Stable Isotope Analysis of Juvenile White Sharks Inside a Nursery Area Reveals Foraging in Demersal-Inshore Habitats and Trophic Overlap With Sympatric Sharks

Abstract

Knowledge about top predators’ trophic ecology is crucial for defining their role in ecosystems, understanding habitat preferences, characterizing life stage-specific feeding habits, and evaluating their interaction with fisheries. In the northeastern Pacific, white sharks (Carcharodon carcharias) occupy coastal habitats during the early life stages, including Bahía Sebastián Vizcaíno (BSV) in Mexico, which is a known nursery area. Although BSV presumably provides high prey abundance, the trophic ecology of immature white sharks is poorly understood. Carbon and nitrogen bulk stable isotope analyses (SIA) were used to explore the trophic relationship of early life stages with their potential prey and to infer dietary overlap with sympatric sharks, while SIA of 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 sampled. Demersal prey and literature-derived isotope ratios for pelagic and offshore species were incorporated into mixing models with a Bayesian framework to estimate their contribution to white shark tissues. Nearshore demersal prey had the highest contribution for all life stages (between 35 and 47%), consistent with previous reports based on gut content analysis. The contribution of pelagic (between 26 and 37%) and offshore (between 14 and 32%) prey was smaller and suggests potential periodic changes in foraging grounds and the presence of a maternal-derived isotopic signature. A high contribution of demersal prey indicates a high level of interaction with local fisheries that target those species and catch white sharks incidentally and is consistent with immature shark movement patterns. Isotope ratios of two sympatric sharks, smooth hammerhead Sphyrna zygaena and copper sharks Carcharhinus brachyurus, were used to estimate the overlap in isotopic niche space. Immature white sharks had the smallest isotopic niche, while the highest was for copper sharks. Overlap was greatest between white sharks and hammerheads (∼45%), while overlap with copper 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.

Inferring habitat use of the Pacific White Shark using vertebral chemistry

“…millennia and persists even during El Niño/La Niña Events (Thunell and Kepple, 2004;Yang et al, 2017). In addition, examination of C and N isotope ratios of young sharks sampled off central Baja California did not show marked differences between (Garcıá-Rodrıǵuez et al, 2021. Multiple studies over time have shown the GC's food web to be enriched in 13 C and 15 N relative to the Pacific (e.g.…”

“…Hussey et al, (2010) reported trophic discrimination factors for vertebral tissues of 1.5 and 3.8 ‰ for carbon and nitrogen, respectively, based on semi-controlled feeding experiments on large sharks. Off central Baja California, small White Sharks (200 cm TL) feed mostly on demersal fishes, with a limited contribution of pelagic fishes (Tamburin et al, 2020, Garcıá-Rodrıǵuez et al, 2021. However, the largest shark caught in the Pacific and those caught in the GC should feed on marine mammals based on their size and known ontogenetic diet shifts (Tricas and McCosker, 1984).…”

“…Young sharks migrate from southern Californian to Mexican waters during late summer and fall, when water temperature decreases (Weng et al, 2007;Weng et al, 2012;White et al, 2019) and some individuals may enter the Gulf of California (GC) (Weng et al, 2012). Immature sharks find high food availability and suitable environmental conditions off the western coast of the Baja California peninsula year-round (White et al, 2019;Anderson et al, 2021;Garcıá-Rodrıǵuez et al, 2021).…”

“…Younger sharks (< 2 m total length; TL) feed mostly on demersal species, like small elasmobranchs and bony fishes (Tricas and McCosker, 1984;Santana-Morales et al, 2012;Garcıá-Rodrıǵuez et al, 2021). Larger juveniles and sub-adults (2-3.5 m TL) switch their diets to larger bony fishes and marine mammals (Carlisle et al, 2012;Hussey et al, 2012).…”

“…An important feature of the tropical eastern Pacific is the presence of a broad oxygen minimum zone (OMZ), which leads to strong denitrification and high d 15 N values of dissolved inorganic nitrogen (Altabet et al, 1999;Sigman et al, 2005). This produces an enriched 15 N baseline within the GC, with consumers feeding within the system having higher tissue d 15 N values that those from the Pacific, including off Baja California (e.g., Aurioles-Gamboa et al, 2013;Whitehead et al, 2020;Garcıá-Rodrıǵuez et al, 2021). If chemical and isotopic differences in regional water masses and associated biogeochemical processes are recorded in White Shark vertebrae during growth, it may be practical to differentiate individuals that have resided either in the northeastern Pacific or GC, or that migrated between these areas.…”