Coral Reefs are important, biologically diverse ecosystems. Due to both human and natural threats, coral reef ecosystems are declining. However, the benefits of coral reefs are enough for many people to try protecting them and some have even tried to build them. Ever since coastal fishermen have noticed that fish have been attracted to both natural material and artificial structures submerged in coastal waters, they have been trying to build artificial reefs to increase their catches.
Artificial reefs have been built with about every type of material possible. Since material costs far outweigh the cost of building reefs, many of the materials used in building reefs are discarded or reused materials. People have used old aircrafts, automobiles, tires, garbage, construction rubble, fright trains, pipes, boats, and appliances (Duedall and Champ, 1991). However, some of these materials can pollute the very water that the builders wanted to enhance. Some even question whether artificial reefs are actually increasing the species populations or whether they are just concentrating them for easier access (Mazurek, 1998). Japan, due to necessity, has been leading the research of innovative designs for artificial reefs that not only attract fish, but also help create habitat in order for them to maintain their population (Duedall and Champ, 1991). Some possible methods are using concrete matrices or stabilized ash in a concrete matrix to build complex habitats for fish species. One of the latest trends has been to topple old oilrigs in the name of reef construction (Whitney, 2000). Some studies have shown that the relief and size of the platform has been successful in recruiting large numbers and diversity of fishes (Rilov and Benayahu, 2000). Some reefs have even been built for purposes other than fishing. In California a reef has been built to recreate wave action on a beach that was stabilized to protect a pipeline (Anonymous, 1999). The hope is that by breaking the waves further offshore, beach erosion will diminish.
It is still unknown what materials are best suited for building artificial reefs. Much of the work done so far has concentrated on locations of reefs and the correct conditions necessary for a successful reef. Factors include size, void-space geometry, and dimensions, water depth, currents, thermocline depth, primary and benthic production. What was focused on in this research project was the type of material. We wanted to know whether there was a preferential difference in recruitment of reefs that were built with natural materials over those that were built by man-made materials. Our hypothesis was that natural materials (conch shells) would be preferred over man made materials (metal) for the construction of artificial reefs resulting in greater populations and diversity of fish and other marine inhabitants.
Methods and Materials
In order to test our hypothesis, two small artificial reefs were built on 18 June, 2001, in the afternoon. These reefs were located 15 meters west of the government dock at the North Point of San Salvador Island, Bahamas. They were built 9.4 meters apart (Figure 1). One was built with more natural materials, primarily conch shells, and the other was built with metal objects (Figures 2-3). These were named Conch Reef and BBQ Reef respectively. Conch Reef consisted of approximately 15 conch shells and one cinderblock. BBQ Reef was constructed by metal objects found along the beach such as a barrel grill, a chair, and some metal grates. The conch reef, because of its structure had numerous small spaces associated with it while the metal reef consisted of one large space. This location was chosen because of the readily available building materials, its proximity to the field station for easy access, and because it was known to have populations of fish and coral in the vicinity.
After building the reefs, they were observed during the day and marine species that inhabited the reefs were noted. The first observation was approximately 24 hours after building the reefs (19 June). The second visit to the reef was over 60 hours after building the reefs (21 June), and the third observation was taken over 96 hours after the construction of the reefs (22 June). After collecting the data we calculated species diversity and sample difference indices.
Figure 1. Diagram of study area and approximate locations of artificial reefs.
Figure 2. Underwater photograph of Conch Reef.
Figure 3. Underwater photograph of BBQ Reef.
On the first day of observation six organisms were observed and identified at the Conch Reef and three organisms were identified at the BBQ Reef (Table 1). They were all juvenile fish from 4 different species with the exception of a coffee bean trivia (snail) found at BBQ Reef. More species and individual organisms were found at Conch reef than were found at BBQ Reef (Tables 1-2). The second observation resulted in 17 organisms being found at Conch Reef and none at BBQ Reef. This was a big jump in number of individuals, but the number of species dropped down to three, with only one species that was common between the first observation and the second. The third day of observation resulted in eight individuals being spotted at Conch Reef and none observed at BBQ Reef. The number of species did increase to six with two species common to those observed on both the other two days.
Table 1. Number of species identified on each day of observation at Conch Reef.
Table 2. Number of species identified on each day of observation at BBQ Reef.
Two indices of species diversity were calculated for each day of observation. These included the Simpson's Index and the Margalef's Index. The equations for these two are as follows:
Simpson's Index: D= 1/(Sum (pi2)
Where D is the diversity and pi is the proportion of the ith species in the total sample.
Margalef's Index: Dm= (S-1)/ln(N)
Where Dm is the diversity, S is the number of species, and N is the total number of individuals in the sample.
For both indices, the highest value of species diversity was found on the third day of observation for Conch Reef (Table 3). However, a linear increase in diversity was not observed because the diversity was actually higher during the first observation than during the second observation. Since BBQ reef only had species found on the first day of observation we could only calculate the species indices for the first day (Table 3).
Table 3. Values of species diversity using the Simpson's and Margalef's Indices for Conch Reef and BBQ Reef.
The Index of similarity between samples was calculated by the following equation:
S = 2C/(A+B)
A= Number of species in Sample A
B= Number of species in Sample B
C= Number of species A and B have in common
The results show that similar differences were found between the first observation and the second and third observations at Conch Reef. However, a larger index was calculated for the difference between the second and third observations. Since species were only observed on one day at BBQ Reef, these calculations could not be conducted for it. However the index of similarity between the two samples on the first day (one at each reef) resulted in a value of 0.33.
Table 3. Index of similarity between samples at Conch Reef.
According to the data that was gathered on the two artificial reefs, it appears that the juvenile fish in the area were much more attracted to inhabiting Conch Reef than they were to BBQ Reef. However, the evidence is not clear whether they chose the more natural reef because of its material or because of its void-space geometry and dimensions. The spaces provided at Conch Reef were closer to the sized of the inhabiting fish and probably offered more protection from predation than would BBQ Reef. Other studies have shown that some fish assemblages sometimes prefer metal artificial reefs to the natural reefs (Rilov and Benayahu, 2000). Thus our results may not be the normal response. This may be due to the size of our reefs. They were not very large and therefore may have different recruitment potential than the larger structures used in other studies.
The pattern of species diversity was quite interesting. When comparing the first and last observations, it appeared that species diversity was increasing over time in Conch Reef. However, the second observation had a lower index of species diversity. This may be due to the observation's time of day. The second observation was made during the morning and the first and third observations were made in the afternoon. Unfortunately we were unable to monitor the reef for a longer period of time to see when the diversity leveled off or if it occurred in a cyclical pattern.
Overall, it would be helpful to monitor artificial reefs of different material types over a long period of time (years) to observe recruitment rates, species abundance, diversity, and reproduction rates. However difficulties exist in monitoring and counting individuals in this type of habitat. Reefs are also linked to their surrounding environment and understanding of the effects of the surrounding areas must be included in a continuing study in order to accurately compare the reefs of differently constructed materials.
Anonymous. 1999. Artificial reef to benefit surfing, protect shoreline. Civil Engineering. Vol. 69(9): 24.
Duedall, Iver W., and Michael A. Champ. 1991. Artificial refs: emerging science and technology. Oceanus. Vol 34: 94-101.
Mazurek, Roberty. 1998. Artificial reefs: another name for ocean dumping? E Magazine. Vol. 9(1): 20-21.
Rilov, G., and Y. Benayahu. 2000. Fish assemblage on natural versus vertical artificial reefs: the rehabilitation perspective. Marine Biology. Volume 136: 931-942.
Whitney, Chris D. 2000. Toppled platform in place creates reef in US Gulf. Oil and Gas Journal. Vol 98(45): 53-59.
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