Artificial Reefs vs Natural Reefs: Which are More Habitable?

 

Maria Gausman

Tropical Marine Ecology 2003

 

 

Background

Artificial reefs are man-made hard substrates purposely or accidentally placed on sea floors so that corals, sponges, algae, and any other reef-building organisms may colonize them. Like natural coral reefs, artificial reefs provide habitat for the same diversity of fish, invertebrates, and plants that natural reefs do.

Hard substrates are anything that is environmentally inert, such as steel and/or concrete. They can range by size anywhere from a few feet to a mile in length. Substrates may be specifically made for artificial reef construction, or they could be scrap construction materials, bridge parts, planes, ships, culvert pipes, etc. For example, Ten Mile Reef, located 10 miles off shore of South Carolina, between Myrtle Beach and Georgetown, is made up of two buoys, one tugboat, one barge, one 200’ boat, and concrete reef balls (Sea Science, 2003).

There are many organizations creating artificial reefs. Non-governmental organizations (NGO) such as Artificial Reef Society of British Columbia and Geographe Bay Artificial Reef Society Australia accept donated retired ships, airplanes, and other suitable scrap for artificial reef development (Scuba About and Michael McFadeyn, 2003). Government organizations such as the Department of Natural Resources South Carolina and the United States Coral Reef Task Force also participate in reef building (Sea Science and USCRTF, 2003).

There are two main reasons why artificial reefs are made: to promote coral reef biodiversity and to promote regional economic growth. Worldwide, the status of coral reef habitats have been declining for many years due to many environmental and anthropogenic stresses. A Johns Hopkins report published in 2000 estimated that nearly 70% of the world’s coral reef communities would die off in the next 40 years (Johns Hopkins School of Public Health, 2000). There has been a need identified by many NGOs and governmental agencies to work towards protection of the existing coral reefs left, and promotion of new coral reef habitat. Artificial reef development is one way to try to create additional coral reef habitat.

Besides benefiting the natural community of reef associated marine life, developing artificial reefs are also a way to enhance or rebuild economic and/or sustenance resources of coastal communities. For example, the Department of Natural Resources South Carolina has created over 100 artificial reefs to attract more sport divers and fishers (Sea Science, 2003). Jagruti, an NGO active in India, has created artificial reefs off the shore of Nellore to enhance the fish catch for the local fishing community (Sustainable Networking Development Programme, 2003).

The effectiveness of artificial reefs in rehabilitation of degrading coral reefs have been well studied. Common endpoints used to evaluate successfulness of artificial reefs are numbers, biomass, and species richness of both fish and coral. The duration of recruitment and the extent of attainment are also important endpoints to consider. It is possible that the same or better level of fish species richness and density found in undisturbed natural coral reefs may be obtained with artificial reefs within 1 year of deployment (Clark and Edwards, 1999). Such statistics indicates that artificial coral reefs are effective tools for rehabilitation, but do they really promote the return of the original community?

 

Research Question: Are artificial reefs more or less habitable than natural reefs?

Hypothesis: Artificial reefs are less habitable than natural reefs, based on the abundance of some key fish species.

 

The Research team: Penny Greenler, Cassie Burns, Kat Byerly, Eli Balken, Heather E. Allen, Maria Gausman

 

Methods

The endpoint that the research team chose to measure was density of 10 key fish species that have been found regularly at natural coral reefs at San Salvador: Blue Tang (Acanthurus coeruleus), Blue-headed Wrasse (Anampses lennardi), Nassau Grouper (Epinephelus striatus), Sergeant Major (Abudefduf saxatilis), Fairy Basslet (Gramma loreto), Spotlight Parrot (Sparisoma viride), Rock Beauty (Holacanthus tricolor), French Grunt (Haemulon flavolineatum), Blue Chromis (Chromis cyanea), Squirrel (Holocentrus sp.)

 

Survey Procedure:

At each reef that was studied, three teams of two researchers either snorkeled or dove about an area that they had selected to survey. Selection criteria were areas with large coral mounds containing at least 2 different coral species, or areas supporting at least 2 or more types of fish. Teams would select areas that were not in close proximity with each other. Using slates to record the number of chosen fish species in an area, one researcher would time the survey allowing for 5-minute observations while the other researcher would count and record.

 

The study included two artificial reefs, Government Pier and Cargo Box, and four natural reefs: Snapshot, Gaulin, Grotto, and Dump.

Government Pier consisted of a long stretch of concrete rubble, metal wiring and fasteners. It was a pier used by the United States Navy, and is now abandoned. It is long but not wide, and the concrete rubble provided enough relief for fish to hide. Some coral was observed growing there, such as Diploria sp.

 

Cargo Box was approximately one kilometer east of Government Pier. It was a 5’x5’ metal box. Some coral had begun colonating, such as Diploria sp. and Porites sp. It is not known how long the Cargo Box has been there.

 

Snapshot Reef is a patch reef located off the western coast of San Salvador, south of Cockburn Town. The contained many mounding coral, such as Montastrea sp, Diploria sp, Porites sp, and many Gorgonians.

 

Gaulin Reef is a patch reef located between Catto Key and the north shore of the island. This reef contained a large colony of Acropora palmata, and some A. cervicornis, as well as mound corals and many Gorgonians.

 

Grotto Reef near Grotto Beach on the south shore of the island is a barrier reef containing very tall mounds a various Diploria, Portites, Montastrea, and many Milliporids.

 

Dump Reef is a fringing reef found in Graham’s Harbor. This reef also contained many mounding corals, A. cervicornis, and some Milliporids.

 

Data Analysis

Two methods to analyze the data were used: Similarity Indices and the Spearman Rank test.

Similarity Indices compares the reefs by focusing on how many of the fish species surveyed were observed at both reefs. The formula (Lynch, 1990):

 

S_xy = 2n_xy/(n_x + n_y)

 

2n_xy = the average number of the species observed for both reefs

(n_x + n_y) = sum of the number of species observed at Reef X and at Reef Y

SI (similarity index) that is closest to 1 indicates that the reefs in the comparison are similar.

 

Using Statview, statistical analysis program for Macintosh, the Spearman Rank test compares the rank order of data collected for each reef and looks for independence of the data sets. A matrix illustrating which reefs were statistically independent from each other is shown in Table 1.

 

Results

Graph 1: Similarity Indices for Government Pier and Cargo Box reefs verses the natural reefs.

 

Similarity indices were high (< 0.8) for all comparisons between the artificial reefs and the natural reefs. For the comparisons with Gaulin Reef, Government Pier was more similar than Cargo Box (0.93 : 0.80). Although the SI indicate that the artificial reefs in our study are very similar in fish species occurrences to the natural reefs, the SI do not reflect the number of individuals per species that occurred at each reef.

 

Table 1: Indepence Matrix

Red numbers are p-values for the comparisons that are not statistically independent from each other (p>0.05).

 

The indepence matrix (table 1) illustrates how each reef compares to each other statistically. According to Spearman Ranking, the artificial reefs, Government Pier (Gob’s) and Cargo Box (Cargo) are not independent of Dump Reef and each other. None of the other natural reefs in the study are similar to the artificial reefs in terms of the ten fish species surveyed. The matrix also indicates that Dump Reef was not independent of Grotto, and Grotto was also similar to Gaulin. Snapshot Reef (Snap) was independent of all of the reefs in this study.

 

Conclusion:

Although two different types of data analysis were employed for this study that indicated two different results, the team concludes that artificial reefs are less habitable than natural reefs. This conclusion is supported by the Spearman Rank analysis, which includes more information provided by the data as well as being a more statistically robust analysis than similarity indices.

 

Discussion

The research team faced several challenges while conducting this study. A better understanding about identifying the fish species that was chosen was needed in some instances. A more uniform and consistent approach to surveying by all of the researchers would have added more validity to the study. Prior to beginning the study, clearer and precise instructions on how to proceed that were agreed upon by the team would have benefited everyone.

Some recommendations to future researchers on this topic are to find more sites within a reef to survey, and/or survey more reefs. Other organisms besides fish would be very informative and perhaps more manageable; such as coral.

Habitat loss to coral reef organisms is the result of many different types of stresses. Artificial reefs are one method that is in serious consideration to be the way to maintain the existing amount of coral reef habitat that is left today. It is also a way to increase habitat in order to rehabilitate. Unfortunately, building artificial reefs don’t solve the problem of what all is causing these vital ecosystems to dwindle in the first place. In order for artificial reefs to be successful, they must overcome the same stresses that natural reefs are dying from today.

           

References

Sea Science: South Carolina Department of Natural Resources (2003) http://www.dnr.state.sc.us/marine/pub/seascience/artreef.html

 

Michael McFadyen’s Scuba Diving Website (2003), http://members.ozemail.com.au/~diving/articles/hmasswan.htm

 

Artificial Reef Society of British Columbia (2003), http://scuba.about.com/gi/dynamic/offsite.htm?site=http%3A%2F%2Fwww.artificialreef.bc.ca%2F

 

United States Coral Reef Task Force (2003), http://coralreef.gov/

 

Population and the Environment: The Global Challenge, Population Reports. Series M, Vol. XXVIII, No. 3, Fall 2000. The Johns Hopkins School of Public Health: http://www.jhuccp.org/pr/m15/m15chap5.shtml

 

Sustainable Network Developmental Programme, India, “Artificial Coral Reefs Regenerate Marine Life”, The Business Line (May 9, 2003), http://sdnp.delhi.nic.in/resources/environment/news/bl-09-05-03-coral.html

 

Clark S. and Edwards A.J. (1999) An evaluation of artificial reef structures as tools for marine habitat rehabilitation in the Maldives. Aquatic Conservation: Marine and Freshwater Ecosystems 9, 5-21.

 

Hardy Jr. J.D. (1998) Coral Reef Fish Species, NOAA\National Oceanographic Data Center, NODC Coral Reef Data and Information Management System, http://www.nodc.noaa.gov/col/projects/coral/hardcoral/coral_fish_all.pdf

 

Lynch, M. (1990) The similarity index and DNA fingerprinting. Molecular Biology and Evolution 7, 478-484.