Abstract
Demographic differences in resource use are key components of population and species ecology across the animal kingdom. White sharks (Carcharodon carcharias) are migratory, apex predators, which have undergone significant population declines across their range. Understanding their ecology is key to ensuring that management strategies are effective. Here, we carry out the first stable isotope analyses of free-swimming white sharks in South Africa. Biopsies were collected in Gansbaai (34.5805°S, 19.3518°E) between February and July 2015. We used Stable Isotope Bayesian Ellipsis in R and traditional statistical analyses to quantify and compare isotopic niches of male and female sharks of two size classes, and analyse relationships between isotopic values and shark length. Our results reveal cryptic trophic differences between the sexes and life stages. Males, but not females, were inferred to feed in more offshore or westerly habitats as they grow larger, and only males exhibited evidence of an ontogenetic niche shift. Lack of relationship between δ13C, δ15N and female shark length may be caused by females exhibiting multiple migration and foraging strategies, and a greater propensity to travel further north. Sharks < 3 m had much wider, and more diverse niches than sharks > 3 m, drivers of which may include individual dietary specialisation and temporal factors. The differences in migratory and foraging behaviour between sexes, life stages, and individuals will affect their exposure to anthropogenic threats, and should be considered in management strategies.
Highlights
Patterns of resource use are a key component in the ecology of species, and such data are vital for ensuring that wildlife management and conservation measures are successful
Male δ15N values showed a decreasing trend with increasing shark length, and δ15N values were significantly related to δ13C for outlier-removed shark data
Percentages are for female (F) or male (M) white sharks less than 3 m or over 3 m in total body length of shark, which were greatly reduced when outliers were removed
Summary
Patterns of resource use are a key component in the ecology of species, and such data are vital for ensuring that wildlife management and conservation measures are successful. There is suggestion of individual dietary specialisation in white sharks (Estrada et al 2006; Hussey et al 2012b; Carlisle et al 2012; Kim et al 2012; Hamady et al 2014; Pethybridge et al 2014; Christiansen et al 2015), evidence of individual variation in predatory behaviours (Huveneers et al 2015; Towner et al 2016), and sexual differences in movement patterns (Pardini et al 2001; Anderson and Pyle 2003; Domeier and Nasby-Lucas 2007; Weng et al 2007; Jorgensen et al 2010; Bruce and Bradford 2012; Domeier and Nasby-Lucas 2012; Robbins and Booth 2012; Kock et al 2013)
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