A beautiful sunset over Grahams Harbor, San Salvador, Bahamas. See other beautiful phenomena from the Bahamas.
Brad Martin - Report Author
Research was conducted to determine the recruitment rates of artificial reefs at varying distances from a source population reef (also artificial). The research took place off the Government Dock located on Graham's Harbor, San Salvador, Bahamas. Observations were made over a four-day period. At each reef we recorded the rate of recruitment, the number of recruits at each observation period, and species diversity. Furthermore, comparison was made with other artificial reefs that are at least one year old and also located in Grahams Harbor. Though, we had no indication of the age of these reefs, we randomly chose reefs that were similar in composition to our conch reefs. All reefs utilized in this study were located on sandy substrate in the midst of algal beds and sea grass at a depth between 1.9 and 2.0 meters. Data are analyzed using similarity and diversity indices.
We hypothesized that those reefs closest to the source population would have the most diversity in terms of fish species and the most fish in terms of population numbers. This hypothesis was proved incorrect. After consulting relevant literature on artificial reef recruitment, the researcher found that results were in accordance with other studies.
Artificial reefs can serve to alleviate the stresses on marine environments caused by natural reef degradation and other forms of habitat loss in the world's oceans. Natural reefs are declining at an unparalleled rate in modern times. As these reefs are disappearing, the species that inhabit them are also disappearing. In an effort to stem these losses and replace lost habitat, researchers and conservationists have constructed artificial reefs using numerous types of materials and placed them in our oceans. It is hoped that these reefs will recruit settlers so that population dynamics on the artificial reef will parallel that found on natural reefs (Shulman, 1983). Increases in fish abundance creates increased resources for commercial and recreational uses, as well as contributes to better ecosystem fitness (Rooker, 1997; Shulman, 198). There are numerous variables to consider when examining artificial reef assemblages. Among these variables, construction materials, recruitment dynamics, refuge availability fish assemblages, as well as spatial and temporal variables.
Reef construction typically utilizes a hard substrate material so that benthic flora and fauna can easily attach to the structures. There are four primary materials used in the construction of artificial reefs (Walker et al. 2002). Walker et al however, do not account for submerged sea-faring vessels in their research, only those reefs that are constructed solely for the purpose of reef building. According to Walker et al (2002), the predominant artificial reef building materials consist of the following:
Limestone/Quarry Rock Boulders
These materials are strong, erosion resistant, cost-effective, closely resemble the natural calcium carbonate rock secreted from coral and other marine organisms. This material can be utilized effectively, however, only at the cost of destroying terrestrial habitat to obtain the material. Furthermore, refuge availability is likely a limiting factor.
Currently the most cost effective of the reef-building materials; chemical composition/texture similar to that of natural coral; sturdy building material; leaches calcium hydroxide, which may affect reef inhabitant; limited underwater durability.
Extremely durable; easily acquired; allow for complex constructions (high refuge availability); lack stability; limited invertebrate recruitment potential.
Remedies ballast problem of tire reefs; patented technology; environmentally positive alternative to use of tire-only reefs; surface of reef is primarily concrete.
Each material has distinct advantages and disadvantages serving as an artificial reef. None of these materials can completely fill the role that natural reefs play in marine ecosystems, however, with continued research better reefs can be designed that lack the flaws of the aforementioned materials.
Refuge Availability, Competition, Predation
When examining an artificial reef, the most important factor in whether or not the reef is successful is the ability of the reef to recruit floral and faunal species. Artificial reefs such as those constructed for this research project can serve as habitats for juvenile fishes. Since adult populations (and therefore reproductive success) are directly related to survival of juvenile populations, it is important that artificial reefs can properly house and support a juvenile population (Shulman et al, 1983). Recruitment and settlement are dependent upon refuge availability, predation, and competition (Shulman et al. 1983). Refuge availability is the amount of safe-space that a reef inhabitant will have from predators. Predators influence recruitment directly, by preying on juveniles, or evolutionarily, by selecting for settlement not occupied by predators (Shulman et al, 1983). Refuge availability indirectly affects species richness through its effect on abundance (Caley and St. John, 1996). There are two types of refugia on a reef, permanent and transient. Caley and St. John (1996) define permanent refugia as those spaces that physically exclude predators (small holes, outcrops, etc.) and transient refugia as resulting from habitat complexity that increases the probability that prey will elude predators. In their study, however, neither type of refuge availability was indicated as more effective or Competition between settlers also influences reef recruitment as juveniles directly compete for habitat and reef resources. Priority effects also influence each of these variables in recruitment (Shulman et al 1983). Priority effects is a hypothesis in which the presence or absence of one species will decrease the probability of invasion by another species. These effects can help to maintain high regional species diversity and determine local species distributions.
Of direct relevance to our study is the spatial variables that are in play in reef recruitment. According to Shulman (1985), distance from a natural reef is an important factor in artificial reef recruitment. As distance from a natural reef increases fish recruitment generally decreases; however, the inverse can be true for an artificial reef. Artificial reefs at a greater distance from a natural reef exhibited higher rates of recruitment than closer reefs of the same design (Shulman, 1985). This difference in recruitment is attributed to increased refuge availability found in the algal grass beds surrounding the reefs of greater distance. The algal beds near major reefs are often grazed to the point of no longer providing refuge from predators. Prey species also encountered predators less frequently because these predators would hunt on major reefs where prey abundance is much higher (Shulman, 1985).
Artificial reefs can serve to replace lost or degraded reef in many habitats throughout the oceans. They can serve to supplement recreational and commercial fishing needs. Many state and federal government have realized the potential benefits that an artificial reef can have, both economically and ecologically. Some researchers have found that artificial reefs can even thrive in low-production zones (Spanier et al. 1985) and that artificial reefs are most successful when placed within 600 meters from a natural reef (Danner et al, 1994).
33 Conch Shells
Underwater writing slates
In conducting this research, our research utilized 33 conch shells that we obtained at the government dock east of the Gerace Research Center to construct 3 artificial reefs (11 shells each) at varying distances from the northernmost part of the docks. The reefs were placed in a linear fashion parallel to the shore in increments of 25 meters. The first reef (Conchstantinople) was placed 25 meters of the government dock at a depth of 1.95 meters. Dimensions: Height-35 cm; Width- 60cm; Length- 55 cm. The second reef (Betty's Boutique) was placed 50 M from the dock at a depth of 1.97 meters. Dimensions: Height-30 cm; Width- 50 cm; Length- 60 cm. The final reef (Bobo's Shells 'N Shakes Shack) was placed at a distance of 75 meters from the dock at a depth of 2.0 meters. Dimensions: Height-25 cm; Width- 60 cm; Length- 70 cm. Measurements were taken once a day, however, the time at which each of the measurements were taken varied due to our busy schedule on the island.
Fish Species that Inhabited our Reefs:
Cocoa Damselfish, Squirrelfish, Sand tile Fish, Surgeon Fish, Blue-Headed Wrasse, Parrotfish, Goatfish.
Population Count By Day
Conchstantinople: 7 3 7 4
Betty's: 11 20 14 15
Bobo's: 10 11 9 19
The data collected was analyzed using the Similarity Index (S= 2C/A+B) for each of the reefs. This equation compares each reef with each of the other reefs in the study to determine the similarities between them. The scale of similarity ranged from 0.0 (no similarity) to 1.0 (very similar).
S= Similarity of species
C= Common Species
A= Sample A
B= Sample B
Betty's: .33 Bobo's: .44 Old Reef 1: .5 Old Reef 2: .85 Old Reef 3: .66
Conchstantinople: .33 Bobo's: .31 Old Reef 1: .5 Old Reef 2: .73 Old Reef 3: .62
Conchstantinople: .44 Betty's: .31 Old Reef 1: .89 Old Reef 2: 1.0 Old Reef 3: .8
The data was also analyzed using the diversity index (D= S-1/log n). This equation is used to compare the diversity of species at each of the reefs. Higher numbers indicate higher diversity on the reef.
D = S-1/log n
S = # of species
N = # of individuals
Old Reef 1: 2.27
Old Reef 2: 4.19
Old Reef 3: 3.19
Recruitment at each of our reefs was primarily by juvenile fishes that most often came form the surrounding sea grass and algal beds, not from the artificial reef at the Government dock or other reefs in the harbor as we hypothesized. The hard substrate habitat provided by the conch shells was used (often within minutes of placement) by juveniles as refugia from the roaming barracudas that were observed throughout the research tract. Though we had no measurement of the refuge space available inside of our constructed reefs, it was observed that at each of the older reefs in the harbor, there was slightly higher diversity and fish populations. This can, perhaps, be attributed to the increased amount of refuge space provided by the older reefs. The influence of waves and time had caused many of these reefs to spread out into a larger area than our constructed reefs. This may have opened up more refuge to settling juveniles. Even though the older reefs had been in place for at least one year, they did not exhibit any statistically significantly differences from our newer reefs. All of the reefs in this study are constrained by a finite size and would not grow any larger as would a natural reef, thus the amount of inhabitants on the reef was also limited. his is in accordance with Spannier et al (1985) who found that after a week of recruitment, settlement began to level off. The amount of refugia and resources available to the reef inhabitants was a limiting factor in recruitment.
The similarity index indicated that there was a higher degree of similarity between the established reefs and our new reefs than there was among the new reefs together (all index were greater than .5). This is attributed to the limited space and resources of the reefs, which were all about the same size.
The diversity index indicated that each of the reefs was also similar. The index ranges from 2.27 to 5.95. This indicates that each of the reefs was relative similar in the amount of diversity found. This is a foreseeable result since each of the reefs is located in a similar sea grass habitat from which to draw settlers.
In conclusion, the results that we obtained form this short research project were in accordance with the relevant literature on artificial reef recruitment. The artificial reefs did recruit juveniles from the surrounding areas that eventually will grow too large for the small reefs we constructed and move on to larger reefs were they will find more resources for survival as well as increased chance for reproduction. As the number of artificial reefs increases around a natural reef, it can be inferred that the influx of settling adults on the larger reefs after growing too large will affect evolutionary pressures. The increases in adult populations on the large reefs will select for those fish species that are more fit, and thus increase ecosystem fitness in the long-term. This however, will require more research to prove or disprove the hypothesis. However, it is argued by this author that artificial reefs are an important addendum to marine habitats, when constructed and placed in a fitting area. If the rate of natural reef continues to decline at the current rates, then artificial reefs will likely become more necessary to maintain biodiversity in the oceans of the world.
Caley, M. Julian and Jill St. John. "Refuge Availability Structures Assemblages of Tropical Reef Fishes." Journal of Animal Ecology. Volume 65. 1996. pp. 414-428.
Danner, EM et al. "Comparison of Rockfish Recruitment of Nearshore Artificial and Natural Reefs off the Coast of Central California." Bulletin of Marine Science. Volume 55. 1994. pp. 333-343.
Shulman, Myra et al. "Priority Effects in the Recruitment of Juvenile Coral Reef Fishes." Ecology. Volume 64. 1983. pp. 1508-1513.
--------------------------- "Recruitment of Coral Reef Fishes: Effects of Distribution of Predators and Shelter." Ecology. Volume 66. 1985. pp. 1056-1066.
Rooker, J.R. et al. "Fish Assemblages on Artificial and Natural Reefs in the Flower Garden Banks National Marine Sanctuary, USA." Coral Reefs. Volume 16. 1997. pp. 83-92.
Spannier, E. et al. "Enhancement of Fish Recruitment by Artificial Enrichment of Man-Made Reefs in the Southeastern Mediterrean." Bulletin of Marine Science. Volume 37. 1985. pp. 356-363.
Walker, Brian K. et al. "Fish Assemblages Associated with Artificial Reefs of Concrete Aggregates or Quarrystone offshore Miami Beach, Florida, USA." Aquatic Living Resources. Volume 15. 2002. pp. 95-105.
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