When a massive reef shark aggregation relies on delivery

Our work was recently published in the journal Current Biology and even made the cover of the journal.CURBIO_26_15.c1.indd

Mourier J, Maynard J,  Parravicini V, Ballesta L, Clua E, Domeier ML, Planes S (2016) Extreme Inverted Trophic Pyramid of Reef Sharks Supported by Spawning GroupersCurrent Biology 26(15): 2011-2016

It is becoming increasingly difficult to understand what a coral reef ecosystem should look like in areas characterized by low human influence, simply because these areas are becoming increasingly rare. Most of the ocean reefs are facing increasing anthropogenic pressure, especially overfishing and habitat degradation. Therefore, our view of the ocean is often not accurate as we get used to overexploited ecosystems in which the top of the food web has been extirpated. Scientists therefore need to use bait or increase hours of observations to sight any large predator. However, rare healthy reefs still exist and may help us to better understand coral reef ecosystem functioning in health conditions, which is critical to help global conservation management of coral reefs.

In this paper we first quantify the abundance and density of an aggregation of reef sharks in the pass of Fakarava, an atoll of the Tuamotu archipelago in French Polynesia. French Polynesia host healthy populations of sharks as they have never been targeted by local human population and have been protected by law under fishing bans since 2006 within which is one of the largest worldwide shark sanctuary! Our work allowed to estimate the population size of this aggregation to reach up to 900 reef sharks (700 grey reef sharks) in a tiny and narrow channel (density 2-3 times what have been found in other health reefs), making for the moment the largest aggregation of reef sharks ever documented.

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Figure above: Up to 700 grey reef sharks live in the pass of Fakarava, a narrow channel in French Polynesia

We therefore questioned how such a high biomass of predator can survive in such a restricted area.

One of the first question was to determine if there were enough fish in the pass to feed such a number of sharks… and the response was obviously NO!

The second question was therefore to understand if these sharks are resident or if it happens that they feed outside the pass. Using acoustic telemetry, we found that sharks were more present and abundant during the winter months while they were roaming in and out of the pass during summer. So how during these winter months this massive number of sharks can be supplied if they don’t search for food outside the pass?

During June and July, camouflage groupers form dense spawning aggregations in the pass as well as less other fish species. Although relatively ephemeral, these spawning aggregations represent a large amount of biomass into the pass. Indeed, this grouper aggregation accounted for more than 17,000 fishes representing about 30 tonnes of potential food, freely brought to the sharks. Sharks use these inputs as trophic subsidies and do not need to conduct costly foraging excursion outside the pass.

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Figure above: Fish spawning aggregations represent important subsidies for sharks.

While they are generally resting in the strong current during the day, our underwater nocturnal observations showed that they were actively hunting in the pass at night, targeting at least 14 species of fish including groupers.

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Figure above: Sharks are hunting at night on spawning grouper as well as up to 13 other fish species.

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Figure above: Nocturnal hunting of grey reef sharks in Fakarava

Generally, typical ecosystem should be represented as a classic pyramid with broad base of primary producer and decreasing biomass as higher trophic levels are being reached. Biological energy flow is lost when predator consume their prey and therefore there is a requirement for more prey than predators. In the pass of Fakarava, the huge abundance of resident reef sharks and lower relative abundance of prey makes the trophic pyramid inverted; a paradox in which predators outnumber their prey which is not supported by the physical laws of thermodynamics.

Our research provides 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 optimizing the exploitation of both spatial and temporal subsidies.

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Figure above: Sharks exploit both spatial (in summer) and temporal (in winter) subsidies.

Finally, our study revealed important considerations for coral reef conservation. Indeed, protection of vulnerable spawning aggregation is important, not only for the fish themselves but also for predators that depend on them for trophic subsidies.

You can also watch our Video Abstract to better understand our findings.

you can also watch our documentary “The Grouper Mystery” that was produced during our scientific project.

Read also Colin Simpfendorfer and Michelle Heupel dispatch related to our study here.

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