Biorock Reef Restoration in Gili Trawangan, North Lombok, Indonesia |

ABSTRACT

Background ��60 Gili Trawangan as a Laboratory for Biorock Reef Restoration ��60 The System and Principle of the Biorock Method �� 61 Methods of Biorock Study in Gili Trawangan �� 63

Selection of Biorock Structures �� 63 Selection of Coral Samples �� 63 Measurements of Coral Growth and Survival Rate �� 63 Measurements of Limiting Factors of Coral Growth ��65 Measurement of Survival Rate ��66 Measurements of Growth and Survival of Sponge Clathria ��66 Observation of Coral Fishes ��68 Measurement of Density of Coral Fish ��69 Index of Diversity (H′), Index of Uniformity (E), and Index of Dominance (C) ��69

Results �� 70 Growth of Acropora formosa on Biorock Substrate �� 70 Growth Rate of Coral Acropora formosa at Three Stations �� 71 Average Growth Rate of Acropora formosa Groups at Three Different Stations �� 71 Survival Rate �� 72 Growth Rate of Acropora formosa and Montipora digitata .......................................................72 Environmental Condition of the Study Area �� 74 Growth of Clathria Sponge on Biorock Structure �� 75 Growth of Transplanted Sponge Fragments �� 76 Rate of Basal Growth of Transplanted Sponge Fragments �� 77 Rate of Survival of Transplanted Sponge �� 77 Density of Pomacentridae and Labridae Coral Fishes at Biorock Substrate �� 78

Conclusions �� 79 References ��80

BACKGROUND

The Gili Islands in Eastern Indonesia lie in the Coral Triangle, the most biodiverse region of the oceans� Unfortunately, the Gili Islands coral reefs have experienced substantial deterioration and destruction within the last two decades, mostly due to human activities causing marine resources overexploitation, destructive fishing methods like bombs and poisons, coral “heatstroke” from global warming, land-based sewage, global sea-level rise, overfishing, and direct physical damage from boats, anchors, tourists, reef harvesting, and coral diseases, compounded by the absence of appropriate management, poor enforcement capacity, and a lack of environmentally sound alternative sources of livelihood�

As a result, renewable marine resources are declining and endangering local food supplies, shorelines, and tourism income� The Biorock coral rehabilitation method can increase the growth rates of coral� Robbe et al� (2011) report the Biorock reef restoration project on the Gili Islands has been regenerating coral reefs for seven years� They add that measurable success can be clearly seen with regard to fish populations, coral growth and survival rates, ecotourism, education, and the halting of beach erosion�

This chapter explains the results of a study conducted to determine the growth rate of colonies of Acropora formosa and sponge Clathria as well as to measure the density of fishes growing on Biorock structures� This study was conducted in Gili Trawangan, one of the three Gili Islands, in 2010 and 2011�

GILI TRAWANGAN AS A LABORATORY FOR BIOROCK REEF RESTORATION

Several studies to assess the effect of Biorock on coral and sponge growth have been carried out in Gili Trawangan, located in the village of Gili Indah, Pemenang subdistrict of North Lombok district, West Nusa Tenggara, Indonesia� This most developed of the Gili Islands covers 342 ha� To the north and west is the Bali Sea, to the south is Lombok strait, and on the east is Tanjung Sire� Gili Indah lies between 8º 20′–8º 23′ South and 116º 00′–116º 08′ East, and the islands are surrounded by white sand� The depth of the bottom is less than 10 m at 20 m from the shore and is more than 20 m depth at 40 m away from the coast�

Since March 15, 2001, until March 4, 2009, the management of Gili Islands has been under the Natural Resources Conservation Unit or Balai KSDA NTB, Department of Forestry (under Ministry of Forestry decree number: 99/Kpts-II/2001)� On March 4, 2009, the authority of the Gili management was handed over to the Department of Ocean and Fisheries following Ministry decrees BA�01/Menhut-IV/2009 and number BA�108/MEN�KP/III/2009� Based on the Ocean and Fishery Ministry decree number 67/MEN/2009 for selection of national water conservation areas, management authority for Gili Air, Gili Meno, and Gili Trawangan in West Nusa Genggara (September 3, 2009) is under the Director General of Ocean, Marine, and Small Islands (BKKPN)�

The Gili Islands are dependent on a healthy marine habitat for their fisheries, tourism, sand supply, shore protection, and marine biodiversity� Unfortunately, the Gili Islands’ coral reefs have experienced a substantial deterioration and destruction within the last two decades, mostly due to human activities but also including severe mortality from high temperatures in 1998 and from storm damage�

As a result, renewable marine resources are declining and endangering local food supplies, tourism income, and shorelines� Without large-scale restoration of degraded habitats to make them capable of supporting larger fish and shellfish populations, there will be fewer fish in the future, and without healthy growing corals, there will be fewer beaches or tourism income, affecting all businesses and residents on the island�

The Biorock reef restoration project on the Gili Islands has been regenerating coral reefs for seven years� Regular training workshops on Biorock restoration are facilitated by Satgas Gili Eco

Trust in collaboration with Centre for Environmental Studies in Mataram at the University of Mataram, with genuine support from all stakeholders in the Gili Islands� Robbe et al� (2011) reports that a measurable success can be seen with regard to fish populations, coral growth and survival rates, ecotourism, education, and the halting of beach erosion� Therefore, Gili Trawangan can be seen as a laboratory of Biorock reef restoration�

THE SYSTEM AND PRINCIPLE OF THE BIOROCK METHOD

The Biorock method was invented, developed, and patented by the late Prof� Wolf Hilbertz and Dr� Thomas J� Goreau� Biorock technology uses low-voltage direct currents (above 1�2 V) passing through a steel structure (Figure 6�1)� The Biorock electrolysis process occurs between two metals receiving electricity in seawater, causing the steel cathode structure to grow solid limestone minerals and the anode to slowly disintegrate� These currents are safe to humans and all marine organisms� There is no limit in principle to the size or shape of Biorock structures, and they could be grown hundreds of kilometers long if funding allowed� The limestone is the best substrate for hard coral� The Biorock process is used to regenerate coral reefs, repopulate damaged reefs with corals and fish, break wave action, and grow beaches�

The formation of mineral deposits is not a direct oxidation reaction such as electroplating, but is an indirect process, in which mineral deposition occurs as a byproduct of changes in pH around the cathode due to the electrolysis of seawater� Oxygen and chlorine are produced at the anode, while hydrogen forms at the anode and dissolved magnesium and calcium, which are abundant in seawater, precipitate on the cathode (Figure 6�2)� This deposited material is composed largely of calcium carbonate, which is the mineral that forms coral reefs (Goreau and Hilbertz 2005)� Biorock corals grow dense branches, have bright colors, can recover from physical damage up to 20 times faster than natural reefs, and also have a survival rate up to 50 times higher after severe high-temperature bleaching events (Goreau and Hilbertz 2005)�

Hard and soft corals, sponges, tunicates, and bivalves are observed to grow on Biorock materials at extraordinary rates� Fish and lobster population growth in these structures is extraordinary, especially juvenile fish, and depends on the shape of Biorock structures, which can be made to provide

an extraordinary density of hiding places� Biorock reefs have turned severely eroding beaches into 15 m of beach growth in a few years by slowing waves, so that instead of eroding sand at the shore, they deposit it� They have been found stable in category 4 hurricanes and the Asian Tsunami because the open frameworks allow large waves to pass through (Goreau 2010; Goreau and Hilbertz 2005, 2007, 2008)�

Biorock structures are made with metal bars, charged by a low-voltage current above 1�25 V� These structures are installed on the ocean floor, and pieces of corals are attached to them� These corals come from reefs in the neighborhood that had been naturally broken by various causes (unaware divers, strong waves, anchor damage, etc�)�

The electric current, which is totally harmless for any organism, leads to electrolysis, causing a calcareous precipitation on the whole structure� This will not only prevent appearance of rust, which would weaken the structure, but since coral skeleton is made of limestone, the structure will, thanks to this reaction, become the best place for coral to grow�

Thus Biorock technology relies on a very simple principle: enhancement by electrolysis of the natural reactions occurring among coral, seawater, sun, and dissolved minerals�

Biorock technology acts to catalyze the natural reaction and enables coral growth two to six times faster than in usual conditions� Normally coral grows only around centimeters per year, so faster growth is a highly efficient way to restore damaged reefs� Moreover, corals on Biorock structures are more resistant to hazards they face�

Hard corals are not the only ones to grow on Biorock structures, but tunicates, bivalves, sponges, and soft corals also develop faster than average� On a Biorock structure, their survival and resistance rate is 20 to 50 times higher than in natural environment following severe high-temperature bleaching events (Goreau and Hilbertz 2005)�

Finally, because Biorock technology relies on electrical fields, it benefits all corals and ecosystems around the metal structure in a perimeter up to about 10 m� These results by W� Hilbertz and T� Goreau are borne out since the 1980s by the efficiency of Biorock structures installed all around the world� These structures contributed to restoration of damaged coral reefs, enlargement of beaches affected by erosion, and repopulation of marine areas with many species of fishes and other sea organisms�

Since 2004, the Gili Eco Trust has launched the Biorock program around the Gili Islands� There are now nearly 75 Biorock structures around the Gili Islands, which fall into three categories of

structure: (1) Biorock reefs to grow corals and provide new fish habitats, thus creating interesting new dive sites; and (2) Biorock antierosion reefs to grow corals and provide fish nurseries while causing sand to accumulate on the beach, (3) good shallow snorkeling sites, and (4) Biorock wavebreaker structures to stop and reverse the erosion�

Every Biorock reef or structure has been showing positive consequences on fish populations, coral biodiversity, and regenerating eroding beach� It is the best technology ever used to restore our corals reefs, in association with education to avoid further damage� Everyone on the Gili Islands now knows about the Biorock project, and it is a big step forward in ecotourism� We should continue to send out information and expand our existing reefs�

The antierosion reefs are working very well as fish nurseries and by accumulating sand� The wavebreaker reefs in Karma Kayak, Gili Eco Villas, and Kokita have shown incredible results very quickly by stopping the erosion process� The beach directly behind the structures has started to grow (Goreau et al� Chapter 3)� The other Biorock reefs are growing coral faster and making divers happy through the biodiversity of fish and creatures they can observe on the structures and the surrounding area�

METHODS OF BIOROCK STUDY IN GILI TRAWANGAN

Selection of Biorock Structures

The research site for the Biorock study can be seen in Figure 6�3� The three stations for monitoring are located in front of “Tir Na Nog” and “Beach House” restaurants (Table 6�1)�

Biorock structures were designed in various shapes and sizes following the seventh Biorock Reef Restoration Training and Workshop in Gili Trawangan on November 2010� They are made with metal bars, charged by a low-voltage current above 1�2 V� Three different structures were selected in this study, namely a Rack structure, which is located under 3 m depth; a Manta (Pari) structure, which is placed under 5 m depth; and a Dolphin (Lumba-lumba) structure, which is located under 8 m depth� These structures were installed on the ocean floor in the coordinate point as shown in Table 6�1, and pieces of corals are attached to them during the workshop� Control corals were grown at 3, 5, and 8 meters depth�

Selection of Coral Samples

The coral employed in this study was Acropora formosa� This coral is relatively fast-growing and is easy to find in the Gili Islands�

This coral comes from reefs in the neighborhood and was broken by various forces (unaware divers, strong waves)� Various sized fragments were attached to the determined structures� Acropora formosa grown in nature was selected as controls�

Measurements of Coral Growth and Survival Rate

Growth of Acropora formosa and Montipora digitata were measured on the Biorock structures (Figure 6�4), which has mineral accretion treatment, and in natural condition on posts (Figure 6�5) as a control� Three coral samples were identified for observation at each different Biorock structures of 3, 5, and 8 m depth�

For the coral growth of Acropora formosa and Montipora digitata, the length of vertical and horizontal branching were measured using calipers at a given interval using the following formula:

α = −L Lt 0

where

while the coral growth rate was measured using the following formula:

β =

−

L L

T T i i

−

where

Measurements of Limiting Factors of Coral Growth

In this study, some limiting factors of coral growth such as temperature, salinity, current velocity, water transparency, and pH were determined during the measurement of coral growth� The water transparency was measured using Secchi disc, while a dive computer was used to measure the temperature at the study area�

Water salinity was measured by hand refractometer� The current velocity was measured using current drogue� For pH of the surface water in the study area, a pH paper was employed� Good transparency is important for photosynthesis because penetration can reach deep water� Clear water is needed by coral for photosynthesis by Symbiodinium� The intensity of solar radiation at depth is related to water transparency (Supriharyono 2007)� This is crucial for the growth of symbiotic or hermatypic reef-building corals� Without enough radiation, the rate of photosynthesis is reduced, thus the capacity of coral to form the reef from limestone precipitation (CaCO3) is also reduced (Dahuri 2003)� To grow well coral reefs require temperatures ranging around 25°C-30°C (Nontji 2002; Soekarno et al� 1983)� Fluctuation of temperatures will influence feeding behavior of corals� Their feeding behavior tends to decrease in extreme temperatures (Nybakken 1988)� Salinity also influences the growth of corals (Thamrin 2006)� Coral reefs grow well in salinities of 30‰–35‰ (Dahuri 2003)� Corals tolerate water salinities of 27‰–40‰ (Nontji 2005)� Current is important to bring food for the polyp of coral� Currents, water mass movement, and waves are needed for the transport of nutrients, larvae, suspended organic matter and sediment, oxygen, and plankton, and help keep the polyp clean (Nontji 2005)� Higher water velocity is much better than still water for coral growth� Indonesian waters have relatively low salinity and pH due to high rainfall� Coral can grow well at low pH and high nutrients (Sadrun 1999)�

Measurement of Survival Rate

The survival rate of transplanted corals on Biorock and in the controls was determined using the following formula (Sadarun 1999):

S N

N =

 

 

×100%

where

Measurements of Growth and survival of Sponge Clathria

The sponge observed in this study is the genus Clathria, which is a member of the class Demospongia�

Concrete cement blocks were made as a substrate placed at a distance from Biorock structure just a week before transplanting the Clathria (Figure 6�6)� The Biorock structure selected in this sponge growth study was designed and installed during the seventh Biorock workshop/training in 2010�

A Clathria fragment from the same parent colony was transplanted on Biorock structure and on concrete block substrates outside the Biorock (Figure 6�7 and 6�8)� Three sponge fragments were attached in the structure and in concrete block substrate using cable ties with distance of 2, 4, 6, 8, and 10 m from the Biorock structure�

The sponge survival rate was determined by counting the fragments surviving from the start until the last observation around the Biorock and outside the Biorock fortnightly� The first reading was taken immediately after transplanting (T0), which is followed by T1, T2, T3, and T4 reading at 2 week intervals�

Measurement of vertical and basal growth rate of sponge was done using the following formula (Fauziyah 2006):

β = −

−

L L

T T i i

where

Rate of survival of sponges was measured using the following formula (Fauziyah 2006):

S N

N =

 

  ×

where

Observation of Coral Fishes

Coral fish monitoring around Biorock structures was carried out by the underwater visual census method at 3 to 8 m depth� Population density of coral fish was monitored weekly at 08�00 to 11�00 a�m�, along a 5 m distance between the Biorock structure and the non-Biorock structure� Observations were made at 5 m sight distance, with observed area of 2 × 5 m2 for the Biorock Rack

structure at 3 m water depth, 5 × 6 m2 for the Manta structure at 5 m water depth, and 2�5 × 7 m2 for Dolphin structure at 8 m water depth� For coral fish density observation outside the Biorock, a transect line, which has similar area with the observed Biorock structure, was made� Monitoring stations, which consist of soft coral, sand, and coral fragments, are located at 5 m distance from each Biorock structure�

Data recorded include all species and number of fish observed around Biorock and non-Biorock of 2 m distance from right and left of the transect line�

Measurement of Density of Coral Fish

A visual census technique was used to identify the populations of coral fishes living around Biorock habitat, for example, quantifying their density, diversity, uniformity, and dominance index� The population density of coral fishes was determined following Odum (1996):

n i

= ∑ where

Index of Diversity (H′), Index of Uniformity (E), and Index of Dominance (C)

H P Pi i i

′ =

∑ ln 1

H =

′

C Pi

∑ ( )2 1

RESULTS

Growth of Acropora formosa on Biorock Substrate

A study was conducted to measure the growth of Acropora formosa on Biorock structures at different depths, to monitor the growth of coral under natural conditions around Biorock as controls, and to assess some environmental parameters that influence coral growth�

Based on the results, the average vertical growth of coral Acropora formosa was 0�293 cm/week at Biorock and 0�072 cm/week for controls (Figure 6�9)� The average horizontal growth of Acropora formosa was 0�078 cm/week on Biorock and 0�027 cm/week for controls (Figure 6�10)� Therefore, vertical and horizontal growth of Acropora formosa was four times and three times higher than controls, respectively�

The data collected at three different d epths also showed a significantly accelerated growth of Acropora formosa transplanted onto Biorock structures compared to controls� At 3 m, the growth rate of Acropora formosa was significantly faster than at deeper structure position (0�315 cm/week)� The corals grown at 5 m and 8 m depth reached about 0�291 cm/week and 0�071 cm/week vertical growth rate, respectively� The depth tends to reduce the growth rate of Acropora formosa� In this study, the coral survival rate with both Biorock and control treatments was 100%� Thus, the growth rate of coral Acropora formosa at Biorock was two to four times higher than the natural controls�

Growth Rate of Coral Acropora formosa at Three Stations

The growth of coral Acropora formosa vertically at Biorock was four times faster than the growth at control treatment�

This significant difference of coral growth is also supported by Soesilo and Budiman (2006), who stated that the growth of some corals may reach 10 times faster due to mineral accretion�

The average vertical and horizontal increment of Acropora formosa varied somewhat over time� This is possibly because of unstable electrical power supply that affects mineral accretion in the Biorock structures� It is normal in Gili Trawangan that the electrical power operated by the state electrical company (PSN) suffers frequent power outages� In turn this may cause varying growth of Acropora formosa�

Sudden changes in temperature may inhibit growth of corals and their feeding behavior (Nybakken 1992)� Another factor influencing growth is turbidity, which is a function of suspended solid and dissolved organic matter concentrations in the water column� When particle content is high, penetration of solar radiation is reduced, lowering the compensation point for photosynthesis� According to Nontji (2005), solar radiation is needed for photosynthesis of symbiotic algae, which then provide energy to the host coral animal� Photosynthesis of stimulates growth of calcareous skeleton (CaCO3) by corals�

Average Growth Rate of Acropora formosa Groups at Three Different Stations

Average vertical growth by Acropora formosa on the Biorock Rak was 0�315 cm/week, which is 4�257 times (425�7%) greater than the growth of control corals (0�074 cm/week)� At Manta Biorock, the growth rate was 0�291 cm/week, which is 4�157 times faster (415�7%) than controls (0�070 cm/ week)� At Dolphin Biorock, the rate was 0�273 cm/week, which is 3�845 times faster (384�5%) faster than controls (0�071 cm/week)� It is clear that the growth of coral is greater on Biorock (Figure 6�11 and 6�12)�

The higher growth rate of coral on Biorock mineral accretion was achieved because coral metabolism is stimulated due to the formation of calcareous substances deposited on the steel structure by the electrical field� The Biorock influenced the growth of Acropora formosa significantly at different water depth� At 3 m depth, the coral transplanted on Biorock tend to grow faster than at 5 and 8 m depths, suggesting that depth factor contributes to the growth of coral on the Biorock� This is probably because of solar radiation intensity penetrating the water and influencing photosynthesis by the symbiotic algae� Solar radiation is needed by the symbiotic algae for photosynthesis (Nontji 2005; Suharsono 2004)� However, unlike the Biorock condition, there was no significant difference in the control coral growth at three different water depths� This suggests the Biorock process may be increasing photosynthetic efficiency�

With 0�074 cm/week average increase of vertical coral growth in the controls, it is estimated that their height increase will be 3�848 cm per year� According to Romimohtarto and Sri (2007), branching corals (Acropora) may grow as much as 5-10 cm or even more per year� Based on this growth result, it can be concluded that the coral growth in Trawangan is slow due to turbidity influencing solar radiation penetration into seawater�

Survival Rate

The survival rate of all corals with mineral accretion and control corals was 100%� There was no case of coral death during the study� Thornton et al� (2000) state that coral rehabilitation is successful if the survival rate ranges from 50% to 100%�

Environmental conditions affect coral survival rates� Fluctuating currents and good water transparency are among factors influencing the development and growth of coral reefs (Bengen 2001)�

Growth Rate of Acropora formosa and Montipora digitata

A comparison study was conducted to determine the growth rates of Acropora formosa and Montipora digitata growing on Biorock substrate and further away (up to 10 m) from a larger Biorock structure in Gili Trawangan� Data collection for the coral growth was taken by direct measurements (Supriharyono 2007) using a caliper� The reading was taken every two weeks on Acropora formosa and Montipora digitata attached on Biorock structure and placed on concrete blocks (Figure 6�2) away from the same Biorock structure (up to 10 m distance)�

Acropora formosa on Biorock substrate showed the greatest increase in growth, 3�42 cm within 8 weeks, while Acropora formosa away from Biorock grew only 0�78 cm� Montipora digitata on the substrate Biorock grew 2�40 cm, while Montipora digitata away from Biorock grew only 0�72 cm (Figure 6�13)� The result of t-test analysis showed significant differences between coral growth on the Biorock structure and away from it� The height of Acropora formosa colonies transplanted on Biorock structure (0�43 cm/week) is four times higher than the growth away from the Biorock structure (0�09 cm/week)� Growth of Montipora digitata colonies on Biorock was 0�3009 cm/week, which is three times faster than the growth away from Biorock (0�009 cm/week)�

The growth of Acropora formosa on Biorock was 4�39 times faster than the growth away from Biorock� This is consistent with research conducted earlier where the vertical growth of Acropora formosa on Biorock was up to four times faster than the growth of control corals� The growth rate of Montipora digitata on Biorock was 3�34 times faster than Montipora digitata controls away from the Biorock substrate�

The growth rate of each species is significantly different depending on species and distance from the Biorock structure, as well as varying over time (Table 6�2)�

Growth of Acropora formosa and Montipora digitata on Biorock substrate is three to seven times faster than the growth on the blocks away from Biorock structure, with Acropora formosa growing faster than Montipora digitata on Biorock, although control rates were similar�

A similar study conducted by Murtawan in 2011 found a significantly higher growth rate of Acropora formosa on Biorock than nearby� The growth rate of Acropora formosa was measured by colony height and colony diameter� The height increase rate of Acropora formosa at the Biorock, north of Biorock, and south of Biorock was 0�57, 0�29, and 0�23 cm/week, respectively, whereas the diameter increase rate was 0�14, 0�48, and 0�24 cm/week� The correlation of colony height increase on the north and south near Biorock is −0�80 and −0�58, respectively, which means that the farther

corals are transplanted from Biorock, the slower they grow� On the diameter growth rate, the correlation degree of northern block is −0�47, while the southern block has −0�46� Based on a graphic analysis of Acropora formosa growth rate, Biorock structure can affect the growth of Acropora formosa that has been transplanted up to 10 m from Biorock�

Environmental Condition of the Study Area

Physical and chemical parameters assessed during this study were geomorphology, substrate condition, water transparency (Figure 6�14), temperature (Figure 6�15), salinity (Figure 6�16), current velocity (Figure 6�17), and pH (Figure 6�18)� Geomorphologically, the study site is a slope, 8 m

deep� The substrate is slightly sandy, which may influence coral and sponge growth� All parameters were considered as being in the healthy range for corals and sponges�

The location of this study has good transparency� The temperature fluctuated slightly at each reading, ranging from 27°C to 29°C� Change in temperature due to weather can be seen in Figure 6�19� The current velocity in the study area also varied at the time of measurements� Water current was between 0�061 and 0�224 m/s� It had a similar pattern with the temperature and rate of sponge growth: when the current velocity was high, the growth of sponge was also high� The pH of water during study is on the low side for growth of coral, but the precision of the measurement was only 1 pH unit�

The physiology of sponges is influenced by water flow, which may carry oxygen, nutrients, food, and metabolic wastes around their body wall (Hickman et al� 2001)� In high-current speed, the water flow containing oxygen and food is increased, speeding up the physiological growth process� When the current speed decreased, then the supply of oxygen and food through water flow also decreases, causing slower growth� The salinity and acidity of the environment were favorable for sponge growth�

Growth of Clathria Sponge on Biorock Structure

A study to observe the effect of Biorock on development and growth of Clathria transplanted inside and outside the Biorock structure was conducted in Gili Trawangan� Based on the results, the growth of Clathria sponges transplanted on Biorock was faster than those sponges transplanted outside the Biorock structures, which were on north and south blocks� But this was not the case for basal growth of sponges� The sponge tends to grow vertically, increasing length and branch

formation� Correlation analysis indicates that Clathria grew slower when they were transplanted farther from the Biorock structure� The survival rate of Clathria transplanted inside and outside Biorock was 100%�

Growth of Transplanted Sponge Fragments

The results showed that the growth rate of branching sponges two weeks after transplantation on Biorock was 2�55 cm and was the highest measured in the north block 1 followed by blocks 3, 4, 5, and block 2 (Figure 6�20)� The same result was found in the south block� Growth rates were slower for sponges away from the Biorock� The slow growth of sponge at block 2 was probably caused by the sedimentation on the block due to diving activities around the Biorock� Sandy substrate on block 2 was monitored during the assessment� This sediment may inhibit the growth of sponge as it covers the surface, thus the ostium of sponge is closed, which may prevent the water flow that transports nutrients and oxygen required by the sponges�

Statistical analysis shows that the growth of sponge on the Biorock structure was significantly different than those sponges growing away from Biorock structure on both north and south blocks�

The correlation analysis gives a correlation coefficient value of −0�60� This means that the distance factor influenced the length growth� The closer the sponge from the Biorock, the faster the growth�

Rate of Basal Growth of Transplanted Sponge Fragments

In general, the basal growth rate of sponge over the substrate is low both inside and outside Biorock� This is normal, as sponge Clathria tends to branch vertically�

Figure 6�21 shows the rate of basal (encrusting) growth of sponges two weeks after transplantation inside and outside the Biorock structure� The distance factor influenced the circle growth rate of sponge� The growth of sponge on the Biorock is faster than any sponge grown in the south block, except for block 5� In the north block, the sponges transplanted outside the Biorock structure grew faster than the transplanted sponge inside the Biorock�

However, statistically there was no significant difference between the basal growth of sponge inside and outside Biorock� The correlation between distance factor and basal growth of sponge is positive, meaning that the farther the sponge from the Biorock, the higher the basal growth rate (0�16)� This small coefficient value suggests that there is little influence of distance factor on the basal growth rate of sponge�

Rate of Survival of Transplanted Sponge

The survival rate of sponge fragments transplanted inside and outside Biorock structure was 100%� The sponges transplanted inside Biorock not only had high growth rate but also had high acclimatory ability� Furthermore, the sponges transplanted inside Biorock had no disease attack during the study, while the sponges transplanted outside Biorock were vulnerable to disease and predator attack (Figure 6�22)�

It is clear that the Biorock influenced the development and growth of Clathria� The vertical growth of Clathria transplanted on the Biorock structure was significantly greater than the vertical growth outside the Biorock structure, but the basal growth was not�

Density of Pomacentridae and Labridae Coral Fishes at Biorock Substrate

This study identified the composition, diversity, density, uniformity, and dominance of coral fishes of Pomacentridae and Labridae family around Biorock structures and determined the effect of Biorock on their population densities by direct monitoring in the field (Figures 6�23 and 6�24)�

The Pomacentridae and Labridae around Biorock in Gili Trawangan consisted of 18 genera and 35 species� The highest density of Pomacentridae was represented by Dasyllus aruanus, while the Labridae was represented by Thallashoma lunare� Ornamental fishes of the genera Amphiprion, Chrysiptera, and Pseudocheilinus had low density�

The study observed that there was a significant difference in biological indices around the Biorock structure (Graph 4)� Index of density, diversity, and dominance index around the Biorock structure were 1�54, 0�53, and 0�10, respectively� Whereas, in the control station the values were 0�97, 0�43, and 0�41, respectively� Hence, community structure at the Biorock is better than the community structure outside Biorock�

The result showed that Biorock was very effective in improving the population of coral fishes� The results indicate that the density of coral fishes found around the Biorock structure was six times greater than the density of coral fishes found outside the Biorock structure�

CONCLUSIONS

The results of this study showed that the growth rate of Acropora formosa colonies transplanted on Biorock structure were four times higher than the growth away from the Biorock structure� Acropora formosa grew faster than Montipora digitata on Biorock, although control rates were similar� Growth of Montipora digitata colonies on Biorock was three times faster than Montipora digitata controls away from Biorock substrate�

The growth of Acropora formosa around Biorock was significantly greater than the growth at 5 and 8 m depths, which is possibly because of solar radiation intensity penetrating the water and influencing photosynthesis by symbiotic algae�

Based on the results, the vertical growth of Clathria, a sponge transplanted on Biorock, was faster than those sponges transplanted outside the Biorock structures� However, this was not the case for basal growth of sponges, which is normal, as the sponge tends to grow vertically, increasing length and branch formation�

This study showed that Biorock was very effective to improve the population of coral fishes� The result indicates that the density of coral fishes found around Biorock structure was six times greater than the density outside the structure� This study also indicates that the survival rate of all corals and sponges with mineral accretion and control corals was 100%�

This regular community-based Biorock reef restoration workshop/training should be continued to enable the Gili Island’s stakeholders to share their opinions and vision toward sustainable ecotourism� It is clear that the Biorock structures installed in Gili Islands have contributed to the restoration of damaged coral reefs and repopulation of marine areas with many species of fishes and other sea organisms�

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