Recreational activities can harm coral reefs through:
- Breakage of coral colonies and tissue damage from direct contact such as walking, touching, kicking, standing, or gear contact
- Breakage or overturning of coral colonies and tissue damage from boat anchors
- Changes in marine life behavior from feeding or harassment by humans
- Water pollution
- Invasive species
- Trash and debris deposited in the marine environment
Snorkel, Scuba and Trampling
A number of studies document impacts to coral reefs from divers and snorkelers. They can be caused by damage from fin kicks, pushing or holding coral, dragging gear, and kneeling/standing on coral. Not all divers cause the same amount of damage.
SCUBA divers typically have more impacts on corals than snorkelers, particularly divers wearing gloves and photographers with equipment. ref This is because snorkelers mainly float above the corals on the surface of the water, and damage to corals is usually limited to shallow water areas where snorkelers can either stand directly on or kick corals. ref
Studies suggest inexperienced divers cause greater damage than more experienced divers, and only a small percentage of divers are responsible for a majority of the impacts. ref Other social factors have been identified which can influence damage to reefs. For example, a recent study showed that the nationality of visitors can have a greater impact on incidences of reef damage than other factors such as dive experience and ability. ref This suggests that user attitudes, perceptions, and beliefs are just as important as the actual impacts that recreational activities can have on reefs. It also suggests that in some places, management strategies may need to be targeted to specific user groups to reduce damage to reefs.
Trampling of corals is also common on shallow, near-shore reef flats and has led to extensive damage in areas with high levels of human use. ref Shoreline access points where people stand or wade to enter or exit the water can be particularly vulnerable to trampling; in such areas coral mortality from substrate contact can reach levels as high as 100%. ref Even in cases where high mortality does not occur, trampling can result in lower reproductive output for corals. ref
The incidences of coral damage from diver contact inspired the concept of diver carrying capacity. ref While this approach has been applied to numerous reefs throughout the tropics, some conservation organizations suggest assessing carrying capacity may have limited practical value. ref Quantifying carrying capacity (pdf, 78k) can be challenging; it varies widely based on ecological conditions of a reef, potential resilience of a reef, and visitor behavior.
Boat anchors can cause considerable damage to coral reefs, including coral breakage and fragmentation. Larger ship anchors and heavy chains can break or dislodge corals, resulting in damaging vast areas of coral reef. While smaller ships cause less damage, areas of heavy recreational boating may also have serious consequences for coral reefs. Anchoring impacts have been partially alleviated due to the installation of mooring buoys in many locations, but anchoring of small vessels on reefs remains a chronic problem over reefs in many developing countries. ref
Anchor damage on coral reefs can persist for many years. For example, anchor damage on a reef in the Virgin Islands resulted in a decrease in live coral cover, compared to coral cover on adjacent undamaged reefs which remained ten years after the damage occurred. ref Anchoring also damages reef-associated habitats such as seagrasses, which provide important nursery and juvenile habitats for many coral reef species.
Vessel groundings can result in catastrophic effects on coral reefs, not only smashing and dislodging corals, but shattering the reef framework (e.g., in Florida and Bermuda ref). While much of the large-scale damage to reefs has been caused by freighters, similar damage is feasible from cruise ships. For example, in the Virgin Islands, a 200 ft. long cruise ship anchored in 4 meters depth had damaged coral communities over an area of 5,300 square meters. ref
Impacts from vessel groundings can range from a few hundred square meters to several hundred thousand square meters of coral reef. ref Such impacts are magnified when combined with associated impacts including fuel spills, leaching of toxins, and even sinking when the vessel cannot be salvaged. Factors that affect the extent of impact include the size of the vessel, the coral cover in the collision area, the socioeconomic capacity of the region to address the collision, weather conditions following the grounding, and ecological conditions affecting recovery (e.g., presence of factors that support coral settlement and growth). ref
Changes in the Behavior of Marine Life
Marine life interactions, such as fish feedings and encounters with charismatic or rare species, are increasingly popular activities for divers and snorkelers. Recreational impacts on marine ecosystems and species may be caused by diver presence or harassment, or from the feeding of marine life. In some cases, diving has been shown to reduce fish abundance at high-use sites (e.g., Kaneohe Bay, Oahu), ref while in other areas, no significant effects on reef fish communities have been observed (e.g., Bonaire). ref
Fish feeding has been shown to result in a number of negative changes in fish behavior, including changes in time spent obtaining food, the size of the animal’s home range, reproductive activity, population density, migration patterns, and species composition due to an increase in the larger, more aggressive species. ref Fish feeding has also been shown to greatly increase the aggressive behavior of the larger species and may result in divers being bitten.
Research demonstrates the adverse effects of pollution from sediment and chemicals on coral reefs, but limited studies address the role of recreational activities in exacerbating these effects. Tour boats can release human waste and gray-water discharge which can damage reefs, particularly in enclosed bays with limited water circulation. Antifouling agents may also cause damage to marine ecosystems. For example, tributyltin (TBT) is used as a paint additive on boat hulls, docks, and fishnets to discourage growth of marine organisms and is extremely toxic to some marine organisms. Sunscreen can also cause significant damage to corals in areas prone to high levels of recreational use by humans. ref
Invasive species can be spread through tourism and recreational activities such as through transportation of ballast water, hull fouling of cruise ships, and fouling from recreational boating (e.g., from hulls, outboard motors, live wells, water lines, fishing gear and debris).
The main approaches to managing recreational activities in coral reef areas include reducing the level of use at certain sites (e.g., through restricting access) and reducing the impacts of use through modifications in human behavior (e.g., educating reef users to discourage destructive actions, and imposing regulations prohibiting certain destructive actions).