This bat was impaled on thorns. How did this happen? Poor Navigation? Where was Air Traffic Control? Guanacaste Province, Costa Rica.
by Carl Howard
In June of 2003, I was part of a team of six students in the Marine Ecology Field Workshop. We conducted a short field experiment in Grahams Harbor at the Gerace Field Research Station on San Salvador in the Bahamas. The purpose of this field study was to observe the recruitment of fish to newly placed
artificial reef structures. We also wished to test an initial hypothesis that recruitment of fish to such artificial reefs would be affected by the distance from a large source population.
An artificial reef is a man-made structure in a marine environment that serves many of the functions of natural reefs in providing a hard substrate for invertebrate organism to attach to and protective shelters for fish and other marine organisms. Man-made structures such as piers, offshore oil platforms, sunken ships, and other large concrete and metal objects submerged in the marine environment eventually become incrusted with marine organisms through natural processes. They serve many of the same functions as naturally occurring hard substrates. Such artifacts in the marine environment often function as artificial reefs even when that was not the original purpose for which they were placed.
Concern over the decline of natural coral reefs and loss of reef ecosystems around the world has prompted the establishment of artificial reefs to counterbalance these losses. Individuals, organizations, and government agencies now have programs to place structures in marine waters for the primary purpose of being artificial reefs. Increasing the area of hard substrates increases the populations of most marine organisms. In addition to improving marine habitats, artificial reefs provide economic values for commercial and recreational fisheries and scuba activities. In some areas, artificial substrates are designed for specific aquaculture (or mariculture) enterprises to produce shellfish, such as oysters, mussels and clams.
The U.S. Fish and Wildlife Service and most coastal states in the U.S. have programs for placing and managing large artificial reefs. These state and federal programs are drivn by the recognition of benefits for commercial fishing and tourism. De-commissioned military and commercial ships, and some smaller vessels, have been deliberately sunk in locations for the intention of providing wreck diving experiences for scuba divers. Commercially important
fish and sports species are frequently attracted to the artificial reefs for the food source and shelter they provide. Most often, artificial reefs are placed over sand or mud bottoms in coastal waters at moderate depths where they will not obstruct shipping traffic, yet are scuba accessible.
De-commissioned offshore oil platforms are routinely toppled into the waters where they were originally placed as an alternative to taking them apart and disposing of them back on land. Already heavily encrusted by marine organisms during their working life, such platforms often are the only hard substrates over largely sand and mud ocean floors. Because they are vertical structures, they provide habitats at different depths for diverse strata of marine communities, similar to a natural reef wall.
In some areas, artificial reefs have been constructed from piles of old tires, wrecked cars, machinery, and construction rubble. While not initially perceived as aesthetically pleasing, these objects are transformed when they become overgrown with marine organisms until the original substrate structures may be barely recognizable. They take on the characteristic of a natural reef community.
Several commercial companies produce artificial reef building units. These concrete units are generally large geometric forms, such as hollow, perforated spherical or pyramidal shapes that can be interlocked with each other to construct extensive, complex underwater structures. Such structures are designed to maximize both internal and external surface areas. The external surfaces become overgrown with organisms that need light for growth and the internal areas
provide holes for fish and species that seek dark protected areas, as well as provide ample water circulation through the structures.
Productivity in the marine benthic (bottom) environment is limited by the amount of solid surface area available for organisms to attach and grow on. Naturally occurring reefs develop over hard, rocky bottoms. Areas between such reefs are often loose sand that relatively few species are well adapted to live on or in. Where these sand bottoms support marine grasses, such as eel grass or turtle grass, they can provide valuable habitats for a variety of mollusks, and nurseries for juvenile fish of many species. Seagrass-covered sand bottoms are important in the ecology of adjacent reef systems, providing foraging and breeding sites for reef fish. In waters too deep or too shallow for marine grasses, or where high energy zones of wave and current action constantly scour the sand bottom, only a few species of fish and invertebrates can tolerate the desert-like environments and productivity is low. Deeper waters and coastal areas that receive high loads of sediments, such as estuaries, generally develop mud bottoms. Few species are well adapted to such
Stable substrates provided by rock beds and underwater outcroppings are important for highly productive marine ecosystems. In temperate and cold waters coralline algaes, kelps, sponges, bivalve mollusks, and anemones encrust the hard surfaces in community successions, often seasonal in character. They do not usually build up thick permanent accretions of dead organisms under living organisms. In warm tropic and subtropic waters hard corals overgrow other corals to form the characteristic coral reef substrate which other organisms overgrow in turn, evolving into major geologic features.
Because both natural and artificial reefs have significant amounts of vertical surfaces, the available surface area of the sea bed onto which organisms can settle is greatly increased. Differing proximities to the surface and depth of light penetration, as well as degrees of exposure to currents and wave action, further enhance the diversity of habitats provided by reefs.
Artificial reefs have been constructed in many types of marine environments from cold northern waters to tropical lagoons, in both shallow and deep waters. Many nations and international conservation organizations promote artificial reef-building projects for their recognized economic benefits. Japan has developed the most extensive network of artificial reefs in the world, primarily to enhance coastal fisheries and mariculture of shellfish.
Artificial reefs are not entirely without problems. Several organizations have raised concerns over the placement of and the intentions of their use in various locations. Artificial reef construction should not be used as a pretence for offshore dumping. Structures used should not contain hazardous chemical and industrial compounds. Careful planning should go into designing and placing large artificial reef structures so as not to present navigational hazards, interfere with naturally occurring beneficial marine habitats, or block normal current flows and circulation of waters in shallow bays.
Some groups have argued that building artificial reefs or sinking ships to increase sports fishing and scuba tourism to some areas can lead to undesirable social and economic impacts on local communities and local fisheries. Poorly designed or poorly placed artificial reefs can lead to over-concentration of marine organisms in shallow embayments, resulting in environmental deterioration through over-nutrification and eutrophication. This is especially true where these structures are used for intensive mariculture practices in waters with restricted circulation and poor tidal flushing, common in many Asian countries and some North American and European areas.
Most artificial reefs built by private and governmental organizations are fairly large structures or networks of structures that cover dozens to thousands of square feet of sea bottom. Any hard-surfaced object, from stones of only a few inches across to sunken battleships, will provide a surface for the recruitment of new colonies of organisms by the settlement of larvae from the plankton.
In Grahams Harbor, San Salvador, the reefs we built were quite small, made from piles of conch shells. Some of the other reef structures placed by research groups in previous years consisted of concrete construction blocks, empty metal barrels, and other discarded metal and concrete objects. Conch shell piles were among the most common reef construction objects. The bottom of Grahams harbor is coralline sand with patches of seagrass beds. It is a moderate energy zone of tidal currents, and occasional high energy storm surge, which cause frequent shifting of the sand. Heavier objects, such as concrete blocks tended to resist being shifted about by the currents. Over time, however, the piles of conch shells are collapsed and dispersed over a wider area. Their lighter weight and rounded shape allows them to be moved about by strong wave action.
In June of 2003 we conducted a short field experiment in Grahams Harbor, Bahamas, at the Gerace Field Research Station. The purpose of this field study was to observe how long it would take to recruit of fish to newly placed artificial reef structures, and determine what species would take up residence. We also wished to test an initial hypothesis that recruitment of fish to such artificial reefs would be affected by the distance from a source population. We evaluated three older conch shell reefs placed by teams in previous years at the end of the observation period to compare diversity and population density of the established reefs with our newly placed ones.
We anticipated that the source of recruitment would come from the large pier near the field station. This pier is a collapsed cement structure, originally about 30 meters long and 3 meters wide. Large sections of the pierŐs surface have collapsed become submerged, forming an artificial reef habitat supporting a great diversity of reef fish species. Natural patch and fringing coral reefs occur about a quarter of a mile and further out into the harbor from the pier.
The artificial reefs we designed for out study consisted of three piles of conch shells placed at intervals of 25, 50, and 75 meters out from and perpendicular to the outermost pilings of the old pier and parallel to the shore line. Each pile consisted of 11 large conch shells. We used only
snorkeling gear during the course of our study. We used a measuring tape and a compass to determine our distance and direction out from the end of the pier so that we could easily relocate the conch piles each day of our observations. We also measured the depth of the water to the top of each conch pile at the time that we initially placed them, which was near low tide. We also measured the height, length and width of each conch reef. The dominant bottom cover over the area where we placed the conch piles was white coralline sand and seagrass beds (Table 1).
TABLE 1. Artificial Reef Descriptions
Dimensions Reef 1 Reef 2 Reef 3
Distance from pier 25 m 50 m 75 m
Water depth (low tide) 1.95 m 1.87 m 2.0 m
No. of conch shells 11 11 11
Height of reef 35 cm 30 cm 60 cm
Width of reef 60 cm 50 cm 60 cm
Length of reef 55 cm 60 cm 70 cm
We visited the conch piles once each day for a period of four days, recording the number and species of fish seen at each conch pile
TABLE 2. Daily counts of individual fish
at reef (all species)
Reef 1 Reef 2 Reef 3
Day 1 7 11 10
Day 2 3 20 11
Day 3 7 14 9
Day 4 4 15 19
Additionally, we visited three older artificial reefs which had been built by research teams in previous years. These older reefs were similar structures of conch shells placed in the seagrass bed. We compared the populations of fish inhabiting these older structures with those that were attracted to the newly placed conch piles. Similarity Index (Table 3) and Diversity Index (Table 4) calculations were performed on the collected values to compare the populations at each reef with the others, both new and old reefs.
TABLE 3. Similarity Index comparison of species
populations at new and old reefs
Similarity Index (S)
S = 2c/A+B
C = Common species
A = Sample 1
B = Sample 2
New New New Old Old Old
Reef 1 Reef 2 Reef 3 Reef 1 Reef 2 Reef 3
Reef 1 - .33 .44 .50 .85 .66
Reef 2 .33 - .31 .50 .73 .62
Reef 3 .44 .31 - .89 1.00 .80
The Diversity Index (Table 4) compares the number of different species observed at each reef with the number seen at each of the other reefs. The most common species seen at the conch reefs were juveniles of sand tilefish, wrasses, parrotfish, fairy basslets, French grunts, various angelfish, occasional yellow goatfish, damselfish, squirrelfish, and surgeon fish.
TABLE 4. DIVERSITY Index comparison of species
diversity at new and old reefs
D = S-1/log n
S = # of species
N = # of individuals
New Reef 1 2.55
New Reef 2 5.95
New Reef 3 3.13
Old Reef 1 2.27
Old Reef 2 4.19
Old Reef 3 3.19
OBSERVATIONS AND DISCUSSION
Within only a few minutes of placing the artificial reefs, we observed small fish approaching the conch piles. Most of these were sand tilefish, which hide in holes in the surrounding sandy bottom amid the manatee grass and turtle grass. In consecutive days we saw a variety of species, primarily juveniles that were only a few inches long. It was obvious that these recruits did not come from the pier which was home to much larger, adult fish. Closer investigation of the surrounding seagrass bed revealed that many small fish lived among the seagrass and frequented any objects that had hard surfaces and crevices, such as shells and objects discarded by human activities.
Artificial reefs built as part of research projects in previous years were predominantly piles of conch shells, similar to the ones that we constructed. Due to the unstable nature of the sandy bottom and wave and current action in the relatively shallow near shore area of Grahams Harbor, such reefs of conch shells became dispersed over a larger area. Reefs built of piles of concrete blocks or other heavy artificial objects stayed intact over longer periods than the piles of conch shells. The scattered conch shells still served a valuable function as protective habitat for small fish. Nearly every conch shell or large objects providing holes or covers were frequented by some population of fish. It was rare to find an old conch shell without one or more fish in or
The conch shells used to build the reef were collected from near the large pier where piles of conch are found. These had been thrown here by local fishermen for whom the conch are an important food item. Very few live conch were seen in Graham harbor because of over fishing, but the considerable quantities of shells piled in the water by the pier attest to previous abundance.
Many of the conch shells gathered to build the reefs were covered with a short, thin growth of fine filamentous algae. It was not observed that the juvenile fish coming to the conch reefs were feeding on this algae, but rather that they were attracted to the holes the conch provided. These juveniles did not remain continuously at the conch piles but swam back and forth between the reefs and the surrounding grass. Most of their foraging appeared to take place within the seagrass.
It became clear over the course of this project that our original expectations about the source of new recruitment were incorrect. Recruitment of fish to the conch piles did not come from the population of large fish inhabiting the old pier but from the sea grass bed around the conch piles. The fish that took up residence in the conch pile reefs were small juveniles. The conch shells with their openings and crevices provide a more secure protective cover than the
sea grass itself. A close inspection of the sea grass reveals that it is inhabited by numerous small fish of many reef species, functioning as a nursery area away from the predation of large fish feeding around the large reefs. Schools of yellow jacks and an occasional barracuda were observed swimming above the seagrass beds. These and other large fish are potential predators on small juveniles.
In keeping with the results of other researchers, the surface area and crevices provided by the conch pile artificial reefs attracted populations of fish. Larger fish that were seen to predominate on natural coral reefs in the waters around San Salvador were more attracted to large structures such as the old pier. Small juvenile fish were attracted to small hiding places such as those provided by the empty conch shells.
The following individuals were members of the a team, calling themselves Team Fun, who carried out this artificial reef project.
Artificial Reefs, Inc. Summer 2001 Newsletter. www.artificial-reefs.com.
Georgia Artificial Reef Program. 2001. A Guide to Georgia's Offshore Artificial Reefs. Georgia Department of Natural Resources, Brunswick, Ga. 30 pages.
Harald Franzen. 2001. The Submerged Subway Reef. Scientific American (August 06, 2001). Posted online at www.sciam.com.
Monte Lee Thorton. 1996. Marine Awarenes: Aquatic Ecology Of Gulf Rigs. In Explore Underwater Internationial Magazine Online. www.exploreuw.com.
R. J. Kern. 2001. Artificial Reefs: Trash to Treasure. National Geographic News (February 5, 2001). Posted online at news.nationalgeographic.com.
U.S. Department of Interior Environmental Information. 2003. Artificial Reefs: Oases for Marine Life in the Gulf. Online article discussing decommisioning of oil-rig platforms in the Gulf of Mexico. www.gomr.mms.gov.
Wilson, D. A., V. R. Van Sickle, and D. L. Pope. 1987. Louisiana Artificial Reef Plan. Louisiana Department of Wildlife and Fisheries Technical Bulletin No. 41. 143 pages.
www.oceanconservancy.org. Artificial Reefs. Online article from Ocean Conservancy conservation organization discusses issues and negative impacts of artifical reefs.
www.reefball.org. Online site of the Reefball Foundation, an international environmental nonprofit group with a mission is to help restore the world's ocean ecosystems.
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