Grey Reef shark (Carcharhinus amblyrhynchos) Carcharhinus amblyrhynchos
By Victoria Caissie
Reef shark species are common residents of coral reefs. They play a crucial role in the ecological process and have lasting effects on the reef’s habitat. These sharks help maintain reef health, balance the food web, and preserve biodiversity. However, in recent decades, an increase in environmental stress has become more clear and caused the time sharks spend at reefs to decline. Warming ocean temperatures, habitat destruction, and ocean acidification are responsible for pushing these reef sharks away from their homes causing a domino effect at the reefs. To gain a deeper understanding of this idea scientists combined roughly eight years of data in which they tracked the Grey Reef Sharks movement to and from reefs. (Williamson et al., 2024)
Why are Sharks Important to the Reef Ecosystem?
Sharks are at the top of the food chain which gives them a lot of responsibility to maintain the balance of biodiversity in the reef ecosystem. The absence of these reef sharks leads to an increase in the number of herbivores on the reef resulting in overgrazing and the potential destruction of the reef structure. On the flip side of things, if there is a lack of shark residency on these reefs the prey of herbivorous fish can thrive causing herbivory to decrease leaving corals overgrown with algae. Another way reef sharks benefit the reef ecosystem is by bringing in new nutrients from their feces. These feces provide tiny microscopic organisms called phytoplankton with the nutrients they need to grow and photosynthesize.
What are the Environmental Stressors affecting sharks and coral reefs?
Environmental stress occurs when a species is no longer able to meet the demands the environment is introducing. This affects an ecosystem at all levels from individuals to communities. It is no surprise that the world is getting warmer, this is due to an increase in carbon emissions into the atmosphere and the remaining being absorbed by the ocean. This is prompting the ocean sea surface temperature to increase forcing sharks to abandon warmer waters and move to areas with a cooler water temperature. On top of warming waters ocean acidification has increased the frequency and intensity of coral bleaching in recent years. This occurs when coral’s symbionts, zooxanthellae, expel out of the coral leaving it a white color. Though coral bleaching does not pose a direct effect on sharks it does cause the reef to decline in health and change the food web making it a displeasing habitat to reside in. Habitat destruction and human activities often go hand in hand when it comes to the decline of pristine coral reefs. Runoff from fertilizers causes excess nutrients in the water. As corals thrive in what is called oligotrophic waters, which means little nutrients, this fails to meet the specific needs of corals. Overfishing is a major human activity affecting marine ecosystems, leading to a reduced level of prey in the water and altering the food web. Various fishing techniques can also destroy the reef’s structure, hindering the availability of shelter, food sources, or areas for reproduction to occur. It is hypothesized that when environmental stressors mentioned above increase in both frequency and intensity, the residency of sharks on reefs will decline. They are likely to find a more suitable habitat with readily available resources.
Results
Scientists found that on average the residency of grey reef sharks decreased when these environmental stressors increased. This data also revealed that the same relationship could be seen during an El Nino year. El-Nino is a climate pattern with behavior typically resulting in the trade winds weakening therefore warming the water temperature in the Pacific Ocean. Environmental stress is often greater during an El-Nino year making it more likely for corals to bleach. The study revealed that there was nearly a fifty percent chance that sharks would remain away from the reefs even after conditions on the reef returned to normal. As shown in the graph below the conditions for both instances, El-Nino and non-EL nino years, are similar regarding return to residency probability.
Fig 1. The mean detections between 122 grey reef sharks and the probability that the sharks would stay away from the reef after longer than the stress occurs. was 0.466. Yellow indicates El-Nino periods and blue indicates Non-El Nino periods.
What does this mean?
Reef sharks that switch their residency away from reefs and towards more open waters are at a higher risk of extinction due to fishing in offshore waters. On the other hand, it is possible that switching their residency may make them less susceptible to being overfished, as fishermen would find less of them in one area on the reefs. As this topic is more recently being explored, the precise outcome of the decline in residency cannot yet be proven. However, this shows the extreme necessity to help prevent the frequency of these environmental stressors occurring due to climate change. These increases are causing sharks to no longer return to reefs they once occupied and in most cases leaving the reef in poor condition. To do this we can decrease individual carbon footprints and press to implement more environmental laws to reduce the overuse of fossil fuels in industries.
References:
Williamson, M. J., Tebbs, E. J., Curnick, D. J., Ferretti, F., Carlisle, A. B., Chapple, T. K., Schallert, R. J., Tickler, D. M., Block, B. A., & Jacoby, D. M. P. (2024b, September 9). Environmental stress reduces shark residency to coral reefs. Nature News. https://www.nature.com/articles/s42003-024-06707-3
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