Apex predators often have strong top-down effects on ecosystem components and are therefore a priority for conservation and management. Due to their large size and conspicuous predatory behaviour, reef sharks are typically assumed to be apex predators, but their functional role is yet to be confirmed. In this study, we used stomach contents and stable isotopes to estimate diet, trophic position and carbon sources for three common species of reef shark (Triaenodon obesus, Carcharhinus melanopterus and C. amblyrhynchos) from the Great Barrier Reef (Australia) and evaluated their assumed functional role as apex predators by qualitative and quantitative comparisons with other sharks and large predatory fishes. We found that reef sharks do not occupy the apex of coral reef food chains, but instead have functional roles similar to those of large predatory fishes such as snappers, emperors and groupers, which are typically regarded as high-level mesopredators. We hypothesise that a degree of functional redundancy exists within this guild of predators, potentially explaining why shark-induced trophic cascades are rare or subtle in coral reef ecosystems. We also found that reef sharks participate in multiple food webs (pelagic and benthic) and are sustained by multiple sources of primary production. We conclude that large conspicuous predators, be they elasmobranchs or any other taxon, should not axiomatically be regarded as apex predators without thorough analysis of their diet. In the case of reef sharks, our dietary analyses suggest they should be reassigned to an alternative trophic group such as high-level mesopredators. This change will facilitate improved understanding of how reef communities function and how removal of predators (e.g., via fishing) might affect ecosystem properties.

Reassessing the trophic role of reef sharks as apex predators on coral reefs

https://onlinelibrary.wiley.com/doi/pdf/10.1111/oik.07026

Trophic niche size and overlap decreases with increasing ecosystem productivity

Expanded trophic complexity among large sharks

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.

/pdf/diet-and-trophic-ecology-of-the-tiger-shark-galeocerdo-1yjc0uj8vx.pdf

Diet and trophic ecology of the tiger shark (Galeocerdo cuvier) from South African waters.

Bulk stable isotope analysis was used to assess the trophic level and foraging habitats of Caribbean reef sharks (Carcharhinus perezi) compared to three large sympatric predatory teleosts (the Nassau grouper Epinephelus striatus, black grouper Mycteroperca bonaci, and great barracuda Sphyraena barracuda) in an isolated Caribbean coral reef ecosystem. Models and empirical studies have suggested that the depletion of large-bodied sharks in coral reef ecosystems triggers a trophic cascade that could affect the benthic community, favoring algae over coral. The hypothesized cascade is based on the premise that sharks prey on large piscivorous teleost fish that in turn prey on key herbivorous fish. Analysis of nitrogen-stable isotopes (δ15N) from white muscle tissue revealed neither adult or juvenile Caribbean reef sharks were significantly enriched in 15N compared with sympatric predatory teleost species. Linear regression found no evidence of an ontogenetic increase in nitrogen with increasing body size for Caribbean reef sharks; however, there was a significant positive relationship between body size and carbon isotope (δ13C) values. These results suggest that Caribbean reef sharks in isolated systems do not act as the apex predator in coral reef ecosystems primarily feeding on large-bodied sympatric teleosts. Instead, Caribbean reef sharks form part of an upper trophic-level predator guild alongside large-bodied teleosts, which makes the predicted trophic cascade as a result of the removal of reef sharks unlikely. Moreover, the body size–δ13C relationship suggests Caribbean reef sharks exhibit ontogenetic and individual variation in where they feed. The ecological role of this species is, therefore, complex and contextual, similar to carcharhinid species in the Indo-Pacific, emphasizing the need to further elucidate the interactions between reef sharks and the overall coral reef ecosystem so as to best inform effective conservation and management of the species.

The trophic ecology of Caribbean reef sharks (Carcharhinus perezi) relative to other large teleost predators on an isolated coral atoll

The authors provide corrections for two equation errors, one in the main manuscript text and one in Supplementary Material File S4 of Hussey et al. (2014). A spelling mistake in one of the figure axes is also corrected. The errors are as follows:


1. Within the main manuscript (page 241; bottom of text column two), the following equation to calculate trophic position (TP) using a scaled Δ15N framework based on a dietary δ15N value‐dependent Δ15N model was presented;


TP=log(δ15Nlim−δ15Nbase)−log(δ15Nlim−δ15NTP)k






The TP of the baseline organism was omitted from this equation and must be added to obtain an absolute TP value for the organism of interest. The corrected equation is consequently;


TP=log(δ15Nlim−δ15Nbase)−log(δ15Nlim−δ15NTP)k+TPbase


where TPbase is the trophic position of the consumer used to define δ15Nbase (i.e. TP = 2 or 3).



2. In the supplementary material file titled, ‘ele12226‐sup‐0004‐SupplementaryMaterialS4’, equation (S3) was written as follows which is incorrect;


k=−log(β0−δ15Nlim)−δ15Nlim






The above equation has now been corrected to;


k=−logβ0−δ15Nlim−δ15Nlim


following the same equation presented in the manuscript text (page 242; top of text column one).


In addition, the equations in Supplementary Material File S4 have been reordered; the above equation is now listed as (S4). The corrected Supplementary Material File S4 has been provided as the online Supporting Information of this corrigendum (entitled ‘ele_12270‐sup‐0001‐SupplementaryMaterialS4 CORRECTED’)

3. In Figure 6, a spelling mistake in the x‐axis legends of (a), (c), (e) and (g) reads ‘basline organism’ and should be corrected to ‘baseline organism’.

Corrigendum to Hussey et al. ()

Trophic position and body mass are traits commonly used to predict organochlorine burdens. Sharks, however, have a variety of feeding and life history strategies and metabolize lipid uniquely. Because of this diversity, and the lipid-association of organochlorines,thedynamicsoforganochlorineaccumulationinsharksmaybepredictedineffectivelybystableisotope-derivedtrophic position and body mass, as is typical for other taxa. The present study compared ontogenetic organochlorine profiles in the dusky shark (Carcharhinus obscurus) and white shark (Carcharodon carcharias), which differ in metabolic thermoregulation and trophic position throughout their ontogeny. Although greater organochlorine concentrations were observed in the larger bodied and higher trophic position white shark (e.g., p,p 0 -dichlorodiphenyldichloroethylene: 20.2 � 2.7ng/g vs 9.3 � 2.2ng/g in the dusky shark), slopes of growth-dilution corrected concentrations with age were equal to those of the dusky shark. Similar ontogenetic trophic position increases in both species, less frequent white shark seal predation than previously assumed, or inaccurate species-specific growth parameters are possible explanations. Inshore habitat use (indicated by d 13 C values) and mass were important predictors in white and dusky sharks, respectively, of both overall compound profiles and select organochlorine concentrations. The present study clarified understanding of trophic position and body mass as reliable predictors of interspecific organochlorine accumulation in sharks, whereas regional endothermyanddietshiftingwereshowntohavelessimpactonoverallratesofaccumulation.EnvironToxicolChem2015;9999:1-10.© 2015 SETAC

Environmental Toxicology COMPARATIVE ORGANOCHLORINE ACCUMULATION IN TWO ECOLOGICALLY SIMILAR SHARK SPECIES (CARCHARODON CARCHARIAS AND CARCHARHINUS OBSCURUS) WITH DIVERGENT UPTAKE BASED ON DIFFERENT LIFE HISTORY

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Corrigendum to Hussey et al. ()

Environmental Toxicology COMPARATIVE ORGANOCHLORINE ACCUMULATION IN TWO ECOLOGICALLY SIMILAR SHARK SPECIES (CARCHARODON CARCHARIAS AND CARCHARHINUS OBSCURUS) WITH DIVERGENT UPTAKE BASED ON DIFFERENT LIFE HISTORY