Sharks surf the slope: current updrafts reduce energy expenditure of aggregating marine predators

This blog post is provided by Yannis P. Papastamatiou, Gil Iosilevskii, Valentina Di Santo, Charlie Huveneers, Tarek Hattab, Serge Planes, Laurent Ballesta and Johann Mourier and tells the #StoryBehindThePaper for the paper ‘Sharks surf the slope: current updrafts reduce energy expenditure for aggregating marine predators, which was recently published in the Journal of Animal Ecology.

Animals must continuously make decisions regarding which habitats to use. They may choose habitats with lots of prey, those where they are safe from predators, or pick locations where the temperature is just right. They may also decide to select habitats which require less energy to move through or reside in. These subsequent ‘energy landscapes’ can explain animal habitat use such as why elephants avoid moving up steep hills, or why birds use the sides of mountains with updrafts. Indeed, it is a familiar sight to see birds soaring in the sky, without having to flap their wings, as they ride these updrafts. Updrafts may also form in the marine environment when water currents hit the seafloor and are deflected, yet their importance in shaping the habitat use of marine animals is not clear. Sharks are negatively buoyant and to prevent sinking must swim continuously to generate enough lift over their stiff pectoral fins and body. Furthermore, many shark species are obligate ram ventilators and must swim continuously for their gills to extract enough oxygen to meet their metabolic needs. Coastal regions with updrafts could be very beneficial for these animals as they could provide lift and reduce the amount of energy the sharks have to spend swimming.

To figure out the potential importance of updrafts, we travelled to the atoll of Fakarava, in French Polynesia. Fakarava has a tidally flushed channel at the south end of the atoll, where up to 500 grey reef sharks reside. At night, these sharks hunt intensively amongst the coral, but during the day they appear in a calm state hanging out together and often swimming into the current, but with apparently minimal effort. We joined the film maker Laurent Ballesta as part of a joint filming/research expedition on the behavior and ecology of the sharks in the channel (the resulting film was ‘700 sharks at night’ that also aired on the National Geographic Channel).

Grey reef sharks use updraft zones within the Fakarava channel during daytime incoming tides. Photo by Yannis Papastamatiou

Indeed sharks select regions of the channel where we predict updrafts to occur, and these locations will switch based on the direction of tides (ingoing or outgoing). During the incoming tides, sharks drift through these updraft zones with minimal swimming effort. They eventually drift through the zone and then have to actively swim against the current. Rather than waste energy, they let the current take them back into the updraft and the process repeats like a giant shark conveyor belt. During the outgoing tides strong turbulence is generated in the channel and shark move shallower but end up bouncing around like airplanes during a bad flight. Despite this, sharks don’t seem to use much muscular activity and it is possible they can slalom between vortices and save energy, although future research will be needed to confirm this.

Although the tidal currents at Fakarava are not particularly strong, we estimate that sharks reduce their energy expenditure by 10-15% by using these areas, which may partially explain why such large numbers of animal reside within the channel during the daytime when they are not feeding. Sharks are capable of taking advantage of updrafts in much the same way as birds select regions adjacent to mountains. This natural phenomenon creates spectacular scenes at locations like Fakarava and attracts tourists from all over the world who come to see 100s of sharks in beautiful, clear water. However, there are other important implications as well. As we modify coastal environments through building, dredging etc., what impact will this have on updraft zones and the animals that use them?

An acoustic receiver in the Fakarava channel. The receiver listens for acoustically tagged sharks and records data to memory. We return each year to download receivers. Photo by Yannis Papastamatiou

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