Beyond threats associated with climate and ocean change, coral reefs are also affected by various local and regional threats. These threats may occur alone or synergistically with climate change adding to the risks to coral reef systems.
Overfishing and Destructive Fishing
Unsustainable fishing has been identified as the most pervasive of all local threats to coral reefs. ref Over 55% of the world’s reefs are threatened by overfishing and/or destructive fishing. Overfishing (i.e., catching more fish than the system can support) leads to declines in fish populations, ecosystem-wide impacts, and impacts on dependent human communities. Destructive fishing is associated with some types of fishing methods including dynamite, gill nets, and beach seines. These harm coral reefs not just through physical impacts but also through by-catch and mortality of non-target species including juveniles. Read more about threats and management strategies in the Reef Fisheries Toolkit.
Traditionally, impacts from wastewater pollution have been associated with human health, but the detrimental effects of wastewater pollution on marine life – and the indirect impacts they have on people – cannot be overlooked. Wastewater transports pathogens, nutrients, contaminants, and solids into the ocean that can cause coral bleaching and disease and mortality for coral, fish, and shellfish. Wastewater pollution can also alter ocean temperature, pH, salinity, and oxygen levels disrupting biological processes and physical environments essential to marine life.
Other sources of pollution to coral reef waters include land-based pollution associated with human activities such as agriculture, mining and coastal development leading to the discharge or leaching of harmful sediments, pollutants, and nutrients. Marine-based pollution associated with commercial, recreational, and passenger vessels can also threaten reefs by discharging contaminated bilge water, fuel, raw sewage, and solid waste, and by spreading invasive species. Learn more in the Wastewater Pollution Toolkit or in the Wastewater Pollution Online Course.
More than 2.5 billion people (40% of the world’s population) live within 100 km of the coast, ref adding increased pressure to coastal ecosystems. Coastal development linked to human settlements, industry, aquaculture, and infrastructure can cause severe impacts on nearshore ecosystems, particularly coral reefs. Coastal development impacts may be direct (e.g., land filling, dredging, and coral and sand mining for construction) or indirect (e.g., increased runoff of sediment, sewage, and pollutants).
Tourism and Recreational Impacts
Recreational activities can harm coral reefs through:
- Breakage of coral colonies and tissue damage with direct contact such as walking, touching, kicking, standing, or gear contact that often happen with SCUBA, snorkelling, and trampling
- Breakage or overturning of coral colonies and tissue damage from negligent boat anchoring
- Changes in marine life behavior from feeding or harassment by humans
- Water pollution by tour boats through the discharge of fuel, human waste, and grey water
- Invasive species which can be spread through transportation of ballast water, hull fouling of cruise ships, and fouling from recreational boating
- Trash and debris deposited in the marine environment
Coral disease is a naturally occurring process on reefs, but certain factors can exacerbate disease and cause outbreaks. Coral disease outbreaks can lead to an overall reduction in live coral cover and reduced colony density. In extreme cases, disease outbreaks can initiate community phase-shifts from coral- to algal-dominated communities. Coral diseases can also result in a restructuring of coral populations.
Disease involves an interaction between the coral host, a pathogen, and the reef environment. Scientists are learning more about the causes of coral disease, especially in terms of identifying the pathogens involved. To date, the most infectious coral diseases are caused by bacteria. Transmission of coral diseases can be facilitated in areas of high coral cover ref as well as through coral predation, as predators can act as vectors by oral or fecal transmission of pathogens. ref
The causes of coral disease outbreaks are complex and not well understood, although research suggests that important drivers of coral disease include climate warming, land-based pollution, sedimentation, overfishing, and physical damage from recreational activities. ref
On coral reefs, marine invasive species include some algae, invertebrates, and fishes. Invasive species are species that are not native to a region. However, not all non-native species are invasive. Species become invasive if they cause ecological and/or economic harm by colonizing and becoming dominant in an ecosystem, due to the loss of natural controls on their populations (e.g., predators).
Pathways of introduction of marine invasive species include:
- Ship traffic, such as ballast water and hull fouling
- Aquaculture operations (shellfish aquaculture is responsible for the spread of marine invasive species through global transport of oyster shells or other shellfish for consumption)
- Fishing gear and SCUBA gear (through transport when moving from place to place)
- Accidental discharge from aquaria through pipes or intentional release
Sargassum are a type of brown, fleshy macroalgae that can have detrimental ecological and economic impacts on coral reefs when overabundant.
In the Indo-Pacific, high percent cover of Sargassum is common on degraded coral reefs and often represents a phase-shift from a coral to algae-dominated reef system. ref Their reproductive biology and morphology make them excellent colonizers of free space and particularly resilient to disturbances such as tropical storms. ref When overabundant, they can negatively impact the reef by shading, limiting space available for coral larvae to recruit, and transmitting pathogens. ref
In the Atlantic, two species of floating sargassum, S. natans and S. fluitans, are responsible for causing large mats of algae blooms which are particularly harmful and prevalent on the Caribbean and West African coastlines. ref Floating algae mats are naturally prevalent in the Northern Atlantic and provide many ecological benefits such as habitat, food, and nursery grounds to many species of fish, crustaceans and even sea turtles. ref However, in the last ten years, a shift in oceanic currents has led to an algae invasion in coral reef areas, causing reduced sunlight required by corals and anoxic and hypoxic conditions on reefs, as well as poor conditions on beaches that are detrimental to the tourism industry. ref
Coral predators (or 'corallivores') are naturally occurring organisms that feed on corals for their polyps, tissue, mucus, or a combination of the above. Such predators typically include echinoderms (starfish, sea urchins), mollusks (snails), and some fish.
Corallivory is a common process that, under normal conditions, allows for natural turnover in the ecosystem. However, when these predators are overly abundant (e.g., outbreak conditions), they can cause significant declines in coral cover.
Common coral predators include:
- Crown-of-Thorns starfish (COTS), which are found throughout the Indo-Pacific region, occurring from the Red Sea and coast of East Africa, across the Pacific and Indian Oceans, to the west coast of Central America. COTS can be a major driver of coral loss in the Indo-Pacific, particularly under outbreak conditions.
- Drupella snails, which are commonly found living on corals in reefs throughout the Indo-Pacific and Western Indian Ocean.
- Coralliophila snails, which are often more problematic for Caribbean reefs, although some species are prevalent in the Pacific.