Stable Isotope Analysis of Juvenile White Sharks Inside a Nursery Area Reveals Foraging in Demersal-Inshore Habitats and Trophic Overlap with Sympatric
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fmars-08-687738 August 5, 2021 Time: 12:53 # 1 ORIGINAL RESEARCH published: 10 August 2021 doi: 10.3389/fmars.2021.687738 Stable Isotope Analysis of Juvenile White Sharks Inside a Nursery Area Reveals Foraging in Demersal-Inshore Habitats and Trophic Overlap With Sympatric Edited by: Sharks J. Marcus Drymon, 1† 1 † 1 † Mississippi State University, Emiliano García-Rodríguez , Sharon Z. Herzka * , Oscar Sosa-Nishizaki * , United States Christopher G. Lowe2 and John B. O’Sullivan3 Reviewed by: 1 Department of Biological Oceanography, Center for Scientific Research and Higher Education of Ensenada (CICESE), Simona Alessandra Ceriani, Ensenada, Mexico, 2 Department of Biological Sciences, California State University, Long Beach, CA, United States, Florida Fish and Wildlife Research 3 Monterey Bay Aquarium, Monterey, CA, United States Institute, United States Richard Reina, Monash University, Australia Knowledge about top predators’ trophic ecology is crucial for defining their role *Correspondence: in ecosystems, understanding habitat preferences, characterizing life stage-specific Sharon Z. Herzka feeding habits, and evaluating their interaction with fisheries. In the northeastern Pacific, [email protected] Oscar Sosa-Nishizaki white sharks (Carcharodon carcharias) occupy coastal habitats during the early life [email protected] stages, including Bahía Sebastián Vizcaíno (BSV) in Mexico, which is a known nursery †ORCID: area. Although BSV presumably provides high prey abundance, the trophic ecology of Emiliano García-Rodríguez orcid.org/0000-0002-9078-1300 immature white sharks is poorly understood. Carbon and nitrogen bulk stable isotope Sharon Z. Herzka analyses (SIA) were used to explore the trophic relationship of early life stages with orcid.org/0000-0001-7091-7656 their potential prey and to infer dietary overlap with sympatric sharks, while SIA of Oscar Sosa-Nishizaki orcid.org/0000-0002-3043-768X amino acids were used to estimate trophic position. Muscle samples from young white sharks and inshore demersal prey commonly found in their stomach contents were Specialty section: sampled. Demersal prey and literature-derived isotope ratios for pelagic and offshore This article was submitted to Marine Megafauna, species were incorporated into mixing models with a Bayesian framework to estimate a section of the journal their contribution to white shark tissues. Nearshore demersal prey had the highest Frontiers in Marine Science contribution for all life stages (between 35 and 47%), consistent with previous reports Received: 30 March 2021 Accepted: 28 May 2021 based on gut content analysis. The contribution of pelagic (between 26 and 37%) Published: 10 August 2021 and offshore (between 14 and 32%) prey was smaller and suggests potential periodic Citation: changes in foraging grounds and the presence of a maternal-derived isotopic signature. García-Rodríguez E, Herzka SZ, A high contribution of demersal prey indicates a high level of interaction with local Sosa-Nishizaki O, Lowe CG and O’Sullivan JB (2021) Stable Isotope fisheries that target those species and catch white sharks incidentally and is consistent Analysis of Juvenile White Sharks with immature shark movement patterns. Isotope ratios of two sympatric sharks, Inside a Nursery Area Reveals Foraging in Demersal-Inshore smooth hammerhead Sphyrna zygaena and copper sharks Carcharhinus brachyurus, Habitats and Trophic Overlap With were used to estimate the overlap in isotopic niche space. Immature white sharks Sympatric Sharks. had the smallest isotopic niche, while the highest was for copper sharks. Overlap was Front. Mar. Sci. 8:687738. doi: 10.3389/fmars.2021.687738 greatest between white sharks and hammerheads (∼45%), while overlap with copper Frontiers in Marine Science| www.frontiersin.org 1 August 2021| Volume 8| Article 687738 fmars-08-687738 August 5, 2021 Time: 12:53 # 2 García-Rodríguez et al. Immature White Sharks Trophic Ecology sharks was limited (<20%), suggesting less potential for competition. Trophic position estimates were similar to those previously reported for the species. These results highlight the importance of coastal demersal prey heavily targeted by local fisheries for the diet of young white sharks and support the importance of BSV as a nursery habitat. Keywords: white shark, trophic ecology, stable isotope analyses, nursery area, demersal foraging, amino acids INTRODUCTION Bahía Sebastián Vizcaíno (BSV) bay is located on the western coast of the Baja California Peninsula, Mexico. BSV is a natal Most shark species are considered top predators occupying high nursery area for white sharks born locally as well as young trophic levels and their presence or absence influences food sharks born in the Southern California Bight, United States, that web structure (Cortés, 1999; Ferretti et al., 2010; Heupel et al., migrate south covering approximately 600 km (Oñate-González 2014). Some species are highly migratory and move between et al., 2017; White et al., 2019). In Mexican waters, the species coastal and pelagic habitats, that are used throughout ontogeny has been under protection since 2007 and the targeted catch is for nursery, feeding, mating, or parturition (Heupel et al., 2007; prohibited (DOF, 2007, 2010, 2014). However, they are caught Block et al., 2011; Domeier and Nasby-Lucas, 2013; Carlisle incidentally in the nearshore areas of BSV as well as within Ojo et al., 2015a; Nasby-Lucas et al., 2019; Nosal et al., 2019). de Liebre Lagoon (OLL), a semi-enclosed hypersaline shallow Ontogenetic habitat changes are commonly accompanied by estuary located within the bay system (García-Rodríguez and switches in their diet (Malpica-Cruz et al., 2013; Carlisle et al., Sosa-Nishizaki, 2020). These catches occur when local artisanal 2015b; Estupiñán-Montaño et al., 2019; Matich et al., 2019). fishers target high-value species that are part of the white shark’s So, characterizing the trophic relationships of sharks throughout diet (Santana-Morales et al., 2012; García-Rodríguez and Sosa- their life cycle is needed for understanding their ecological role in Nishizaki, 2020). each ecosystem they inhabit. In addition to the direct mortality caused by incidental Adult white sharks (Carcharodon carcharias) are distributed catches, the potential competition between artisanal fisheries and mostly in offshore pelagic waters, including oceanic islands young white sharks (and other shark species) for prey could (Klimley et al., 1992; Domeier and Nasby-Lucas, 2007; Jorgensen have a negative effect on regional conservation efforts. For top et al., 2010; Francis et al., 2015; Boldrocchi et al., 2017; Skomal predators like turtles, birds, and marine mammals, it has been et al., 2017; Moro et al., 2020), while juveniles occupy coastal areas well documented that competition could have adverse effects on as nurseries (Bruce and Bradford, 2012; Harasti et al., 2017; Curtis their abundance by limiting prey’s spatial availability (Huss et al., et al., 2018; Bruce et al., 2019). In the northeast Pacific, newborn, 2014; Hilborn et al., 2017; Hansson et al., 2018). For example, young of the year, and juvenile white shark occupy coastal areas a switch in the diet resulting from prey overfishing has been until they become sub-adults and can migrate between spatially reported for the spiny dogfish Squalus acanthias (Koen Alonso separate nurseries (Klimley et al., 1992; Dewar et al., 2004; Weng et al., 2002). Also, high overlap between target fishery species and et al., 2007, 2012; Lowe et al., 2012; Oñate-González et al., 2017; shark prey has been documented for the spiny dogfish (Lucifora White et al., 2019). Coastal nurseries harbor high food abundance et al., 2009). In sand tiger sharks (Carcharias taurus), spatial and provide shelter from predators, allowing young white sharks overlap between prey and fisheries increased their vulnerability to grow faster, which is crucial for their juvenile survival and to overfishing (Gracanˇ et al., 2017). Changes in prey availability recruitment to adult population growth (Mollet and Cailliet, due to spatial overlap with fisheries in nursery areas could 2002; Heupel et al., 2007). decrease the benefits these provide for the early life stages of The white shark is a top predator that exhibits ontogenetic sharks, potentially changing their diets, limiting their growth, and changes in its feeding habits and habitat use (Estrada et al., 2006; affecting juvenile survival. Carlisle et al., 2012; Kim et al., 2012; French et al., 2018). Whereas Limitation in prey availability due to fisheries could increase the feeding preferences of adults have been more widely studied the competition between shark species with similar dietary (Estrada et al., 2006; Hussey et al., 2012), dietary information for preferences. Interspecific competition between young white the early life stages is more limited and suggest a more benthic- sharks and sympatric shark species that inhabit BSV, like oriented foraging (Santana-Morales et al., 2012; Grainger et al., smooth hammerheads (Sphyrna zygaena) or Carcharhinids, is 2020). Juveniles shift their diet from feeding on smaller bony possible. Previous studies have reported resource partitioning fishes and elasmobranchs (mainly rays) to larger fishes as they in communal areas to reduce interspecific competition, which grow. After becoming sub-adults, white sharks start to expand could be associated with changes in the distribution and habitat their diet to include marine mammals (Tricas and McCosker, use of sympatric shark species with high dietary