Reefs in peril
  Reefs in peril
Proximal causes of decline in health of coral reefs hot buttons for peril part of BIOLOGY OF CARIBBEAN CORAL REEFS overfishing/reef collecting on Caribbean coral reefs disease on Caribbean coral reefs SCUBA/snorkeling recreation on Caribbean coral reefs future of Caribbean coral reefs pollution on Caribbean coral reefs eutrophication of Caribbean coral reefs
There are several major proximal causes for reef decline. The topic of SCUBA/snorkeling/recreation is dealt with here, while other topics are accessible via the "hot" buttons.
hot button for bleaching part of Biology of Caribbean Coral Reefs
title button for SCUBA/snorkeling recreation section of BIOLOGY OF CARIBBEAN CORAL REEFS SCUBA/snorkeling/recreation

photograph of diver-damaged sponge complete with hair-band evidence
No matter how careful are the dive operators and their customers, damage to coral reefs from SCUBA-divers and to a lesser extent, snorkelers, is inevitable. Studies in Bonaire Marine Park on the effects of SCUBA-diving on health of the reefs show that while implementation of user regulations such as prohibition of spearfishing, use of mandatory mooring buoys, and establishment of defined activity zones have been beneficial, some heavily used dive sites (up to 6,000 dives per year) are increasingly showing signs of wear.

A broken-off piece of sponge caused by a a SCUBA-
diver, complete with incriminatory hairband

diver-caused damage to reef

photograph showing diseased boulder coralMost notable in a study in Bonaire of recreational effects on reef vitality is an indication of photograph showing diseased boulder coral Montastrea sp. increase in disease in corals, in some cases exacerbated by diver-caused breakage.

Two examples of diseased boulder coral Montastrea sp. Note that there are many causes of disease in corals,
not just from breakage


photograph showing a black grouper being fed by a diver
A minor but sometimes contentious issue is whether fishes should be fed by divers.  Those against the practice point to alteration of natural composition of the reef community, possibly favouring predatory species such as groupers and morays, but also altering coral and other invertebrate compositions through food pollution.photograph of a SCUBA-diver feeding fishes in St. Lucia

Black grouper Mycteroperca bonaci being
fed by a diver
in Cozumel

Diver feeding sergeant majors Abudefduf saxatilis
in St. Lucia


Snorkeler in Roatan honduras feeding bread to
Bermuda Chub and Sergeant-Major fishes

photograph of snorkellers in Roatan, Honduras feeding fishes with bread

photograph of diver feeding fishes in Moorea, South PacificIn addition to nutrient enrichment through feeding of cheeses, sandwich meat, frozen peas, bread, and similar foods, SCUBA-divers can negatively affect water quality through urine, sunscreens, body lotions, and insect repellants, all of which can create localised enrichment or contamination.  Boat-transports can add to pollution through spilled gasoline and oil.





Diver at feeding station in Moorea, French Polynesia
feeding fishes with a left-over fish carcass from dinner


Those in favour of fish-feeding argue that because it is popular it can be used to concentrate diver activity away from sensitive areas of the reef, with effects being minimized by offering only natural fish-foods rather than miscellaneous human foods.  An example of how effective this can be is Stingray City, Grand Cayman Island, and other similarly well-managed feeding "stations" around the Caribbean. Hawkins et al. 1999 Conserv Biol 13: 888. 

feeding rays at Stingray City, Grand Cayman Island

A study of more than 200 SCUBA-divers during dives in Florida showed that the average diver touched or finned living coral 10 times per dive trip.  In diver-damage to elkhorn coralthe particular high-use area photograph of dive leader with barrel sponge, Roatanstudied, this was equivalent to about 5% of all corals being touched each week by the diver population.  Talge 1992  Proc 7th Int Coral Reef Symp 2: 1077.   

Diver-damage to elkhorn coral Acropora palmata in Bonaire


Dive-leader in Roatan, Honduras waits beside a giant barrel
sponge Xestospongia muta
for other divers in the group
to have a look inside, but to ensure they don't touch


photograph showing divers close to reefThe divers participating in the foregoing Florida study were diving in protected waters, had been instructed NOT to touch the corals, and additionally were being closely monitored.  Without these restrictions, it is estimated that up to 20% of the live corals in high-use areas would be touched weekly. Talge 1992  Proc 7th Int Coral Reef Symp 2: 1077. 





In rough-water conditions, as shown here,
divers stay closer to the bottom and are more
inclined to hang on to parts of the reef for stability


So, what is the problem with touching a coral?

diagram of a coral polyp showing parts
Corals are comprised of many interjoined polyps, each made up of a calcareous skeleton, fleshy tentacles, and a mouth (see diagram on Left). The tentacles can retract into the skeleton for protection or expand out of it for feeding and reproduction. The photograph shows several expanded polyps, with central mouths visible, and other polyps with tentacles completely retracted. The skeletons are not naked; rather, are covered by a thin epidermis or skin. In some species, as that shown here, the skin is thin and transparent; in others, as in the boulder coral Montastrea featured several photos above, the skin may be thicker and heavily pigmented. A layer of mucus lightly covers the polyps.

Touching can be harmless or harmful, as described below:

photograph of polyps opening on a coral

Harmless effects:
include contraction/expansion of the polyps, production of mucous, opening of mouth, and extrusion of mesenterial filaments. These are a coral's normal behaviours.

NOTE these filaments, also known as acontia, are defensive and can be protruded from pores in the skin or, more commonly, from the mouth, depending upon species. They bear highly potent stinging cells

Harmful effects:
include breakage with appearance of bare skeleton, removal of mucus, and loss of zooxanthellae. Tissue damage can lead to disease and predators are attracted.

NOTE the zooxanthellae (pronounced "zoozanthellay") are single-celled photosynthetic organisms living symbiotically within the gut tissues of the coral. Their photosynthetic activities provide the polyp with sugars and other nutrients. Their presence adds colour to the polyps. More on zooxanthellae in corals and other reef organisms is available at NUTRITION: CORALS: A CASE STUDY: PHOTOSYNTHESIS


photograph of divers in reef tunnel, Belizephotograph showing diver's bubble forming on the roof of an underwater cave
Divers enjoy crevices, caves, and tunnels, but if bubbles accumulate to form air pockets that last a long time, they may be lethal to sessile invertebrate and plant inhabitants. 

This community of sessile invertebrates on the walls of
a tunnel in Belize would
likely be safe from air-
bubble damage


A large, potentially lethal bubble accumulates on
the roof of a cave in St. Thomas. All sessile
invertebrates will be suffocated in the air pocket


photograph showing several SCUBA-divers with cameras
“Take only photographs and leave only footprints?” is part of the title of a study that considers the impact of underwater photographers on coral-reef dive sites.  As a result of approaching the substratum more closely and of poorer control of buoyancy while taking photographs, divers with cameras, especially those with bulkier and more specialized equipment, are found to damage corals 5 times more than divers without cameras. Rouphael & Inglis  2001 Biol Conserv 100: 281.
photograph showing divers with photographic equipment on a wreck


The study also reveals that male diver-photographers are almost 5 times more likely than female diver-photographers to damage corals. Here, photographing a wreck in Barbados

in Barbados


photograph of girl "riding" a dolphinThere is growing evidence that “bonding” with dolphins can create stress-syndrome in the dolphins, manifested in some cases in stomach photograph of dolphin Tursiops truncatus being petted by a touristinflammation and ulcers.  Many aquatic parks and resorts in the Caribbean are adopting a policy of limiting a dolphin’s workday to short spells and providing people-free sanctuary between work-times.

Bottlenose dolphins
Tursiops truncatus

  graph showing % damaged corals in relation to number of SCUBA-dives per year in the Caribbean and Red-Sea areasA study summarising diver-caused damage to corals at several Caribbean and Red-Sea diving sites suggests that 6000 dives per year may be the limit to a reef's carrying capacity. The precise number will depend upon whether the divers are naive or experienced, photographers or not, and so on. One Red-sea site included in the study is subjected to 50,000 dives annually. Hawkins & Roberts 1997 Proc 8th Int Coral Reef Symp 2: 1923.