Cassie Burns

Tropical Marine Ecology

 

Natural Reefs vs. Artificial Reefs

            Do artificial reefs have the ability to act as natural reefs in terms of providing an environment that would allow for fish species diversity?

            As the human population increases, there are more and more anthropogenic stresses imposed on natural coral reefs on a global scale. As these aquatic ecosystems are destroyed, there are huge declines in species richness and diversity of fish species (Clark S. et al, 1999). There have been several studies to show that artificial reefs could be potential tools to restore the marine habitats that are currently being destroyed (Abelson A. et al, 2001). These studies look at existing artificial reefs and also the creation of new, well-planned artificial reefs. In tropical waters, artificial reefs have been used to study many fish community structures, the recruitment of fish, and settling preferences (Rilov G. et al, 1998).

            A study completed in Eilat, Israel (Gulf of Eilat, Red Sea) demonstrated that size, relief, surface area, complexity, and location were all important factors that had the potential to influence the success of an artificial reef in attracting fish species. Vertical pillars that supported oil jetties were used as a model for artificial reefs (Rilov G. et al, 1998). Visual fish censuses were taken around the concrete pillars using SCUBA and using varying lengths of transects on the pillars to record the fish. This particular study utilized relative abundance of fish and frequency of appearance in each census. The average values of absolute fish abundance, the species richness, and diversity were calculated for the entire pillar and for the various transects on the pillar. The results calculated from this study indicated that fish abundance and species richness was affected positively by the height of the pillar and the amount of wire that covered the pillar (Rilov G. et al, 1998). Also, the results indicated that species richness increased the deeper the pillar or transect on the pillar was. There were some differences in fish abundance depending on the time of year that the pillars were sampled.

            From this study, it was determined that a diverse and abundant community of fish occurred at the oil jetties.

            The Gulf also contains an abundant supply of offshore petroleum production platforms. These structures could also provide a haven for fish. A visual census was conducted on these structures to investigate the species richness, abundance, and diversity by fish species on various depths of the platform (Rooker J.R. et al, 1996). The results from this census indicated that abundance and diversity were the lowest on the upper portions of the platforms and the greatest in the middle of the platforms. This study demonstrated that species richness was higher on the natural coral reefs versus the platforms. On both artificial and natural reefs, there were several families of fish that dominated the reef.

            Other studies have occurred to indicate if architectural complexity of an artificial substrate has an effect on fish habitation, specifically the damselfish (Pomacentrus amboinensis). Several different types of artificial reefs were constructed from concrete bases and various heights of polyvinyl chloride bars were placed on the base (Kawasaki H. et al, 2002). The fish were counted in a visual census and the data was analyzed among all four artificial reefs. The data suggested that some of the artificial reefs provided suitable habitats for the recruiting damselfish. The greatest number of recruits was found on the reefs with the most bars and artificial reefs without any bars had the least amount of damselfish. This indicated that differences in structure among the four types of reefs could attract different amounts of damselfish (Kawasaki H. et al, 2002). There is still much question if artificial reefs are comparable to natural coral reefs.

            On the island of San Salvador, we asked the question: Do natural coral reefs or artificial reefs contain more diversity in regards to species of fish? We hypothesized that natural reefs contain more diversity with regards to fish species.

Six members made up the group that studied the diversity of natural coral reefs vs. artificial reefs. Ten fish species that were considered common amongst the reefs were chosen to act as indicators of a diverse reef. The more abundant each of the ten fish was at the reef, the diversity of the reef increased and the more habitable it was considered.

Six reefs were chosen to study: two artificial reefs and four natural reefs. The two artificial reefs were Cargo Reef (a small reef consisting of a metal square) and Government Pier (a larger reef consisting of a sunken dock and mounds of broken conch shells). The four natural coral reefs chosen were Dump Reef, Grotto Reef, Gaulin Reef, and Snapshot Reef. Two members of the group paired together to take a random survey at each site chosen. Therefore, a total of three random survey sites occurred at each reef. Each sample survey consisted of a five minute period where the pair of divers would record the number of times a common species of fish was seen. We also used a visual census to record fish count. After all the sites had been studied, the data was recorded and analyzed in Statview and in Excel.

The abundance of common fish species was determined at each reef. Figure 1 is an example of the common fish species found at Snapshot Reef.

Figure 1

Blue Chromis, Grunt, and Squirrelfish seem to have the largest numbers at Snapshot. Grouper (1%) and Rock Beauty (not shown because there were none at the reef) have the lowest numbers.

Figure 2

Figure 2 illustrates the percentage of common species found at Government Pier. This differed from Snapshot because there were no Blue Chromis present and the highest abundance of species were the Fairy Basslet, Blue Tang, and Grunt.

The Spearman Rank test was completed on the data using the Statview program. This test compares and ranks the data according to the abundance and presence of species. This rank test has a null hypothesis that the rank order of one set of data is independent of the rank order of another set of data. If p>0.05, then the outcome would be to fail to reject the null hypothesis. In other words, the rank orders of abundance for one set of data is independent of another set (Hays Cummins, 2003). The below table is a comparison among all reefs. When a number is given in the table, this indicates that the two sets of data are independent of one another. Therefore, they are not comparable to one another in terms of types and number of fish species. If there is an X in the box, then the two reefs being compared have similar species in them. Snapshot Reef has the most occurring number of common species with the most abundance of each species. It is common to the other reefs because it does have every species that occurs in one or more of the other reefs.

Table 1

 

The index of similarity is another test that was performed on the data to compare the fish abundance and diversity of the artificial and natural reefs. The two artificial reefs, Government Pier and Cargo reef, were compared to each of the natural reefs. Figure 1 illustrates that both artificial reefs are similar in the comparisons to the natural reefs. One exception is Government Pier shows more similarity to Gaulin than Cargo reef does.

 

 

            In conclusion, artificial reefs do not have the same type or abundance of the ten common fish species as natural coral reefs do. The artificial reefs seem to allow for some habitation of fish species, but not as many species and the numbers within a species is also lower. There are many studies that try to determine the same question that was asked in San Salvador. We used a visual census for fish count like many other studies do. However, there are several things that need to be taken into account for the next study in San Salvador. The time of year and the season of the year need to be taken into account. There definitely needs to be more samples taken at more survey sites to accumulate more data. Instead of having paired members, individuals should complete a sample survey so that fish are not counted more than once. Also, the use of transects could be used for the next study.

            There was an immense amount of information and data collected over a one week period, but there is a definite need for more data collection to answer this question.

 

References

Abelson, A. et al. ICES Journal of Marine Science. “Comparison of the

Development of Coral and Fish Communities on Rock-aggregated Artificial

Reefs in Eilat, Red Sea”. 2001.

Clark, S. et al. Aquatic Conservation: Marine and Freshwater Ecosystems.

“An Evaluation of Artificial Reef Structures as Tools for Marine Habitat

Rehabilitation in the Maldives”. 1999.

Kawasaki, H. et al. Ichthyological Research. “The Relationship

            Between Habitat Physical Complexity and Recruitment of the Coral Reef

            Damselfish, Pomacentrus amboinensis: an Experimental Study Using Small-scale

            Artificial Reefs”. 2002.

Rilov, G. et al. Marine Environmental Research. “Vertical Artificial Structures

as an Alternative Habitat for Coral Reef Fishes in Disturbed Environments”. Vol.

45. 1998.

Rooker, J.R. et al. Coral Reefs. “Fish Assemblages on Artificial and Natural Reefs in

the Flower Garden Banks National Marine Sanctuary, USA”. 1996.