Community structure of elasmobranchs in estuaries along the northwest Gulf of Mexico
Graphical abstract
Introduction
Estuarine systems are vitally important and provide essential habitat to both freshwater and marine fishes (Beck et al., 2001). Estuaries are shaped by considerable spatial and temporal variability in habitat composition and environmental conditions, a characteristic of ecotone regions, which can drive species presence/absence and community structure responsible for food web stability (Akin et al., 2003). For marine predators, including coastal elasmobranchs, estuaries can serve as areas for parturition (Castro, 1993), nurseries (Heupel et al., 2007), and foraging grounds (Bethea et al., 2004). Degradation of estuarine systems, coinciding with species declines, illustrate the importance of estuaries to elasmobranch fishes across different life history stages (Knip et al., 2010). Because of the important role elasmobranchs play in their respective food webs (Heithaus et al., 2008) monitoring spatial and temporal shifts in elasmobranch community structure can improve our understanding of the effects environmental variability and habitat degradation have on estuaries and coastal ecosystems.
Many elasmobranch species use coastal estuaries throughout their various life history stages, and for some species, estuaries serve as neonate and juvenile refuges because of the food resources and protection they provide (Heupel et al., 2007; Heupel and Simpfendorfer, 2011; Parsons and Hoffmayer, 2007). Specifically, estuaries in the northwestern Gulf of Mexico (NW GoM) have been identified as primary and secondary nurseries for elasmobranchs species such as bull sharks (Carcharhinus leucas) (Froeschke et al., 2010b). In addition to the role estuaries serve as nurseries, several species of elasmobranchs exhibit residency in estuaries throughout mature life stages, including Atlantic stingrays (Dasyatis sabina) (Snelson et al., 1988), bonnetheads (Sphyrna tiburo), Atlantic sharpnose sharks (Rhizoprionodon terraenovae) (Bethea et al., 2015), and cownose rays (Rhinoptera bonasus) (Collins et al., 2007), highlighting the importance of these ecosystems throughout ontogeny for coastal elasmobranchs.
The distribution of coastal elasmobranchs in the GoM has been correlated with physical, (salinity, temperature, depth) (Drymon et al., 2010; Froeschke et al., 2010a) and biological parameters (chlorophyll a, fish and crustacean biomass) (Drymon et al., 2013), which are hypothesized to work in conjunction to create suitable habitat for individual species (Heithaus et al., 2007). However, many environmental parameters are transient and change over timescales ranging from days to years, potentially altering individual distributions, species niche widths, ecological roles, and community structure. In the northeastern GoM (NE GoM), Bethea et al. (2015) found that shark species diversity was greatest in coastal habitats where salinity was highly variable, including river mouths and barrier islands, suggesting that temporally and spatially dynamic estuaries may support highly diverse elasmobranch communities in other parts of the GoM.
Within the NW GoM, the coast of Texas consists of barrier islands that span 591 km (Texas Natural Resources Information System, 2015), and eight major estuarine systems that include Sabine Lake, Galveston Bay, Matagorda Bay, San Antonio Bay, Aransas Bay, Corpus Christi Bay, Upper Laguna Madre, and Lower Laguna Madre, covering an area of 5911 square kilometers (Texas Natural Resources Information System, 2014a) (Fig. 1). Texas estuaries are sourced from 10 major rivers; Neches, Trinity, San Jacinto, Brazos, Colorado, Lavaca, Guadalupe, San Antonio, Nueces, Rio Grande that drain 510,263 square kilometers of watershed, with regional variability in freshwater inflow rates (Texas Natural Resources Information System, 2014b). A longitudinal precipitation gradient increasing from <40 cm3 yr−1 in the west to >140 cm3 yr−1 in the east (Lyons, 1990), and a latitudinal salinity gradient increasing from 0 salinity in the north to > 40 salinity in the south creates environmental variability along the Texas coast (Mohan and Walther, 2015; Tolan, 2007). Climatic gradients within Texas estuaries likely influence the distribution and relative abundance of at least some elasmobranch species (Bethea et al., 2015; Drymon et al., 2014; Froeschke et al., 2010a; Wiley and Simpfendorfer, 2007).
Previous research has investigated long-term trends in the distributions of several elasmobranch species in the NW GoM (Froeschke et al., 2010a; Matich et al., 2017). However, patterns and drivers of spatial and temporal variability in elasmobranch community structure are unclear, despite their importance to state-wide biodiversity management and conservation, as well as gaining a broader geographic understanding of elasmobranch communities in the GoM (Bethea et al., 2015; Drymon et al., 2013; Grace and Henwood, 1997; McCandless et al., 2007). Describing regional differences in elasmobranch community structure is needed in order to gain a more comprehensive understanding of recent and projected shifts in species abundances (Bubley and Carlson, 2012; Froeschke et al., 2012), and to weigh the importance of abiotic and biotic parameters in maintaining ecosystem stability.
The objectives of this study were to 1) examine spatial and temporal patterns of elasmobranch community structure in estuaries along the NW GoM coast, and 2) identify the physical drivers of elasmobranch community structure within estuaries in the NW GoM.
Section snippets
Materials and methods
All analyses were performed on catch data obtained from the Texas Parks and Wildlife Department (TPWD) long-term gillnet monitoring program. Data collection consisted of 45 gillnets set each spring (April–June) and 45 each fall (August–November) in each major estuary system, following a stratified clustered sampling design (see Martinez-Andrade et al. (2009)). One caveat to this sampling program is the size selectivity of the gillnets used. Sharks with a total length (TL) > 2 m typically were
Results
The dataset consisted of 21,597 individual gillnet sets, with 6,032 (27.9%) gillnets containing elasmobranchs, and 20,091 individuals caught over 30 years (1985–2014). The percent distribution of the 10 species caught in NW GoM estuaries were 35.3% bull sharks (n = 7,089), 18.1% bonnetheads (n = 3,640), 17.0% cownose rays (n = 3,421), 13.4% blacktip sharks (n = 2,699), 5.9% Atlantic stingrays (n = 1,185), 3.1% Atlantic sharpnose sharks (n = 620), 2.7% spinner sharks (n = 532), 2.1% scalloped
Discussion
Coast-wide diversity in elasmobranchs was comparable to other estuaries in the GoM (Bethea et al., 2008; Wiley and Simpfendorfer, 2007) and the Western Atlantic (Bangley and Rulifson, 2017). Of the 10 elasmobranch species that were found in estuaries of the NW GoM, there were six species of requiem sharks (Carcharhinidae), two species of hammerhead sharks (Sphyrnidae), one whiptail ray (Dasyatidae), and one eagle ray (Myliobatidae). Using fishery independent gillnet surveys, Bethea et al. (2015)
Conclusion
The results of this study show that NW GoM estuaries have diverse elasmobranch communities that vary geographically, and have significantly changed over time. The northern estuaries had communities dominated by bull sharks, while the southern estuaries were shown to have significantly different community structures dominated by cownose rays. Salinity and depth were the most important variables driving estuarine community structure in the NW GoM. Elasmobranch diversity peaked in the central
Acknowledgements
We thank the Marine Biology Department at Texas A&M University at Galveston for providing logistical resources during manuscript preparation. Also, Michael Dance and Blair Sterba-Boatwright for their careful review of our methodology. All data were provided by the Texas Parks and Wildlife Department.
References (73)
- et al.
Seasonal and spatial variations in fish and macrocrustacean assemblage structure in Mad Island Marsh estuary, Texas
Estuar. Coast Shelf Sci.
(2003) - et al.
Spatial and temporal shifts in suitable habitat of juvenile southern flounder (Paralichthys lethostigma)
J. Sea Res.
(2013) - et al.
Predicting ecological consequences of marine top predator declines
Trends Ecol. Evol.
(2008) El Niño-Southern Oscillation impacts translated to the watershed scale: estuarine salinity patterns along the Texas Gulf Coast, 1982 to 2004
Estuar. Coast Shelf Sci.
(2007)- et al.
Habitat-specific feeding by cownose rays (Rhinoptera bonasus) of the northern Gulf of Mexico
Environ. Biol. Fish.
(2012) - et al.
Environmental Drivers of the Spatial and Temporal Distribution of Spawning Blue Crabs (Callinectes sapidus) in the Western Gulf of Mexico
(2017) - et al.
Habitat partitioning and diurnal-nocturnal transition in the elasmobranch community of a North Carolina estuary
Bull. Mar. Sci.
(2017) - et al.
The identification, conservation, and management of estuarine and marine nurseries for fish and invertebrates
BioScience
(2001) - et al.
Distribution and community structure of coastal sharks in the northeastern Gulf of Mexico
Environ. Biol. Fish.
(2015) - et al.
Foraging ecology of the early life stages of four sympatric shark species
Mar. Ecol. Prog. Ser.
(2004)
A comparison of the foraging ecology and bioenergetics of the early life-stages of two sympatric hammerhead sharks
Bull. Mar. Sci.
Geographic and ontogenetic variation in the diet and daily ration of the bonnethead shark, Sphyrna tiburo, from the eastern Gulf of Mexico
Mar. Biol.
Factors affecting the distribution of juvenile estuarine and inshore fish
J. Fish. Biol.
Age, growth and reproductive biology of the silky shark, Carcharhinus falciformis, and the scalloped hammerhead, Sphyrna lewini, from the northwestern Gulf of Mexico
Environ. Biol. Fish.
Age and growth estimates of the bull shark, Carcharhinus leucas, from the northern Gulf of Mexico
Environ. Biol. Fish.
Science and Information Theory
Age assessment of the lemon shark, Negaprion brevirostris, using tetracycline validated vertebral centra
Copeia
Relative Abundance of Blacktip Shark Based on a Fishery Independent Gillnet Survey off Texas
Life history and population dynamics of the finetooth shark (Carcharhinus isodon) in the northeastern Gulf of Mexico
Fish. Bull.
The shark nursery of Bulls Bay, South Carolina, with a review of the shark nurseries of the southeastern coast of the United States
Environ. Biol. Fish.
Biology of the blacktip shark, Carcharhinus limbatus, off the southeastern United States
Bull. Mar. Sci.
PRIMER v.7: User Manual and Tutorial PRIMER-E
Residence and movement patterns of cownose rays Rhinoptera bonasus within a south-west Florida estuary
J. Fish. Biol.
Sharks of the World. An Annotated and Illustrated Catalogue of Shark Species to Date. Part I (Hexanchiformes to Lamniformes)
Sharks of the World: an Annotated and Illustrated Catalogue of Shark Species Known to Date
Diet, feeding habits, and diel feeding chronology of the bonnethead shark, Sphyrna tiburo, in southwest Florida
Bull. Mar. Sci.
Stage-specific variability in habitat associations of juvenile red drum across a latitudinal gradient
Mar. Ecol. Prog. Ser.
Distribution and dynamic habitat use of young bull sharks Carcharhinus leucas in a highly stratified northern gulf of Mexico estuary
PLoS One
Multiscale analysis of factors that affect the distribution of sharks throughout the northern Gulf of Mexico
Fish. Bull.
Trophic plasticity in the Atlantic sharpnose shark (Rhizoprionodon terraenovae) from the north central Gulf of Mexico
Environ. Biol. Fish.
Distributions of sharks across a continental shelf in the northern gulf of Mexico
Marine and Coastal Fisheries
Extinction risk and conservation of the world's sharks and rays
eLife
Trophic downgrading of Planet Earth
Science
Environmental influences on the occurrence of coastal sharks in estuarine waters
Mar. Ecol. Prog. Ser.
Long-term trends of bull shark (Carcharhinus leucas) in estuarine waters of Texas, USA
Can. J. Fish. Aquat. Sci.
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