyliorhinidae). Thus, feeding mode in carcharinoid sharks seems to reflect their ecology and does not appear to have been phylogenetically constrained.
ii This work is dedicated to my family and friends for their endless love and support.
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Papers by Matthew Ajemian
was quantified across 10 geographic areas in the
northeastern Gulf of Mexico using fishery-independent
gillnet data from 2003 to 2011. A total of 3,205 sets were
made in which 14,244 carcharhiniform sharks, primarily
juveniles, were caught comprising 11 species from three
families. The three most abundant species, Atlantic
sharpnose Rhizoprionodon terraenovae, bonnethead
Sphyrna tiburo and blacktip sharks Carcharhinus
limbatus, were consistently captured over all sampling
sites regardless of environmental conditions; however,
some species (e.g., bull C. leucas, blacknose
C. acrontous, finetooth C. isodon, and sandbar sharks
C. plumbeus) were restricted to a specific area or a range
of areas. Two-way crossed analysis of similarity
(ANOSIM) found geographic area to significantly influence
shark species-life stage assemblages while season did
not. Resemblance matrices between environmental data
and shark community assemblage found the two were
weakly but significantly correlated, with the combination
of salinity and water clarity producing the highest
Spearman rank correlation value. Species diversity varied
by geographic area, but was generally highest in areas with
the greatest amount of fresh and saltwater fluctuations. Our
results suggest that estuarine conditions adjacent to river
mouths may affect juvenile shark assemblages across similar
latitudes and some areas of the northeastern Gulf of
Mexico may be considered important nursery areas for
select shark species. This study provides important insight
into the habitat use of a variety of coastal shark species and
can be used to better manage these species through the
determination of critical habitat.
was quantified across 10 geographic areas in the
northeastern Gulf of Mexico using fishery-independent
gillnet data from 2003 to 2011. A total of 3,205 sets were
made in which 14,244 carcharhiniform sharks, primarily
juveniles, were caught comprising 11 species from three
families. The three most abundant species, Atlantic
sharpnose Rhizoprionodon terraenovae, bonnethead
Sphyrna tiburo and blacktip sharks Carcharhinus
limbatus, were consistently captured over all sampling
sites regardless of environmental conditions; however,
some species (e.g., bull C. leucas, blacknose
C. acrontous, finetooth C. isodon, and sandbar sharks
C. plumbeus) were restricted to a specific area or a range
of areas. Two-way crossed analysis of similarity
(ANOSIM) found geographic area to significantly influence
shark species-life stage assemblages while season did
not. Resemblance matrices between environmental data
and shark community assemblage found the two were
weakly but significantly correlated, with the combination
of salinity and water clarity producing the highest
Spearman rank correlation value. Species diversity varied
by geographic area, but was generally highest in areas with
the greatest amount of fresh and saltwater fluctuations. Our
results suggest that estuarine conditions adjacent to river
mouths may affect juvenile shark assemblages across similar
latitudes and some areas of the northeastern Gulf of
Mexico may be considered important nursery areas for
select shark species. This study provides important insight
into the habitat use of a variety of coastal shark species and
can be used to better manage these species through the
determination of critical habitat.