Bull Mar Sci. 93(2):319–338. 2017 research paper https://doi.org/10.5343/bms.2016.1038 Habitat partitioning and diurnal-nocturnal transition in the elasmobranch community of a North Carolina estuary 1, 2 * 1 Institute for Coastal Science Charles W Bangley and Policy, East Carolina Roger A Rulifson 1 University, East 5th St., Greenville, North Carolina 27858. 2 Present address: Smithsonian Environmental Research ABSTRACT.—In marine communities, resource Center, 647 Contees Wharf Rd, partitioning can be as important as abiotic environmental Edgewater, Maryland 21037. preferences in determining habitat use patterns. * Corresponding author email: Elasmobranchs are generally assumed to be crepuscular or <[email protected]>. nocturnal, but diel temporal habitat partitioning is poorly studied in this group. We attempted to identify habitat preferences and find evidence of resource partitioning among the elasmobranch community in Back and Core Sounds, North Carolina, using a multi-gear, fishery- independent survey with a temporal focus on the diurnal- nocturnal transition. Gillnet, longline, drumline, and rod- and-reel sampling captured a total of 160 elasmobranchs, representing 12 species within the estuary, and differences between the seven most abundant species were assessed in terms of temporal, environmental, and spatial habitat factors. The elasmobranch community was broadly divided into cool and warm temperature assemblages. Most species showed evidence of generalist habitat preferences, but spatial overlap between species was generally low. Blacknose sharks [Carcharhinus acronotus (Poey, 1860)] appeared to be nocturnal, and aggregations of smooth dogfish Mustelus[ canis (Mitchill, 1815)] and spiny dogfish Squalus( acanthias Linnaeus, 1758) were found during mid-afternoon hours. Blacknose sharks and blacktip sharks [Carcharhinus limbatus (Müller and Henle, 1839)] showed evidence of spatial resource partitioning based on distance from the nearest inlet. Temperature appears to be a strong influence on the presence of elasmobranch species within Back and Core Sounds, but behavioral interspecific avoidance may be a Date Submitted: 1 March, 2016. greater influence on fine-scale habitat use by elasmobranchs Date Accepted: 31 August, 2016. in this estuarine system. Available Online: 22 N0vember, 2016. Marine predators can significantly influence the distributions of other species and local community dynamics (Heithaus et al. 2012). Elasmobranchs typically oc- cupy high trophic levels within marine communities, and some species function as apex predators in these ecosystems (Cortés 1999, Heithaus et al. 2010, Hussey et al. 2015, Shaw et al. 2016). Even at relatively small spatial and temporal scales, elasmobranchs can have significant, population-level, top-down effects on prey spe- cies (Beamish et al. 1992, Lacroix and Fleming 2014). The habitat use patterns of Bulletin of Marine Science 319 © 2017 Rosenstiel School of Marine & Atmospheric Science of the University of Miami 320 Bulletin of Marine Science. Vol 93, No 2. 2017 elasmobranchs, particularly apex predator shark species, can have direct predato- ry and indirect behavioral effects throughout the food web to the level of primary producers (Burkholder et al. 2013, Vaudo and Heithaus 2013, Heithaus et al. 2014). However, elasmobranchs may not necessarily function as keystone apex predators in ecosystems with other large, mobile predators (Kitchell et al. 2002, Frisch et al. 2016), though some species may still occupy the highest trophic levels even when co- occurring with other predators (Shaw et al. 2016). Juvenile and small-bodied sharks can be among the most abundant predatory species within estuaries (Grabowski et al. 2005). Given the importance of top-down interspecific interactions in structuring estuarine ecosystems (Heck and Valentine 2007), habitat use by elasmobranchs in estuaries is deserving of detailed study. Habitat use by sharks is likely driven by a combination of abiotic factors in the form of environmental preferences, and biotic factors in the form of interspecific relationships with prey, competitors, or predators (Heithaus 2007). Depending on body size, sharks can function as apex predators or mesopredators, though local en- vironmental conditions may determine the trophic role of a given species in a given habitat (Heupel et al. 2014). In nearshore and estuarine environments, multiple shark species may coexist (Simpfendorfer and Milward 1993) and competition may lead to resource partitioning among sympatric elasmobranchs and other high tropic-level predators (Heithaus and Vaudo 2012). Resource partitioning is widespread among fishes and can be expressed as trophic, spatial, or temporal differences in habitat use between species occupying the same system (Ross 1986). Different types of re- source partitioning have been observed among elasmobranchs: species with similar prey preferences showed differences in spatial habitat use within Apalachicola Bay (Bethea et al. 2004), while different species occupying the same spatial area within Cleveland Bay showed evidence of trophic resource partitioning (Kinney et al. 2011). Temporal separation was the primary form of resource partitioning between fishes in 11% of studies assessed by Ross (1986) and may be important in structuring elas- mobranch communities, but diel temporal influences on habitat use and resource partitioning are not well-known in elasmobranchs (Hammerschlag et al. 2017). A variety of methods can be used to investigate habitat use by elasmobranchs, and each approach has its own unique set of requirements and limitations (Simpfendorfer and Heupel 2012). While catch rates from fishery-independent surveys can cover a broad spatial area, diel differences in habitat use patterns have been assessed most- ly using various forms of telemetry to cover the entire 24-hr cycle (Simpfendorfer and Heupel 2012, Donaldson et al. 2014). Few standardized fishery-independent surveys have assessed diel habitat use by elasmobranchs; in one such study, four of six Carcharhinid shark species captured in longline surveys along the southeastern coast of the United States were caught primarily at night (Driggers et al. 2012). To help fill this knowledge gap, we incorporated set time, with a focus on the transition from diurnal to nocturnal periods, among the variables recorded during fishery-in- dependent sampling to account for potential temporal effects on habitat preferences and resource partitioning among the elasmobranch community of a relatively small, warm-temperate estuary. Bangley and Rulifson: Habitat partitioning in a North Carolina elasmobranch community 321 Figure 1. Fishery-independent gillnet, longline, rod-and-reel, and drumline sampling locations, known seagrass extent, and sampling strata boundaries from 2014 to 2015 elasmobranch surveys within Back and Core Sounds, North Carolina. Bold numbers identify sampling strata. Methods Elasmobranchs were captured during a shark survey conducted in Back and Core Sounds, North Carolina, using gillnet, longline, drumline, and rod-and-reel gear (Fig. 1). Each gear was intended to target different species groups or size classes. Gillnet gear was used to capture small-bodied species (300–1000 mm TL) and species unlikely to feed on longline bait. Longline gear targeted mid-sized species (700–1800 mm TL), and drumline gear was used to account for large, apex predatory species (>1800 mm TL). Gillnet gear measured 45 m in length and 2 m in height, and was comprised of five 9-m panels ranging from 2.5- to 14-cm stretched mesh. Longline gear consisted of a 274-m mainline with 20–30 gangions constructed of 1 m of 136 kg-test monofilament line and a 12/0 circle hook. Drumline gear consisted of a single 15-m long, 408.23 kg-test monofilament leader with a 15/0 circle hook mounted on a 18.14-kg weight. Rod-and-reel gear was used to supplement shark catches during gillnet and longline soak times and in conditions unsafe for deployment of the other gear types, and was comprised of a single fishing rod with 18.14-kg test braided line and a 0.5-m wire leader with a 12/0 circle hook. Longline and rod-and-reel hooks were baited with cut Atlantic mackerel (Scomber scombrus Linnaeus, 1758) or Atlantic menhaden [Brevoortia tyrannus (Latrobe, 1802)], supplemented by locally available baitfish, and the drumline was baited with cut sections of spiny dogfish (Squalus acanthias Linnaeus, 1758), Atlantic sharpnose shark [Rhizoprionodon terraenovae (Richardson, 1836)], or striped bass [Morone saxatilis (Walbaum, 1792)]. We assumed that bait type would not significantly influence species composition among elasmobranchs caught using longline and drumline gear. When possible, 322 Bulletin of Marine Science. Vol 93, No 2. 2017 a combination of multiple gear types was deployed simultaneously within 100 m of each other. Soak time was limited to 30 min for all gears, though longline and drumline soak times up to 45 min occasionally occurred when these gears were simultaneously deployed with gillnet sets. Sampling occurred from March 21 to November 16, 2014, and from April 13 to October 15, 2015. Sampling was not attempted during winter months due to the expected low abundance of sharks within the estuary at that time (Grabowski et al. 2005). In an attempt to focus on the crepuscular period when elasmobranchs are thought to be most active, most sampling occurred between 12:00 and 22:00, though some sets were deployed opportunistically during other hours. Sampling sites were chosen using a stratified-random