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

Emiliano García-Rodríguez,Oscar Sosa-Nishizaki,Christopher G Lowe,John B O’Sullivan,Sharon Z Herzka

doi:10.3389/fmars.2021.687738

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 (&lt;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.

Highlights

Most shark species are considered top predators occupying high trophic levels and their presence or absence influences food web structure (Cortés, 1999; Ferretti et al, 2010; Heupel et al, 2014)
Muscle tissue samples from 31 immature white sharks were collected inside Bahía Sebastián Vizcaíno (BSV)
In the northeastern Pacific, adult white sharks are known to feed in offshore areas, including oceanic islands like Guadalupe island (Jorgensen et al, 2010; Hoyos-Padilla et al, 2016), where the isotopic baseline is depleted in 13C relative to coastal and benthic habitats (Carlisle et al, 2012; Malpica-Cruz et al, 2013)

Summary

Introduction

Most shark species are considered top predators occupying high trophic levels and their presence or absence influences food web structure (Cortés, 1999; Ferretti et al, 2010; Heupel et al, 2014). Adult white sharks (Carcharodon carcharias) are distributed mostly in offshore pelagic waters, including oceanic islands (Klimley et al, 1992; Domeier and Nasby-Lucas, 2007; Jorgensen et al, 2010; Francis et al, 2015; Boldrocchi et al, 2017; Skomal et al, 2017; Moro et al, 2020), while juveniles occupy coastal areas as nurseries (Bruce and Bradford, 2012; Harasti et al, 2017; Curtis et al, 2018; Bruce et al, 2019). Coastal nurseries harbor high food abundance and provide shelter from predators, allowing young white sharks to grow faster, which is crucial for their juvenile survival and recruitment to adult population growth (Mollet and Cailliet, 2002; Heupel et al, 2007)

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