Community structure of elasmobranchs in estuaries along the northwest Gulf of Mexico

https://doi.org/10.1016/j.ecss.2018.02.023Get rights and content

Highlights

  • Estuaries in the northwest Gulf of Mexico (NW GoM) have a diverse elasmobranch community assemblage.

  • Higher latitude estuaries were dominated by bull sharks while lower latitude estuaries were dominated by cownose rays.

  • SSalinity and depth were the two most important environmental variables shaping community structure in NW GoM estuaries.

  • Elasmobranch diversity was primarily driven by salinity, with moderate salinity estuaries exhibiting the highest diversity.

Abstract

Estuaries promote high levels of productivity and biodiversity by providing habitat for many biological communities due to their wide range of environmental conditions. Estuarine systems serve as nurseries, areas for parturition, and feeding grounds for elasmobranchs. However, estuaries face an array of anthropogenic pressures, including overfishing, altered flow regimes, pollution, and habitat destruction. Given the vulnerability of estuarine ecosystems, observing long-term changes in community structure is essential to understanding the effects of anthropogenic stressors. Elasmobranch community structure was analyzed among eight estuaries in the northwest Gulf of Mexico to evaluate spatial and temporal variability in species abundance and diversity using bi-annual fisheries independent gillnet survey data over three decades (1985–2014). Ten species comprised 99.4% of elasmobranchs caught which included 35.3% bull sharks (Carcharhinus leucas), 18.1% bonnetheads (Sphyrna tiburo), 17.0% cownose rays (Rhinoptera bonasus), 13.4% blacktip sharks (Carcharhinus limbatus), 5.9% Atlantic stingrays (Dasyatis sabina), 3.1% Atlantic sharpnose sharks (Rhizoprionodon terraenovae), 2.7% spinner sharks (Carcharhinus brevipinna), 2.1% scalloped hammerheads (Sphyrna lewini), 1.7% finetooth sharks (Carcharhinus isodon), and 0.7% lemon sharks (Negaprion brevirostris). During the study period, elasmobranch community structure changed among estuaries and among decades. Bull sharks, bonnetheads, cownose rays, blacktip sharks, and spinner sharks all increased in abundance during the study period, whereas finetooth sharks and lemon sharks decreased over time. Higher latitude estuaries were dominated by bull sharks while lower latitude estuaries were dominated by cownose rays. Salinity was the most important environmental variable in predicting individual elasmobranch species abundance (deviance explained: 14.4 ± 6.5 SD), while temperature and depth also played a role in shaping community structure. Diversity was greatest in mid-latitudinal estuaries with spatially and temporally dynamic salinity regimes. As environmental change and human impacts persist across much of the world, understanding environmental drivers of community structure using long-term datasets will provide insight to how these changes influence coastal ecosystems, and enable more comprehensive and scale-independent models to be developed for the management and conservation of coastal ecotones.

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.

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