Experimental assessment of coral reef rehabilitation following blast fishing

Summary

Study 1

Throughout the coral reefs of South-east Asia there has been extensive coral damage due to illegal fishing with explosives, termed blast or dynamite fishing. An explosive device is thrown into the water above a reef. The explosion kills or stuns fish, which float to the waters surface, but it also shatters coral skeletons on the reef below. As a further problem, areas of broken coral shift in the water currents abrading or covering new coral colonies, which slows or prevents reef recovery. In this study, the effect of three cost effective reef rehabilitation methods: rock piles, cement slabs and netting pinned to rubble, on coral colonisation was investigated.

Study site: The study was undertaken in the Komodo National Park around several larger and smaller islands in the Nusa Tenggara archipelago of Indonesia.

Restoration plots and  treatments: Nine blast sites were selected, within which 1 x 1 m treatment and control plots were established. At each site, two to four replicate plots of each treatment were created using locally and readily available materials:

1) Rock piles – rocks of 20-30 cm diameter were used to make a 20-40 cm high pile on top of the rubble.

2) Cement slabs – slabs were pinned to remnant coral rubble.

3) Netting – wide-mesh fishing net (5 cm mesh) was attached to remnant coral rubble with U-shaped re-bar pins.

Additionally, four untreated coral rubble control plots were created. Plots were created at three sites in March and April 1998, and at six sites in October and November 1998.

Monitoring: Sites were measured every six months until Spring 2001. The location, size, life-form, and taxon of all hard corals colonising the plots were recorded. Additionally, cover of soft coral and other dominant benthos was estimated.

Coral growth: During the first three years, the rock stabilisation plots had the highest hard coral recruitment and cover, followed by cement slabs and netting. After two and a half years, some coral colonies were 20-30 cm in diameter on rock piles and cement slabs. In contrast, in the untreated coral rubble control plots the number and area covered by hard coral did not increase over the study period.

Treatment degradation: Over the course of the experiment many treatment plots became degraded: many netting plots were scoured or buried by shifting rubble, some rock piles were scattered and cement slabs turned over by the current. All treatments had at least some growth of soft corals.

Conclusions: The results suggest that addition of rock piles to blast damaged reefs provides the best substrate for colonisation by hard coral growth. However, the small size of rock piles in this experiment meant that currents degraded the rock piles.

 

Study 2

Throughout South-east Asia there has been extensive damage to coral reefs due to illegal fishing with explosives, termed blast or dynamite fishing. An explosive device is thrown into the water above a reef. The explosion kills or stuns fish, which float to the waters surface, but it also shatters coral skeletons on the reef below. As a further problem, areas of broken coral shift in the water currents abrading or covering new coral colonies, which slows or prevents reef recovery. In this study, the effect of reef rehabilitation by creating rock piles on coral colonisation was investigated.

Study site: The study was undertaken in the Komodo National Park around several larger and smaller islands in the Nusa Tenggara archipelago of Indonesia. Nine blast sites were selected, and three to four replicate rock piles were installed within each 10 x 10 m site in April and May 2000. Rocks were of limestone and lithic sandstone quarried from the island of Flores.

At each site, rocks were thrown overboard from a small boat, and scuba divers positioned and consolidated them to form 3-4 rock piles (0.5-2.0 m³ volume, 70-90 cm high, spaced 2-4 m apart). Control coral rubble quadrats (using remnant coral from the surroundings) were also created at this time.

Coral monitoring: Sites were measured every six months after installation until May 2002 and again in March 2003. The number, size, life-form and taxon of all hard (scleractinian) corals were recorded in six 1 x 1 m quadrats per site (1-3 quadrats per rock pile). Additionally, cover of soft coral and other dominant benthos was estimated.

Rock pile monitoring: To measure the persistence of rock piles, the size of each pile (length, width, height and circumference) was measured every six months after installation until autumn 2001, and again in March 2003.

Hard coral colonisation: The rock piles rapidly developed a ‘biofilm’ of coralline algae and other encrusting organisms. Hard coral recruits were recorded in the first survey. Within one year there were many recruits of 2-4 cm diameter in size. Hard coral recruits continued to increase at most sites until 18 months after installation of the rock piles, after which the number stopped increasing and in some cases, started to decrease. The hard coral recruits were mainly branching corals, dominated by the family Pocilloporidae and the genus Acropora, with fewer velvet corals Montipora, stony coral Poritidae, and other large corals.

Hard coral area: Despite the decrease in coral numbers towards the end of the study, the total area of coral continued to increase, reaching its highest in spring 2003. Area increased by 464% from spring 2001 until autumn 2001, by 77% from autumn 2001 to spring 2002, and by 216% from spring 2002 to spring 2003. In contrast, there was no increase in coral cover in rubble control plots over this time period.

Soft coral and other recruits: Soft coral (primarily Xenia spp.) colonised and grew very quickly at some sites, although there was relationship between the amount of soft and hard corals. Other sessile organisms also colonised the rock piles, including: algae; sponges; jellyfish (tunicates); echinoderms - sea lilies; (crinoids), sea urchins (echinoids), and sea cucumbers (holothurians); Trochus snails; day octopus Octopus cyanea, and various fish species.

High current sites: At the highest current sites, there was a decrease in hard coral area in the 2002 and 2003 surveys.

Rock piles: Volume of rock piles did not decrease significantly with time. However, there was a decrease in rock pile volume at the highest current sites.

Conclusions: Rock piles were successful in aiding hard coral growth and therefore reef rehabilitation at most sites, except those where there were strong currents.


Study 3

Throughout the coral reefs of South-east Asia there has been extensive coral damage due to illegal fishing with explosives, termed blast or dynamite fishing. An explosive device is thrown into the water above a reef. The explosion kills or stuns fish, which float to the waters surface, but it also shatters coral skeletons on the reef below. As a further problem, areas of broken coral shift in the water currents abrading or covering new coral colonies, which slows or prevents reef recovery. In this study, the effectiveness of four rock pile designs on coral reef rehabilitation was investigated.

Study site: The study was undertaken in the Komodo National Park around several larger and smaller islands in the Nusa Tenggara archipelago of Indonesia.

Four blast sites were selected, and one replicate of each of four rock pile designs (each 140 m³ in volume) was installed at every site from March to September 2002. At each site there was also a coral rubble control. Rock pile designs were as follows:

1) Complete coverage of rocks built ~75 cm high.

2) Rock piles of 1-2 m³ spaced every 2-3 m.

3) 'Spur and groove' morphology, with ridges and valleys, parallel to the prevailing current.

4) Spur and groove morphology, perpendicular to the prevailing current.

Monitoring: Sites were monitored in March 2003. The area covered by the treatments was measured. Using a video recorded, transects were filmed to ascertain coral cover on each treatment and the control. Also, coral recruitment (number, size, life-form and taxon of all hard (scleractinian) corals) was surveyed in six 1 x 1 m quadrats. Only one treatment was surveyed per site, and a treatment was not repeat surveyed at another site. Finally, a stationary video ‘point-counts’ were used to assess fish populations at each treatment and control plot.

Hard coral recruits: Hard coral recruits quickly settled on rock piles, and after 1 year there was a average of 7.3 recruits/m² and a average coral size of 7.5 cm² across all sites.

Fish populations: There was a higher number and diversity of fish on rock treatments than the coral rubble controls. Taxa present included: grouper (Serranidae), anthias (Anthiinae), damselfish and chromis (Pomacentridae), surgeonfish (Acanthuridae), parrotfish (Scaridae), stonefish (Scorpaenidae), fusilirs (Caesionidae), and moorish idols Zanclus cornutus.

Conclusions: Rock piles were successful in aiding hard coral growth and therefore reef rehabilitation. Furthermore, rock piles had a positive effect on fish populations, with far more present at rock piles than control sites. However, there was no clear differences between the rock pile designs in this experiment, perhaps due to the short length of post-rock pile creation monitoring. 

Note: If using or referring to this published study please read and quote the original paper, this is available at: http://www.blackwellpublishing.com/journal.asp?ref=0888-8892