Intrapopulation Variations in Diet and Habitat Use in a Marine Apex Predator, the Broadnose Sevengill Shark Notorynchus Cepedianus
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Vol. 442: 133–148, 2011 MARINE ECOLOGY PROGRESS SERIES Published December 5 doi: 10.3354/meps09395 Mar Ecol Prog Ser Intrapopulation variations in diet and habitat use in a marine apex predator, the broadnose sevengill shark Notorynchus cepedianus Kátya G. Abrantes1,3,*, Adam Barnett2 1Coastal and Estuary Ecosystem Ecology Laboratory, School of Marine and Tropical Biology, James Cook University, Townsville, Queensland 4811, Australia 2Tasmanian Aquaculture and Fisheries Institute, Marine Research Laboratories, Hobart, Tasmania 7053, Australia 3Present address: Department of Earth and Environmental Sciences, Katholieke Universiteit Leuven, Celestijnenlaan 200E, 3001 Heverlee, Belgium ABSTRACT: Intrapopulation differences in diet and/or movement are important for understand- ing the role mobile predators play in different systems. However, ecological studies traditionally overlook individual differences. δ13C and δ15N were used in conjunction with diet and movement information to identify intrapopulation differences in diet and movement patterns of the apex predator broadnose sevengill shark Notorynchus cepedianus in southeast Tasmania. Sevengill samples from 3 inshore and 3 offshore sites were collected, and δ13C and δ15N compared between sites, sizes and sexes. Individuals captured offshore had lower δ15N than those captured inshore, indicating some degree of spatial segregation. Sevengills also had variable δ13C and δ15N within coastal habitats, suggesting intrapopulation differences in diet or migration schedules. In compar- ison to their main prey, most individuals had δ15N lower than expected for a top predator, also sug- gesting that they do not reside permanently in these areas, as their tissue was not in isotopic equi- librium with their known prey. This is in agreement with tracking data that showed seasonal use of coastal areas, with most animals leaving for the colder months but returning the following year. There was also a group of females with relatively high δ13C that suggests greater association to coastal habitats, again in agreement with tracking data, as some tagged females remained in the coastal areas over winter. Overall, together with diet and tracking information, results indicate that there are differences in movement and possibly diet in this sevengill population. This multi- methods approach allowed a better understanding of sevengill ecology than the use of any one of the techniques alone. KEY WORDS: Chondrichthyans · Coastal · Diet · Movement · Stable isotope · Tracking Resale or republication not permitted without written consent of the publisher INTRODUCTION namics and stability in more than one system (Estes et al. 1998, Quevedo et al. 2009, Matich et al. 2011). Large mobile marine predators are often elusive, Hence, knowledge of both movement patterns and have large and/or unknown home ranges and fre- trophic ecology across their distribution range is cru- quently move between distanced habitats, making cial to understanding their importance in the differ- the study of movement patterns and food web dy - ent communities they occupy. namics difficult. These predators can link different Electronic tagging techniques such as acoustic systems by connecting spatially separated food webs, tele metry and satellite technology are useful for therefore transferring energy between distant habi- studying animal movement. However, acoustic track- tats and playing an important role in food web dy- ing can normally only cover a portion of the range of *Email: [email protected] © Inter-Research 2011 · www.int-res.com 134 Mar Ecol Prog Ser 442: 133–148, 2011 habitats used due to the high costs involved in setting They are seasonally abundant in shallow coastal up and maintaining an appropriate acoustic array. habitats (Ebert 1989, Lucifora et al. 2005, Barnett et Satellite tracking also has some limitations, mainly al. 2010a), where they feed mostly on other elasmo- related to the limited battery life, and for marine ani- branchs, marine mammals and teleosts (Ebert 1989, mals it is most appropriate for species that are at the 1991, Lucifora et al. 2005, Braccini 2008, Barnett et al. surface long enough for the information to be trans- 2010b). Recently, an intensive 3-yr study on sevengill mitted to the satellites. Different groups of individu- shark ecology was conducted in 2 coastal habitats of als can also have different movement patterns, sea- southeast Tasmania, Australia, where information on sonally moving between different habitats (Hindell diet, population structure, habitat use and movement et al. 2003, Austin et al. 2004, Weng et al. 2005), and patterns was collected (Barnett et al. 2010a,b,c, hence a large number of individuals of both sexes 2011). Thus, the current study provides a unique and all size classes need to be tagged for an accurate opportunity to use stable isotope analysis in conjunc- estimation of habitat use and movement patterns of tion with diet and movement information to further a population. study the ecology and intrapopulation variations in Stable isotope analysis has also been used to study diet and/or movement for these large apex predators. animal movement (Hobson 1999). This fast and easy- Understanding the causes and consequences of intra - to-use method gives time-integrated information on population differences in movement and dietary the assimilated diet of a consumer. Although it is tra- patterns can provide important insights into factors ditionally used to study diets or to determine the ulti- that influence the spatial dynamics and resource use mate sources of nutrition supporting food webs (e.g. of apex predator populations (Jorgensen et al. 2009, Connolly et al. 2005, Alfaro et al. 2006), it is also use- Matich et al. 2011). However, population ecology ful to study movement between areas and habitats, studies have traditionally overlooked individual dif- when the food sources in the different areas have dis- ferences in diet and movement. tinct stable isotopic composition (Herzka 2005). Sevengills from the 2 main coastal locations consid- Unlike other tracking techniques, which look at ered in the present study (Norfolk Bay and the Der- where the animal goes after the tagging event, stable went Estuary) show relatively low dietary and spatial isotope analysis provides information on where the overlap, suggesting resource partitioning and/or animal has been in the time period before capture. localised site-fidelity over relatively fine spatial Stable isotopes have been successfully used in the scales (~30 km) (Barnett et al. 2010b, 2011). Preferen- study of animal movement in a range of systems, tial or specialist foraging behavior was also sus- both terrestrial (e.g. Cerling et al. 2006) and aquatic pected in some individuals, but since most (76%) (e.g. Best & Schell 1996). For aquatic systems, most stomachs sampled only contained one or 2 prey studies available were conducted in freshwater or items, and because sevengills in general were found estuarine habitats (e.g. Cunjak et al. 2005, Herzka to consume a wide range of prey, individual varia- 2005), and studies on marine systems are still some- tions in foraging behavior were hard to determine what limited. Examples are however available, from stomach content data alone (Barnett et al. mainly related to offshore versus inshore foraging or 2010b). Since stable isotope analysis provides long- to migrations through well defined isoscapes. These term diet information, as opposed to the short-term are mainly related to marine mammals and birds snapshot that is acquired by the analysis of stomach (e.g. Best & Schell 1996, Cherel et al. 2009), and stud- contents, this technique may be more appropriate ies on movement patterns of fish and sharks in partic- to investigate intrapopulation differences in diet ular are still lacking. In addition, with the exception (Vander Zanden & Rasmussen 2002, Matthews & of marine birds (e.g. Ramos et al. 2009, Jaeger et al. Mazumder 2004). Intrapopulation differences in diet 2010), few studies have combined stable isotope and and habitat use by sevengill sharks can have impor- tracking techniques to address movement and/or tant consequences for the local community and food- trophic ecology for marine animals (Cunjak et al. web dynamics, and as such must be considered when 2005, Papastamatiou et al. 2010, McClellen et al. determining the functional role of this species 2010). (Matich et al. 2011). Broadnose sevengill sharks Notorynchus cepedi- The objective of this study was to use stable isotope anus are large mobile predators with a wide temper- analysis of carbon and nitrogen in conjunction with ate distribution, that generally occur from inshore diet and movement information to identify any intra - coastal areas to the continental shelf at depths up to population differences in diet and residency patterns 200 m (Last & Stevens 2009, A. Barnett unpubl. data). of sevengill sharks in southeast Tasmania. A large Abrantes & Barnett: Intrapopulation differences in diet and movement 135 sample size is considered, making this a comprehen- consistently reaches depths of 20 to 30 m. Norfolk sive study of habitat and resource use of the sevengill Bay is a relatively shallow (average depth 15 m; max- shark population in southeast Tasmania. The specific imum depth 20 m), semi-enclosed bay with an area of aims are: (1) to determine if there are detectable ~180 km2, that connects to Frederick Henry and differences in stable isotope composition between Storm Bay (Fig. 1). Storm Bay, a more open area, was sites, including inshore and offshore sites, which also sampled at 20 to 30 m depth. The Maatsuyker can be used as indicators of the level of connectivity site is located close to the Maatsuyker Islands, between areas; and (2) to use stable isotopes in ~10 km off the south coast of Tasmania, at ~90 m conjunction with stomach content and tracking data depth. The Pedra Branca site was located in the prox- to identify intrapopulation differences in diet and imity of a steep rocky outcrop of the same name, movement patterns in 2 coastal locations of southeast ~26 km south of Tasmania (Fig. 1). Here, we sampled Tasmania. sharks from the edge of the continental shelf, at depths of ~150 m.