SMF Article : The role of sharks in coral reefs and in oxygen production

The role of sharks in coral reefs and in oxygen production


Super predators, sharks sometimes have unsuspected roles. Their disappearance would have terrible consequences for the ecosystem, and would threaten the lives of many other species. As a reminder, no less than 17 species of sharks and rays are currently considered at risk of extinction according to IUCN. They play a key role in the life of corals, helping them regenerate and even fertilize them. But not only. They also participate, as a species regulator, in maintaining good oxygen production.


Sharks, coral fertilizers


Movement of animals has a positive ecological impact and facilitates certain processes. This is what a study by a team of British and American researchers, published March 21, 2018 on The Royal Society Publishing, discusses. She explains how the movements of gray reef sharks (Carcharhinus amblyrhynchos) play a role in coral fertilization. The latter took place in the Palmyra Atoll, not exploited from the fishing point of view, in the North Pacific Ocean. Sharks deposit nutrients through their movements between the pelagic zone and the reef. Which would naturally fertilize the coral. « We applied network theory to four years of acoustic telemetry data for gray sharks (Carcharhinus amblyrhynchos) […] to assess their potential role in nutrient dynamics in this remote ecosystem », we can read. The telemetry mentioned here, in fact corresponds to a technique allowing the realization of projects over large geographic distances. Tracks are then transmitted in 2D and 3D. This study conducted on individuals marked at the population level « suggests that the consumption of prey and their subsequent excrement » result in nitrogen deposition. This is indeed 94.5kg of nitrogen that would have been deposited daily around the atoll. This plays an important fertilizing role. One more example of the fundamental importance of sharks in the marine food web at the top of the food chain.


A help with regeneration


Once again, the disappearance of sharks would have an impact on corals. Indeed, by cascade effect, if there are fewer sharks there will be less herbivorous fish. And for good reason, these are predated by carnivorous fish, whose population is regulated by sharks. In other words, fewer sharks = more carnivorous fish = less herbivorous fish. However, these are essential in the coral re-establishment. When it dies, because of a natural phenomenon (like a cyclone for example) or by anthropogenic causes, algae come to settle there. These smother the coral and prevent it from recovering. A problem is also posed at the time of fixation and development of scleractinians (formerly madreporary). Corals of the order Scleractinia, which includes hard corals. The herbivorous fish in question here, eat this seaweed and thus allow the coral regeneration. Which would not be possible if they were to disappear, decimated by carnivorous fish that would then be in excess. A study published in 2013, in the journal PlusOne, is part of it. It reveals that to do this, scientists have used surveillance programs over a period of 10 years. The combined effects of chronic disturbances (shark removal) and impulses (cyclones, bleaching) on ​​the trophic structure of coral reef fish on two isolated atolls off the north-west coast of Australia were studied. . Coherent evidence of the assumption that « shark loss can have an impact spreading through the food chain » is provided. This loss would potentially contribute to the « release of mesopolders and change the number of primary consumers ». As for sharks, « Given that their presence may favor the abundance of herbivores, the elimination of sharks by fishing has consequences for natural and anthropogenic disturbances resulting in the loss of corals, because herbivores are essential for progress and to the result of coral recovery « .


Green lungs, blue lungs


Certainly, it is the expression « green lungs » of the planet that is most heard. That said, although terrestrial plants and forests produce oxygen, the same goes for the ocean. This, along with phytoplankton (ie, cyanobacteria and microalgae). Its etymology speaks volumes. The word is indeed composed of « phyto », plant, and « planktos », wandering. They live in suspension. In addition to being at the base of the marine food chain, he is therefore an oxygen producer. In fact, phytoplankton absorb CO2 and then produce and release oxygen when there is light. It plays a vital role in the production of oxygen and provides the planet at least 50% of it (the figures vary depending on the studies). Overfishing and the disappearance of sharks would have, here again, catastrophic consequences. The equation is simple, fewer or more top predators would result in a disrupted ecosystem as well as the proliferation and disappearance of other species. At the end of the chain, we find the plankton. It is absolutely essential to keep the top predators so that the balance is preserved and the oxygen production is not changed.