Every day you mitigate threats, and you probably don’t realize it. You password protect online accounts so that your identity isn’t stolen, or maybe you get a flu shot to prevent getting certain strains of the flu. Coral reefs also face many different threats. If possible, it is best to prevent these threats from happening. Managers can put measures in place to aid in prevention and reduction of threats to coral reefs. Below, learn about the different threats to coral reefs, and actions that can be taken to mitigate them.
Climate Change & Ocean Acidification
The biggest threat to coral reefs around the world is climate change. Rising ocean temperatures and more severe weather events, caused by climate change, are devastating coral reefs around the world. Higher than normal water temperatures stress out corals to the point where they lose their symbiotic algae, a process called coral bleaching (figure 22-5). Sometimes corals can regain their symbiotic algae and recover, but bleaching can lead to the death of a coral colony after a few days.
Figure 22-5. These Acropora corals are bleaching, revealing their white calcium carbonate skeletons.
When many coral reefs bleach around the world, it is called a mass bleaching event. These devasting events are becoming more frequent as ocean temperatures continue to rise, and corals become more stressed by other threats. Coral bleaching is now five times more likely to occur than only 40 years ago.
Ocean Acidification (OA) is also caused by too much carbon dioxide (CO2) in the atmosphere, which dissolves in seawater becoming carbonic acid (H2CO2) and breaks into hydrogen ions (H+) that lowers the pH (higher acidity) of seawater. Ocean acidification reduces the amount of carbonate available for corals and other organisms (figure 22-6) to create their skeletons and shells. In other words, as the pH of the water decreases, it becomes more difficult for corals to build their skeletons. This will cause corals to grow more slowly and form weaker skeletons that are more susceptible to breakage. As OA worsens, corals may no longer be able to grow at all. This will have major impacts not only for the corals, but also for the entire ecosystem. A loss of corals means there will be a loss of habitat for the millions of species that rely on them.
Figure 22-6. OA will eventually make it harder for shelled organisms to create their shells, such as a) the Triton’s trumpet and b) the giant clam.
Photo Credit: a-b) Ken Marks
The only thing that can be done on a global scale to address climate change and ocean acidification is to reduce the amount of CO2 in the atmosphere, by cutting emissions and sequestering carbon. Without reduction in carbon emissions, coral reefs as we know them today may no longer exist as functioning ecosystems by the end of the century (UN Climate report).
However, there are things that can be done locally to protect and preserve coral reefs. We can reduce the pressure reefs face from other threats, such as overfishing and poor water quality, which may improve their ability to survive and recover from a bleaching event. We can also identify and protect resilient reefs, that may be more likely to survive the coming changes to their environment.
Overfishing & Destructive Fishing Practices
Aside from climate change, overfishing (figure 22-7a) may be the greatest threat to coral reefs. Healthy reefs depend upon fish and other animals for a variety of functions. For example, one group of organisms called grazers (e.g., sea urchins, parrotfish, surgeonfish; figure 22-7b), keep the reef clear of algae, allowing new corals to settle and grow. The effects of removing organisms from a reef have far-reaching implications that go well beyond each organism’s population and effect the entire food web (see Unit 16: Food Web). In addition to overfishing, many harmful fishing practices cause severe damage to the reef itself.
Figure 22-7. a) Some areas in the South Pacific have overfished their sea cucumbers (as shown by the large number of dried sea cucumbers in this photo), which play an important role recycling nutrients on coral reefs. b) Parrotfish are important grazers helping to keep algae in check on the reef, but in some locations, they have been overfished.
Photo Credit: a) Amy Heemsoth; b) Stefan Andrews
Derelict fishing gear is lost, abandoned, or discarded fishing gear (e.g., pots, nets, lines) that is left in the ocean. It is also referred to as “ghost gear” (figure 22-8) because it continues to “fish,” trap, and entangle marine life and smother marine habitats despite being abandoned. In addition to killing fish, ghost gear can cause physical damage to corals. Derelict fishing gear can harm or kill other marine life that becomes entangled or trapped in the gear. This is especially dangerous for organisms that are endangered like sea turtles, sharks, and marine mammals. Removing derelict fishing gear from a reef and the surrounding habitats can have a compounding effect on the health of the reef over time.
Figure 22-8. Ghost gear can entangle various animals that all live on or near coral reefs including this a) Hawaiian monk seal, b) crab, and c) sea turtle.
Various fishing rules and regulations can be implemented to reduce the threat of destructive and unsustainable fishing practices to aid in rebuilding overfished coral reefs. Here are some regulations that can be implemented:
- Area closures: Banning fishing activities in a defined area. Area closures may be temporary, seasonal, or permanent.
- Size limitation: A minimum and/or maximum size restriction of an organism that may be caught legally.
- No take zones: Designated areas where no extraction of any resource is allowed. Activities may include fishing, collecting, mining, or drilling.
- Fishing quota: A specified maximum number or amount of organisms that can be taken within a certain time period by an individual or company.
- Species bans: Identifies certain species (such as protected and endangered species) that are not allowed to be taken.
- Catch and release: Some recreational fisheries allow fish to be caught, but not taken. When certain fish species are captured, the fish must be unhooked and returned to the water unharmed.
- License or permit: A document from a regulating authority that provides the bearer the ability to fish according to the established terms. Fishing licenses vary depending on the individual (e.g., resident, age). There are also different fishing licenses depending on where the fishing is occurring (e.g., saltwater, freshwater). Even with the license, all other regulations remain in effect (e.g., size limits, species bans).
- Fishing gear restrictions: There are some types of gear that are especially harmful to coral reef fisheries. There are two types of fishing techniques that are extremely destructive to coral reefs – dynamite fishing and fish poisoning. Banning these types of fishing can aid in reducing destructive and unsustainable fishing.
Pollution & Coastal Development
Water pollution can be harmful to corals and other marine life in a coral reef ecosystem. Most water pollution stems from land-based sources. There are many different forms of land-based pollution that enter the ocean. Run-off carries excess nutrients, pathogens, toxins, and sediment from waterways to the ocean, reducing water quality (see Unit 8: Environmental Conditions) and harming coral reefs (figure 22-9). These pollutants come from farming, sewage, coastal development and road construction, deforestation, oil and chemical spills, industrial waste, and stormwater.
Figure 22-9. These rivers carry sediment and other forms of run-off into the ocean in a) New Caledonia and b) Western Australia, which both have nearby coral reef ecosystems.
Photo Credit: a) Andrew Bruckner
Coastal development often worsens these impacts. Hard structures like roads and roofs cause less water to be absorbed into the ground, resulting in more run-off to the ocean. Removal of trees and vegetation causes sedimentation, where more sediment runs off into the ocean from land, clouding the water column, and making it more difficult for corals to get the sunlight they need for their zooxanthellae (see Unit 4: Coral Feeding). Eutrophication, where excess nutrients (often from fertilizers on farmland or golf courses), can cause algal blooms in the usually clear waters of the coral reef (figure 22-10). This not only clouds the water column making it harder for corals to grow, but it also fuels the growth of macroalgae on the reef, making it more difficult for new corals to settle and grow (see Unit 9: Coral Growth) as corals and algae compete for limited space on the reef. Eutrophication can also fuel harmful algal blooms, which can cause massive die-offs of reef fish and other marine life.
Figure 22-10. a) Pictured is a small dinoflagellate, Kryptoperidinium foliaceum. When run-off increases, it can cause algal blooms. Though not toxic, these blooms deplete the water of oxygen, killing fish and other organisms. b) In the Philippines, this algal bloom has been caused by poor waste management.
Destruction of coastal habitats like mangrove forests, salt marshes, and seagrass beds, reduces the water quality further, as these natural habitats act as a filter for the water running into the ocean. Mangroves and seagrasses stabilize sediment, prevent erosion, and filter excess nutrients, heavy metals, and pollutants. They help to improve water quality before it reaches coral reefs, which need low nutrient waters to survive (see Unit 8: Environmental Conditions). They also sequester (store) carbon dioxide, which reduces the amount of greenhouse gases that enters our atmosphere. As mentioned previously, carbon dioxide contributes to climate change, which in turn causes ocean acidification.
One way that managers can mitigate run-off, sedimentation, and loss of coastal habitats is by working with other government and non-government organizations to implement a watershed management plan. These plans not only focus on managing the coral reefs, but all the land use upstream that could contribute to poor water quality on a reef.
Marine debris is any human-made object that is discarded or disposed of that enters the ocean (figure 22-11). All kinds of marine debris end up in the ocean, including trash such as plastic bottles, glass jars, plastic bags, and soda cans, but it can also include everyday items such as flip flops, fishing gear, and tires. Derelict fishing gear is also considered marine debris.
Figure 22-11. a) After marine debris enters the ocean, currents can carry the debris far distances, where it washes up on beaches like this one in the Papahānaumokuākea Marine National Monument. b) Some marine debris sinks to the seafloor, like this can of motor oil, which landed on top of a coral. As time goes on, the coral polyps will die that are underneath the can.
Photo Credit: b) Annelise Hagan
Managers can organize different marine debris cleanup programs or work with other organizations to remove marine debris from the land and the ocean (figure 22-12a & b), but the best way to reduce marine debris is to stop trash from entering the ocean in the first place. Most marine debris enters the ocean from improperly disposed of trash, so limiting your use of single-use plastic (such as plastic bottles, bags, and straws), and disposing of trash and recyclables properly can help keep these items from ever entering the ocean (figure 22-12c).
Figure 22-12. Cleanup programs remove marine debris from the land and ocean but limiting the use of single-use plastics is another solution.
Invasive species are not native to a specific location, and when introduced they often have no natural predators and can cause a major threat to an ecosystem. In a coral reef ecosystem, invasives can spread rapidly and become the dominant species on the reef. When this occurs, their presence can reduce the number of native species and alter the entire ecosystem.
Lionfish are a good example of an invasive species that has done considerable damage to coral reefs in the Atlantic Ocean (figure 22-13a). On their native reefs in the Indian and Pacific Oceans, lionfish are a healthy part of the coral reef ecosystem. On Atlantic reefs, they have few predators and can reduce native fish populations by throwing off the balance of the coral reef food web (see Unit 16: Food Webs). Removing these fish is one effective method used to mitigate the threat of lionfish (figure 22-13b). Various organizations have created fishing tournaments to remove as many of these fish from the reef as possible. Grocery stores are selling lionfish and local restaurants have also started serving it on their menus to help with removal and create awareness. Although removal helps reduce the lionfish’s population size, it is unlikely that all lionfish will ever be completely removed from the Atlantic Ocean.
Figure 22-13. Lionfish are invasive species in the Atlantic Ocean. Removal programs can help to mitigate the problem, but lionfish will never fully be removed from the Atlantic Ocean.
Photo Credit: a) Andrew Bruckner
Crown-of-thorns starfish (Acanthaster planci), also known as COTS, is a corallivore species in the Indian and Pacific Oceans that can have population outbreaks (figure 22-14). A corallivore is a predator that feeds on coral. They are an important coral reef functional group or a group of organisms that perform the same function in an ecosystem; however, booms in their population size can cause the loss of corals over large areas in a short period of time. Scientists have suggested that excess nutrients could trigger plankton production (Birkeland 1982; Fabricius et al. 2010), which COTS larvae feed on. More food for COTS larvae means that more may survive to adulthood. The loss of COTS predators, like the Triton’s trumpet (Charonia tritonis; Figure 22-15a) and titan triggerfish (Balistoides viridescens), may worsen the issue by reducing the number of predators that could help control the COTS population. Triton trumpet shells are commonly overharvested for artisanal trade and titan triggerfish are overfished on some reefs.
Figure 22-14. During this COTS outbreak, numerous starfish are seen feeding on an individual coral.
Photo Credit: a) Ken Marks; b) Annelise Hagan
Measures can be put in place to reduce the likelihood of a COTS outbreak, by reducing land-based pollution and implementing fisheries regulations (see Mitigating Threats). Once an outbreak occurs though, the only way to address it is by carefully removing these animals from the reef (figure 22-15). For more information, see Managing a COTS Outbreak.
Figure 22-15. Triton trumpets are both a food source for people and their beautiful shells are sold in the seashell market. By implementing fishing regulations for COTS predators, like the a) Triton trumpet, and b) removing COTS from the reef, these actions can aid in mitigating these outbreaks.
Photo Credit: a & b) Ken Marks
Coral reefs are used for different recreational activities including diving, snorkeling, and boating. These activities can cause physical damage to coral reefs. There are regulations that can be put into place to help reduce these pressures.
Snorkelers and scuba divers can cause damage to coral reefs by standing, walking, touching, dragging their gear over them, and kicking corals with their fins (figure 22-16). They can also kick up sediment that covers corals reducing the amount of sunlight their zooxanthellae need to photosynthesize (see Unit 4: Coral Feeding). One way to reduce this threat is to properly educate people before entering the water so they know how to enjoy snorkeling and diving on the reef without damaging it. Another way to regulate this threat is to only allow recreational scuba diving and snorkeling in certain areas, or to regulate the number of people allowed to conduct these activities in a specific area. This can be done by providing a certain number of licenses to operators, a person or company that runs a business related to these activities, such as a dive shop.
Figure 22-16. Scuba divers who stand and sit on corals and the sea floor, kick up sediment, which can all cause damage or even loss of these corals.
Boat anchors can physically damage corals by breaking them apart or displacing them (figure 22-17a). The anchor chain can scrape the live tissue off the coral and/or become entangled, breaking the coral into pieces. Anchors can also harm other sessile or slow-moving reef organisms such as sponges and sea cucumbers. Mooring balls can be used to reduce or eliminate boats from anchoring (figure 22-17b & c). Mooring balls are round buoys that float on the surface of the water and are attached to a heavy permanent anchor on the seafloor. Instead of anchoring a boat to the seafloor, a line can be attached from the boat to the mooring ball to hold it in place.
Figure 22-17. b & c) Mooring balls prevent boats from anchoring which can cause a) damage to coral reefs.
Photo Credit: a) Ken Marks