Coral reef rehabilitation through transplantation of staghorn corals: effects of artificial stabilization and mechanical damages
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- Lindahl, U. Coral Reefs (2003) 22: 217. doi:10.1007/s00338-003-0305-6
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In order to develop and test a low-cost method of coral reef rehabilitation, the staghorn corals Acropora muricata and A. vaughani were transplanted to a shallow site with unstable substrate. To avoid abrasion, dislodgement and transport due to water movement, the transplanted corals were tied to string sections, which were connected at the seabed to form a grid. This created stability and improved the survival of the corals. The average increase in weight of live coral over 1 year was 56%, eight times more than the control treatment with unattached coral branches. This difference was mainly due to a reduced partial mortality among smaller coral fragments in the stabilized treatment. Survival was positively related to initial size among the loosely placed coral branches, whereas the attached treatment showed a negative relation between size and relative increase in weight of the surviving parts of the coral branches. Coral fragments were not significantly affected by severe physical damage simulating the effects of handling.
KeywordsCoral reef rehabilitation Coral transplantation Coral growth Acropora muricata Acropora vaughani
Numerous experiments on coral transplantation have been carried out, aimed at assessing the feasibility of various methods of reef rehabilitation. Provided that the attachment is sufficient and that environmental factors, such as substrate and water quality, are favorable, a wide variety of coral species has been shown to survive transplantation well (Maragos 1974; Birkeland et al. 1979; Harriott and Fisk 1988a; Hudson and Diaz 1988; Guzman 1991; Kaly 1995; Berker and Mueller 1999; Tomlinson and Pratt 1999; Hudson 2000). Poor survival or loss of transplanted corals can often be explained by factors such as wave-induced dislodgement (Birkeland et al. 1979; Auberson 1982; Plucer-Rosario and Randall 1987; Harriot and Fisk 1988a; Newman and Chuan 1994; Clark and Edwards 1995; Bowden-Kerby 1997), human disturbances (Auberson 1982), burial and smothering by loose substrate (Auberson 1982; Harriott and Fisk 1988a; Bowden-Kerby 1997; Nagelkerken et al. 2000), or elevated temperatures (Yap and Gomez 1984; Yap et al. 1992; Lindahl et al. 2001). Coral reef rehabilitation is controversial, mainly because of the high costs involved, the damages to source populations, and the questionable long-term survival of the transplanted corals (Harriott and Fisk 1988b; Edwards and Clark 1999; Spurgeon and Lindahl 2000). One of the most difficult problems is to prevent dislodgement of the transplanted corals due to water movement. Different attachment methods can be used, depending on the substrate, wave exposure, and growth form of the coral.
Corals killed by natural or anthropogenic disturbances are often degraded to rubble (Carpenter and Alcala 1977; Alcala and Gomez 1987; Sano et al. 1987; Blanchon et al. 1997). This substrate is often inhospitable for natural re-colonization by corals (Alcala and Gomez 1979; Brown and Dunne 1988; Riegl and Luke 1998; Fox et al. 2000) and is therefore of primary interest for studies of reef rehabilitation. Corals transplanted to areas with unconsolidated sediment and exposure to water movements can be secured to artificial substrates (Maragos 1974; Schumacher and Schillak 1994; Clark and Edwards 1995). However, these structures are probably too expensive for a wider application in developing countries (Edwards and Clark 1999; Spurgeon and Lindahl 2000).
Methods for transplanting unattached fragments of staghorn corals (Acropora spp) to shallow backreef areas have been developed in order to keep the costs low (Bowden-Kerby 1997; Lindahl 1998). These methods are quick and do not require SCUBA divers or expensive materials and equipment. In sufficiently protected habitats, survival of unattached fragments can be nearly 100% (Bowden-Kerby 1997). However, water movements can cause severe mortality. When growing naturally, staghorn corals are mainly restricted to protected or moderately exposed habitats. Dense monoclonal thickets of staghorn corals, formed through vegetative reproduction and fragmentation, can often dominate the coral community in shallow back-reef areas. Individual colonies often gain stability and support by direct contact with surrounding colonies rather than by attachment to the substrate, and fusions between adjacent branches are common (Gilmore and Hall 1976; Highsmith 1982). This way of growth enables a thicket of staghorn corals to encroach over loose substrate, where the recruitment and growth of most other corals is prevented (Tunnicliffe 1981).
The aim of this study was to test a new low-cost method of artificial stabilization of staghorn coral fragments transplanted to loose substrate. In addition, the effects of mechanical damages on transplanted coral fragments were assessed.
Materials and methods
The study site
The study was carried out at Tutia Reef near Mafia Island, Tanzania (8°08′S, 39°40′E). The experimental site has a depth of 3 m and is situated in a sound between two reefs, approximately 100 m north of the crest of Tutia Reef. Oceanic waves entering the sound from the east are somewhat reduced before reaching the study site. Therefore, the site is never exposed to surf or breaking waves. The seabed is covered with a mix of coral rubble, sand, and rhodoliths. Tutia Reef is relatively unaffected by human disturbance, although some blast fishing and seine netting have occurred. The rubble fields are probably of natural origin and in the process of being colonized by adjacent thickets of branching corals (Acropora spp). Staghorn corals and other branching corals in the genus Acropora, of which approximately 80% were killed during the 1998 ENSO event, dominate the surrounding coral community.
Artificial stabilization of transplanted corals
"Partial mortality": the estimated proportion (by weight) of the parts of each branch that were regarded as dead, as defined above. Totally dead branches were regarded as having 100% partial mortality.
"Relative weight of whole coral": final total weight of each branch divided by its initial weight. Total weight includes all parts (dead and live) of the branch.
"Relative weight of surviving parts": the weight of the proportion that was regarded as alive on each branch divided by the initial weight of the branch.
Initial weight =100 g, final total weight = 150 g
"Partial mortality" = 20% (visually estimated)
"Relative weight of whole coral" = 150/100=1.5
"Relative weight of surviving parts" = 0.8×150/100=1.2
Fragments of A. muricata with an average weight of 83±27 g (SD, n=216) were collected from 12 different thickets, each assumed to be a distinct clone. The 18 fragments collected from each clone were randomly distributed to two treatments: "damage" and control (each treatment with 108 branches). Thus, for each of the 12 clones, nine branches were allocated to each of the two treatments. The damage treatment simulated the kind of injuries that occur during handling and transport of collected fragments, and involved forceful scraping with a knife twice along all main branches and trimming of all branch tips by 2 cm. The scraping removed the protruding corallites and soft tissue, creating a 3- to 5-mm-wide and 1-mm-deep scar. All branches were attached in vertical positions to a PVC-rack at 3 m depth, using cable ties. The weight of each branch was recorded immediately after the treatment and again after 8 months. Also partial mortality was estimated as in the experiment on artificial stabilization.
The results of the two experiments were analyzed with ANOVA, using the program Statistica (StatSoft Inc.). A separate ANOVA was done on each of the variables "partial mortality," "relative weight of whole coral," and "relative weight of surviving parts." The data were subjected to Cochran's test of homogeneity of variances prior to analysis. In order to get a balanced number of replicates, one randomly selected clone of A. muricata was excluded in the experiment on artificial stabilization. The average from all 10 branches from each string section was considered as one replicate, giving n=2. The unattached branches were randomly pooled into two groups per clone, and the average for each group was regarded as one replicate. Species was regarded as a fixed factor and the random factor clone was nested under species. The factors species and treatment were orthogonal. The partial mortality and relative change in weight of surviving parts on each of the transplanted branches were regressed on the initial weights of the branches. Regressions were carried out on each combination of species and treatment (attached or unattached) as well as on the two species pooled in each treatment. In order to facilitate comparison between the branch weights used in this paper and the more commonly presented type of results based on branch length, weight was regressed on length, yielding a polynomial curve for the relation between the two measurements.
ANOVA table of the effects of treatment, species, and clones on partial mortality and relative weight of whole corals and surviving parts in the experiment on artificial stabilization. Asterisks indicate p levels below 0.05
Clone (in species)
Relative weight of whole coral
Clone (in species)
Relative weight of surviving parts
Clone (in species)
Effects of initial weight
The tested method of artificial stabilization is intended to protect the transplanted fragments from two types of mortality: breakage and loss of live tissue. In a long-term field study such as this, growth and different kinds of mortality are difficult to measure separately, because these factors have a combined effect on the relative weight of surviving parts at the end of the experiment. However, this measure gives a good indication of the effectiveness of the stabilization method. The partial mortality found on the upper surfaces of the unattached coral branches was probably caused by contact with the sediment, indicating that they had been overturned by water movements during the course of the experiment. This significant mortality of coral branches close to an area where staghorn corals were proliferating naturally in dense thickets demonstrates the vulnerability of small, unattached corals. The study showed that staghorn coral branches could be successfully transplanted to a moderately wave-exposed habitat by using the "string-grid" method. The attachment to strings did not fix the corals entirely, but helped to maintain their orientation. Contacts between adjacent branches also prevented movement and frequently resulted in fusion, which added extra stability. The attachment resulted in a net increase in weight of surviving parts eight times greater than for the unattached corals. This is probably an effect of reduced partial mortality among the smallest, fast-growing fragments in the attached treatment.
Apart from the effect of burial and abrasion, the corals attached to strings were probably severely affected by competition between adjacent branches. Lindahl (1998) found that sparsely transplanted A. muricata had a greater growth rate than those that were placed in close proximity, indicating an adverse effect of intra-specific competition for light and space. Hence, there is a trade-off between the stability provided in dense populations and the increased availability of light and water circulation among more sparsely growing corals. The average increase in weight of surviving parts among the branches placed in racks (damage experiment) was 157% over 8 months. This would correspond to an annual increase considerably greater than the 82% that was found for branches of similar size among the corals tied to strings. Hence, it is clear that the placement on the seabed disturbed the corals even when they were attached to strings. A defensive response can be triggered through the contact of two colonies of different genetic origin (Hildemann et al. 1975). In order to avoid aggressive reactions, which may result in reduced rates of growth and reproduction (Rinkevich and Loya 1985), coral branches attached to the same string section should optimally originate from the same clone.
The combined weight of the corals attached to the string-grid was sufficient to anchor the structure and prevent deformation of the grid, without attachment to the seabed. Further experiments are necessary to assess the usefulness of this method on sites with stronger water movements, different substrate types or more steeply sloping seabed. It is important to note that this kind of attachment can only provide a temporary stabilization, since the strings will degrade with time. Even intact strings would be of less use after some time, as the transplanted corals grow and start to spread over the seabed. Since the natural proliferation of artificially created thickets is essential for the usefulness of the method, it should only be used in areas where thickets of staghorn corals are known to be able to grow naturally.
From the present results, it appears that transplanted corals in a wide range of fragment sizes will survive and grow well. Two effects of the initial size of transplanted coral branches remained when outliers had been removed from the regressions. First, larger initial size reduced the mortality among branches placed loosely on the seabed. Second, branches secured by strings or placed in racks showed a negative relation between relative weight of surviving parts and initial size. The low r2 values in the regressions indicate that several factors other than initial size influenced the survival and weight increase of the corals. These factors may include differences in growth form and genetic composition as well as external factors such as predation, overgrowth, diseases, and sediment interaction. Several studies have reported a positive correlation between fragment size and survivorship in corals (Highsmith et al. 1980; Hughes and Jackson 1985; Liddle and Kay 1987; Harriott and Fisk 1988a; Knowlton et al. 1988; Bowden-Kerby 1997; Smith and Hughes 1999), whereas others have not found such a correlation (Rogers et al. 1982; Lewis 1991; Bruno 1998). Most studies have not taken into account partial mortality. Had this not been done in the present study, no size-dependence would have been found, since very few corals were entirely dead. The positive relation between size and survival in the present study may be related to larger reserves and an increased capacity to repair damages (Connell 1973) or to increased resistance to water movements and ability to overgrow competitors. The greater relative growth rate for smaller fragments was probably related to a greater surface to volume ratio in smaller corals. This effect may have been present also among the unattached branches, but was probably overshadowed by the greater partial mortality among smaller branches. The relation between initial branch length and weight (Fig. 4) shows that the proportions of the transplanted branches were different depending on their length, since proportionality would have resulted in a linear relationship between the weight and the cube of the linear dimension (Maragos 1978). It is important to note that the branches used were fragments of colonies, and thus not representative of the natural growth form of the species.
The experiment on mechanical damage showed a more than five-fold difference in growth rate between some of the clones of A. muricata. Similarly, Rinkevich (2000) found a significant difference in growth rate between clones of Stylophora pistillata in the Red Sea. The magnitude of the variation in the present study was larger than expected and has important implications for coral reef rehabilitation. Clark and Edwards (1995) suggested that some corals may suffer from mortality and reduced growth as a consequence of the transplantation procedure. However, the present study did not show a significant effect of mechanical damage, even though the inflicted injuries were more severe than can be expected to occur during collection and transport. The capacity to repair damages is highly variable among coral species (Hall 1997), and the ability of A. muricata and other staghorn corals to withstand injuries is probably related to their strategy for reproduction through fragmentation (Tunnicliffe 1981; Kobayashi 1984).
The string-grid method is extremely simple, and could easily be carried out by snorkelers down to a depth of 5-10 m after some basic training. The method is therefore well suited for community participation projects in developing countries, involving the local artisanal fishermen and other reef users. However, damages to source populations, recruitment limitation, and other ecological circumstances must be carefully considered by experts. Using artificial attachment allows transplantation of smaller fragments and results in greater rates of live-weight increase than would be possible with loosely placed branches in exposed conditions. There is no reason to assume that the method used in this study was optimal under the present circumstances. Therefore, future experimentation and new inventions are likely to produce better results. The string-grid method can be modified by altering the size and spacing of the corals and of the string sections, as well as by using other species or fast-growing clones. More research is also needed to assess the general applicability of the method in a wide range of habitats and to study effects of coral collection on source populations. Although transplanted staghorn corals have been shown to enhance fish abundance and diversity (Lindahl et al. 2001), it is important to bear in mind that transplantation of a few species of corals can never entirely replace a degraded coral reef ecosystem, and reef rehabilitation should never be viewed as an alternative to preserving existing coral reefs.
This study was carried out in co-operation with the Institute of Marine Sciences, Tanzania. I would also like to thank D. DeVilliers, K. Gallop, J. Greupner, H. Trattner, and M. Willson for assistance in the field. Dr. K. Johannesson, O. Lindén, J.-O. Strömberg, and anonymous referees gave valuable comments on the manuscript. Funding was provided by the Swedish International Development Co-operation Agency (Sida) through the Sarec Marine Science Program and by Göteborg University.