Abstract
Knowledge of the diet and trophic ecology of apex predators is key for the implementation of effective ecosystem as well as species-based management initiatives. Using a combination of stomach content data and stable isotope analysis (δ15N and δ13C) the current study provides information on size-based and sex-specific variations in diet, trophic position (TP) and foraging habitat of tiger sharks (Galeocerdo cuvier) caught in the KwaZulu-Natal Sharks Board bather protection program. This study presents the longest time-series and most detailed analysis of stomach content data for G. cuvier worldwide. Prey identified from 628 non-empty stomachs revealed a size-based shift in diet. Reptiles, birds, mysticetes, and large shark species increased in dietary importance with G. cuvier size, concomitant with a decrease in smaller prey such as batoids and teleosts. Seasonal and decadal shifts in diet driven primarily by changes in the importance of elasmobranchs and mammal (cetacean) prey were recorded for medium sized (150–220 cm) G. cuvier. Both stomach content and stable isotope analysis indicated that G. cuvier is a generalist feeder at the population level. Size-based δ13C profiles indicated a movement to offshore foraging habitats by larger G. cuvier. Calculated TP varied by method ranging from 4.0 to 5.0 (TPSCA for stomach contents) and from 3.6 to 4.5 (TPscaled and TPadditive for δ15N). Large (> 220 cm) G. cuvier did not feed at discrete trophic levels, but rather throughout the food web. These data provide key information on the ecological role of G. cuvier to improve the accuracy of regional food web modelling. This will enable a better understanding of the ecological impacts related to changes in the abundance of this predator.
Highlights
Large sharks are one of the most ecologically important group of animals in coastal and open ocean systems [1, 2, 3]
Through access to long term data on tiger shark stomach contents (1983 to 2014) combined with multiple tissues sampled from recent captures (2006 to 2014), this paper provides a detailed investigation of the diet and trophic ecology of G. cuvier off KwaZulu-Natal (KZN), South Africa
Prey items ranged in size from small unidentified shrimps and bivalves to various large whale species including Physeter macrocephalus and Megaptera novaeangliae
Summary
Large sharks are one of the most ecologically important group of animals in coastal and open ocean systems [1, 2, 3]. Diet and trophic ecology of the tiger shark ecosystem at the community level through both direct predation and risk effects [1, 7]. As a result, understanding the potential of top predator removal is critical. Some studies suggest their potential for driving large scale cascading effects within food webs [2, 3, 8] while more recent analysis suggests this may not be the case [9, 10, 11]. To improve our understanding of the wider ecological consequences of predator removal, detailed information is required on species’ ecological roles. Information is required on diet and dietary switches with body size and sex as these determine the species trophic position [2, 12, 13]
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