Trophic Ecology

This axis of research aims to better understand the trophic ecology of marine predators and their role in marine food webs.

Two approaches are taken:

1) Direct observations of predation and interspecific interactions - the come-back of natural history observations

Rare predation can be observed during underwater surveys providing direct observations of what predators can eat. It therefore provides information of direct trophic interactions between predator and prey and can help in gaining insight into the complexity of the marine food web. Some shark species which are believed to be apex predator can be also the prey of other larger shark species which in turn make them being mesopredators or lower-level predators such as shown with the picture below with a great hammerhead shark hunting and predating a grey reef shark (Mourier et al. 2013).

A 4.5 m Great Hammerhead shark predates on a 1.5 m grey reef shark in the atoll of Fakarava, French Polynesia (Mourier et al. 2013).

Smaller shark species can opportunistically target fish aggregation in order to easily predate on their prey. This is the case in the picture below showing an aggregation of surgeonfish attracting a large number (> 50 individuals) blacktip reef sharks (Weideli et al. 2015).

A group of ~50 blacktip reef sharks hunt around a large school of spawning surgeon fish in the lagoon of Moorea, French Polynesia (Weideli et al. 2015)

Underwater nocturnal observations allowed to document for the first time natural predatory events and reveal the diet and hunting behaviour of reef sharks.

Observations of the natural predatory behaviour of grey reef sharks at night in the atoll of Fakarava, French Polynesia (from Mourier et al. 2016).

Using underwater monitoring coupled to a bioenergetic model, we were able to provide an empiric example demonstrating that inverted trophic pyramids, where predators outnumber their prey, is only possible at local scales if predators find a way of optimising the exploitation of both spatial and temporal subsidies.

Sharks exploit both spatial (in summer) and temporal (in winter) subsidies that feeds a local inverted trophic pyramid (Mourier et al. 2016)

2) Use of stable isotope analysis

This section is conducted in collaboration with the Heithaus lab (Florida International University). Stable isotopes are naturally occurring biomarkers and provide tools to quantify temporal changes in animal diets. Stable isotope analysis is increasingly being used to investigate the trophic interactions of large-bodied marine predators, including sharks. We use different tissues (plasma, skin, muscles) sampled from free-living reef sharks in a non-invasive way to investigate the trophic ecology of sharks at the community level but also at the species level including between sex and along the ontogeny of animals.

Using isotope analysis taken from plasma samples (high turnover rate) of recaptured individuals provide valuable information on how individual shark diet change over time.

Shift in trophic interactions in juveniles of 3 different shark species as they grow (Matich et al. 2015).

Using stable carbon and nitrogen isotope values from concurrently sampled tissues, we also quantified the direction and magnitude of intraspecific variation in trophic shifts among three shark species, and how these changed with body size. This work suggested that environmental stability may affect variability within populations, and ecosystems with greater spatial and/or temporal variability in environmental conditions, and those with more diverse food webs may facilitate greater individual differences in trophic interactions, and thus ontogenetic trophic shifts. In light of concerns over environmental disturbance, elucidating the contexts that promote or dampen phenotypic variability is invaluable for predicting population- and community-level responses to environmental changes.

Potential factors that may lead to differences in intraspecific variation within populations (Matich et al. 2019).

In examining stable isotope values of carbon and azote in juvenile sharks, we demonstrated that sharing a nursery with another species led the species to trophic niche partitioning. While they feed on a similar trophic level when they are alone in a nursery, lemon sharks appear to feed on prey with higher trophic level than blacktip reef sharks when they share the same nursery. This partitioning allows the co-existence of both predator species during the first life stages. This plasticity show the capacity of sharks to adapt to competition and more broadly to their environment.

In the shark nurseries of Moorea, juveniles adapt their diet depending on if they share it with another species (Matich et al. 2017).