LOW-TECH CORAL REEF RESTORATION ...

BULLETIN OF MARINE SCIENCE, 69(2): 915–931, 2001

LOW-TECH CORAL REEF RESTORATION METHODS MODELED AFTER NATURAL FRAGMENTATION PROCESSES Austin Bowden-Kerby ABSTRACT Low-cost and environmentally sustainable coral reef restoration methods modeled after natural coral reef recovery processes and appropriate for use in developing countries were investigated. The study focused on post-fragmentation processes important to natural coral reef recovery and to successful transplantation, quantifying size-specific, substrate-specific, site-specific, and species-specific survival, growth, and self-attachment of coral fragments. Acropora cervicornis and A. prolifera, with distinct morphotypes from high and low energy environments were used for all experiments. Coral fragments of similar size from axial and basal regions were also tested to determine if senescence affects survival. Results indicate that the mortality and growth of unattached coral fragments are strongly size and substrate dependent, with insignificant differences between morphotypes, species, and sites. Axial fragments had significantly lower mortality than did inner/older fragments. Back reef and reef front morphotypes of A. cervicornis grown together in the sheltered back reef for 1 yr continued to differ significantly in branch diameter, relative growth, and self-attachment ability, indicating a genetic basis to morphology and adaptation to specific reef environments. Scattering coral fragments onto unstable rubble or attaching fragments to simple frames on sand proved effective for restoring coral cover to substrates where natural larval-based recruitment processes are inhibited.

The fact that coral reefs are declining due to numerous anthropogenic threats is wellestablished (Dight and Scherl, 1997; Gomez, 1997; Jackson, 1997; McManus, 1997). Direct human intervention may be necessary to restore degraded reef habitats (Pratt, 1994), and coral reef restoration is beginning to be viewed as requiring research on a ‘monumental scale’ (NRC report, 1995 cited in Jackson, 1997). The restoration emphasis to date has focused on the priorities of the developed world, reviewed by Harriott and fisk (1988a) and Lindahl (1998). These methods require long hours underwater reattaching broken corals and stabilizing substrates. Low-tech restoration methods addressing the types of reef damage most common in the developing world, and appropriate for use by rural communities, have not yet received much attention (Bowden-Kerby, 1997; Lindahl, 1998). Rural fishing communities are primary users of coral reefs on a global scale, and as such are major forces of coral reef degradation (Gomez, 1997; McManus, 1997). Destructive fishing often lowers coral cover, and coral cover is an essential habitat for reef fish, directly related to fish abundance (Bell and Galzin, 1984; Sale, 1991). Overfishing may inhibit the natural recovery of degraded reefs, as lack of herbivorous reef fish can cause an increase in algal cover, inhibiting the recruitment of coral larvae (Birkeland, 1988; Wittenberg and Hunte, 1992; Hughes, 1994; Szmant, 1997). Problems such as dynamite and destructive fishing, coral harvesting, reef flat dredging, and severe storms destroy and crush corals, often leaving behind extensive areas of unstable and often silty rubble unfavorable for coral recruitment and reef recovery (Alcala and Gomez, 1979; Brown and Dunne, 1988). Even though much of this reef damage is in lower energy lagoonal situations, very little of the previous restoration work has been attempted in such depositional environments dominated by unconsolidated rubble and 915

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BULLETIN OF MARINE SCIENCE, VOL. 69, NO. 2, 2001

sand (Harriott and fisk, 1988b; Marigos, 1992; Bowden-Kerby, 1997; Lindahl, 1998). Recent work (Bowden-Kerby, 1997; Lindahl, 1998) has shown that transplanting coral fragments to unstable lower energy areas preludes the necessity of substrate stabilization or securing transplants with cement, lowering cost and effort considerably. These inexpensive and simple “Johnny coral seed” restoration methods are based on mimicking natural coral fragmentation and transport processes that lead to coral reef recovery and reef development. Coral fragmentation is important to the basic ecological structuring of coral reefs, and fragmentation has been incorporated into the life history of branching corals (Highsmith, 1982; Wallace, 1985). To a large extent coral fragmentation determines the local abundance and distribution patterns of branching coral species (Tunnicliffe, 1981; Highsmith, 1982; Riegl and Riegl, 1996). The recovery of coral reefs subsequent to destruction is largely dependent on the asexual fragmentation process (reviewed by Highsmith 1982). Indeed, the recovery of coral reefs from disturbances such as hurricanes appears to be more related to the survival of coral fragments than to the settlement of coral larvae (Edmunds and Whitman 1991; Shinn 1976). Storms have even been called ‘major reproductive events’ for corals, provided the storm is not so severe as to kill the resulting fragments (Highsmith, 1982). Coral reef recovery may occur in as little as 5–10 yrs where numerous corals and coral fragments survive (Endean, 1973; Shinn, 1976; Highsmith et al, 1980; Highsmith, 1982), but recovery may take as long as 20–50 yrs when few coral fragments survive (Randall, 1973; Grigg and Maragos, 1974; Stoddart, 1974; Pearson, 1981; Huges, 1994; Dulvy et al., 1995). Larger size gives coral fragments superior survival ability over tiny larval recruits (Bothwell, 1981; Tunnicliffe, 1981; Hughes and Jackson, 1985). Connell (1973) points out the relationship between coral fragment size and mortality. This size-dependent mortality is confirmed by several other researchers (Highsmith et al., 1980; Knowlton et al., 1981; Hughes and Jackson, 1985: Plucer-Rosario and Randall, 1987). However, size-dependent mortality appears to be related to substrate. Contact with sand and silt has been shown to be problematic for corals (Rogers, 1983, 1990; Heyward and Collins 1985; Abdel-Salam and Porter, 1988; Rice and Hunter, 1992). Heyward and Collins (1985) found size-dependent mortality for coral fragments placed on rubble and sand substrates, but no size-dependent mortality in controlled aquarium conditions. Kobayashi (1984) found no size specific differences in mortality for coral fragments attached to nets well above the substrate. Lewis (1991) found no size-dependent difference in mortality for the hydrocoral Millepora complanata on rocky substrate. In my preliminary work (Bowden-Kerby, 1997) small fragments (8–12 cm) of Acropora cervicornis (Caribbean) and Acropora aspera (Pacific) almost always perished on sand, while larger 3-dimensional fragments (>30 cm) had low mortality and grew rapidly on sandy substrates. Coral fragments of at least some species have been shown to cement themselves naturally onto hard substrates within a few weeks (Gilmore and Hall, 1976; Bak and Criens, 1981; Kobayashi, 1984; Wallace, 1985; Bowden-Kerby, 1997). Transplanting coral fragments without attachment could therefore be a possibility for lower energy reef areas or for areas only periodically exposed to strong wave and current action. Variability between coral species in their ability to attach to the substrate has been reported (Wallace, 1985; Bowden-Kerby, 1997), but there are no publications dealing with attachment in any detail, and very little information exists on self-attachment times or attachment variability

BOWDEN-KERBY: REEF RESTORATION BASED ON NATURAL FRAGMENTATION PROCESSES

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within and among coral species. Such information could be vital for the selection of transplants used to restore areas periodically exposed to rough seasons or storms. Senescence has been demonstrated in the lower regions of branching coral colonies (Meesters and Bak, 1995). Senescence of older colony portions might lower survival of fragments originating from these regions, potentially affecting the outcome of transplanting efforts where entire colonies are fragmented for use. The effect of fragment origin within a coral colony on transplant survival has apparently not yet been investigated. Genetic adaptation to a specific environment would likely affect an individual coral’s ability to thrive in a particular environment, and could thus affect the outcome of transplantation efforts. Intraspecific differences have already been demonstrated for growth (Potts, 1984; Edmunds, 1993; Takabayashi and Hoegh-Guldberg, 1995; Hoegh-Guldberg et al., 1997), survival (Potts, 1984), and bleaching susceptibility (Edmunds, 1994). The relative importance of obtaining coral transplants from areas similar in environment to restoration sites has not yet been investigated. The following questions were deemed important in the investigation and development of effective low-tech restoration methods for lower energy sites, and are addressed by this study: (1) Is the mortality and growth of unattached coral fragments on back reef rubble dependent on fragment size, transplant site, or coral species? (2) Is the mortality of small (8–12 cm) coral fragments dependent on substrate (rubble vs various degrees of sand contact)? (3) Do coral fragments from axial (younger) regions of coral colonies differ in mortality when compared to fragments of similar size obtained from inner (older) colony regions >10–25 cm below the apex? (4) What are and how variable are the rates of selfattachment and overgrowth of coral fragments onto solid substrate? (5) Do Acropora transplants taken from high energy (reef front) environments survive, attach, and grow significantly differently in low energy (back reef) environments than do corals of the same species originating from the back reef zone? (6) Do simple low-cost methods (attachment to lines or frames) increase fragment survival significantly in the back reef over simply scattering unattached fragments? STUDY SITES Sites were located on five different reefs within the La Parguera reef system, in southwestern Puerto Rico (Fig. 1): Media Luna, Laurel, San Cristobal, La India, and Margarita. The largest back reef rubble area on each reef was used, with the various experimental replicates randomly selected within these shallow (0.5–0.75 m MLW) areas. Lagoonal transplant sites were located in sand flats directly behind the reef flat rubble sites. Due to the natural bathymetry of the back reef, these sandy sites were 0.5 to 1 m deeper than the reef rubble sites and were more sheltered, with less tidal and storm-driven current action. At 6 mo (September 1996), Margarita Reef, the most exposed site, was lost to Hurricane Hortense, being buried under 0.3 m of rubble sediment. METHODS EXPERIMENTAL CORAL SPECIES AND MORPHOTYPES.—The coral species used in all experiments were Acropora cervicornis and A. prolifera, species with an erect arborescent ‘staghorn’ growth form. Staghorn coral species are particularly prone to fragmentation, have rapid growth rates, and exhibit

Figure 1. Site map of the study reefs. Reef flat rubble beds are shaded black, back reef sand flats and seagrass beds are stippled.

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the ability to colonize unstable substrates (Alcala et al, 1982; Lindahl, 1998). Staghorn Acropora species also serve as important fish shelter habitat (Dubin and Baker, 1982; Roos, 1982; Patton, 1994). The coral species chosen are similar in form and habitat preferences to common Indo-Pacific staghorn species such as A. formosa and A. aspera, and were in-part chosen for this reason, potentially yielding results of more pantropical interest. In Puerto Rico, the morphology of A. cervicornis and A. prolifera is distinctly different in reef front versus back reef environments: robust morphotypes are normally found in higher energy reef front areas and more slender morphotypes are found in the back reef. In this study corals from both back reef and reef front populations of Acropora were grown together in lower energy back reef sites, to test if the source environment from which transplants are obtained is important in mortality, growth, and attachment in calm back reef environments. The experimental species/morphotypes are henceforth referred to as ‘coral varieties.’ The reef front variety of A. prolifera was not abundant on the study reefs, and the shortage of experimental material prevented its inclusion in many of the experiments, as indicated below. SIZE-SPECIFIC FRAGMENT MORTALITY AND GROWTH ON REEF FLAT RUBBLE.—To test for the effect of size on coral fragment mortality and growth, I used apical coral branches of three size-classes, 3–5, 8–12 and 15–22 cm, obtained from the three more common coral varieties. The experiment was set up in a 3 ¥ 3 ¥ 4 factoral (split plot) block design with three coral varieties, three size classes, and four replicate sites (discounting the hurricane-destroyed site). To control for possible genetic effects, coral fragments for all size classes per replicate were obtained from the same source colonies. To help control for possible temporal effects, all coral varieties and treatments were begun within 2 wks of each other. Coral fragments were placed on-deck in buckets filled with seawater, trimmed to the specified size, and measured to the nearest mm along the growth axis. All fragments were exposed to air briefly during this time. To ensure accurate initial measurement and to simplify subsequent growth measurements, only fragments with one linear axis and growth apex were used (for the largest size class, side branches were trimmed off as necessary). For each of the coral varieties and size classes, fragments were attached to a 60 lb-test monofilament line with 10 cm plastic cable ties, threading the cable tie through a double loop on the line to prevent fragments from sliding on the line. The resulting 5–6 m lines had 30 fragments each, ten per size class, and with each fragment about 10– 30 cm apart. One such line was prepared for each coral variety per site. Mortality and partial mortality was determined for each fragment on the lines subsequently at 2, 3, and (for lines which survived the hurricane) at 6 and 7 mo. Unattached coral fragments of each coral variety were scattered onto the rubble substrate adjacent to the experimental replicates as controls, in staked 5 ¥ 5 m areas, to determine if attachment to the monofilament lines was needed to retain coral fragments within the sites over time. To counter a concern for the potential confusion of data (90 tagged fragments per site), each size class of each coral variety was assigned a specific color of tie. An alternating non-random arrangement of size classes was also used down the line, either large/medium/small, or small/medium/ large. This arrangement helped facilitate fragment identification during field measurements regardless of neighboring fragment loss, breakage, or tie loss over time. To counter potential bias related to the ordering of subsamples, the fragments were secured to the lines while still in the boat, and anywhere between 10–30 cm of line was placed between each fragment, to avoid interactions between fragments. Each completed line was secured at one end to a metal stake that had been driven into a randomly selected point in the rubble bed. After attachment, each line of fragments was loosely stretched out in the direction of the prevailing currents, so that all fragments were in close contact with the substrate, but not held securely in place. The rubble zone sites were diverse at the fine scale, with small sandy or algal covered patches, gradations in rubble size, the presence of cobbles, etc. Based on the combination of these multiple factors, the final positioning of fragments onto the substrate was completely haphazard and is assumed to approach the random condition, particularly at the microenvironmental scale.

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THE EFFECT OF RELATIVE AGE (WITHIN COLONY FRAGMENT ORIGIN) ON MORTALITY.—The relative age of tissues in branching corals is related to their proximal-distal position along the coral growth axis, with fragments originating from outer apical portions relatively younger than fragments originating deeper within a coral colony. This experiment compared the mortality of apical fragments to those from regions deeper within the coral colony. For this experiment a line of coral fragments of the medium size class (8–12 cm) was constructed for each coral variety in the same manner as and in association with the size-specific experiment above, but with fragments >10–15 cm from branch apices. For each of these fragments, the distal end originated 10–15 cm below the apex (proximal end 18–27 cm below). Coral fragments from even deeper within coral colonies were included as well, with distal ends 20–25 cm below and proximal ends 28–37 cm below the apices. Due to a shortage of living experimental material, only three replicate lines of these >20–25 cm fragments were constructed, branches alternating on the lines with the sub-apical treatment. Completed lines were attached to the metal bars, immediately adjacent to the size-specific experimental line of its variety, and at a 15–20∞ angle to avoid overlap. Fragment mortality was recorded on the same schedule as for the size-specific experiment. SUBSTRATE-DEPENDENT FRAGMENT MORTALITY, GROWTH, AND ATTACHMENT IN THE BACK REEF.—The substrate dependent experiment was set up as a block design, with four sites, three coral treatments (varieties) and four substrate contact treatments. Relatively small (8–12 cm) unbranched apical coral fragments were used. Fragments were placed either directly on sand, supported on frames with only the fragment bases in direct contact with sand, or supported on frames 5–10 cm above the sand. The 8–12 cm size class of the rubble experiment described above was used to compare the relative effect of direct rubble contact on mortality with that of the three sand contact treatments. The treatment above the sand was added to control for possible environmental effects of the sandy back reef not related to direct substrate contact, a well as to test a potential methodology for increasing the survival of small transplants. The growth frames were constructed by bending a 0.5 ¥ 1 m piece of PVC-coated 2.5 ¥ 5 cm wire mesh into A-shaped frames 1 m long and standing about 25 cm high. A separate frame was used for each coral variety at each site, and frames were located directly adjacent to one another on the sand substrate, within 0.5–3 m of each other at a site. Fragments of the treatment placed on sand were scattered unattached in a prone position, each beside the frame of its variety. The four sites were located on lagoonal sand flats, within 5 m to the leeward of the rubble sites. In addition to the four main sites, an ancillary site with all four coral varieties was located on a sand bar >100 m from Media Luna reef. Even though this site survived the hurricane, all fragments on all frames suffered breakage, and so this site was excluded from the growth comparisons. However, mortality was not greatly affected and overgrowth of the surviving fragments to the frames was exceptional, allowing the site to be used in the analysis of these factors. These experiments used twelve fragments per treatment per replicate for each of the coral varieties. Two sites had all frames of all four coral varieties, while two did not, allowing for some partial comparisons of the data. Fragments of the sand contact and above sand treatments were attached with 10 cm plastic cable-ties to the wire frames in a vertical position. To separate possible genetic components in experimental outcome from site effects, all fragments of each coral variety were obtained from distinct coral thickets located on four different reefs, and these fragments were used in equal numbers in the replicates at all sites (4 clones ¥ 3 ea per treatment). To control for season as a factor in fragment survival and growth, all replicates were begun within days of each other and within a few weeks of the rubble experiment. During experimental set-up, fragments were transported to the UPR Department of Marine Sciences Isla Magüeyes Laboratory in buckets of seawater and held in large tanks in the flowing seawater system. These coral fragments were attached within several hours to the wire frames, which were submerged in the seawater tanks. The completed frames were kept overnight in seawater, suspended from the Isla Magüeyes dock, while the fragments for the unattached treatment were maintained in the flowing seawater system overnight. The completed frames and unattached fragments were transported to the field sites the next day, submerged in a plastic 55 gal drum of seawater.


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In the field, the frames were weighted with short 30–40 cm sections of 0.75 in diameter metal bars and placed into the sites, with the lower part of the frames pushed several cm into the sand. This resulted in the sand contact fragments having 0.5–1 cm of their bases covered by sand. Within each site, frames were oriented in the direction of the prevailing wave-generated currents flowing over the reef flats, in a general north-south direction and perpendicular to the reefs. This was done to minimize current drag during potential storms. A large amount of handling and aerial exposure was inevitable during experimental set-up, however the few (10–15 cm below the apex treatment were lost to Hurricane Hortense shortly before data was taken at 6 mo. A paired t-test (n = 10) excluding all incomplete replicates and using pooled data from all of the coral varieties as replicates indicates a highly significant statistical probability (P = 0.003). For the three additional lines of A. cervicornis that included fragments from both >10–15 cm and >20– 25 cm below the apex as treatments, analysis at 6 mo using a paired t-test indicates that mortality is related to age for even the oldest, most proximal colony regions, having sig-

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Figure 2. Mean percent mortality of three Acropora varieties of three fragment size classes on rubble at 6 mo (n=40: 4 reefs, each with 10 fragments per size-class).

nificantly greater mortality (P = 0.008) when compared to the subapical fragments (4, 3, 5 vs 0, 0, 1). HURRICANE EFFECTS ON THE RUBBLE ZONE EXPERIMENTS.—Most of the experimental replicates in the four sites surviving the hurricane were held securely in-place by the monofilament line during the storm, but suffered extensive abrasion and breakage of fragile tips and secondary branches. Healing of wounds was rapidly progressing at the rubble sites 2– 3 wks after the storm, when a heavy bloom of the filamentous green alga Trichosolen began smothering numerous surviving fragments and some entire replicates. Replicate lines at San Cristobal (SC), the most inshore reef site, were less affected by storm abrasion and breakage. Of 276 unattached fragments adjacent to the lines at this site in a 5 ¥ 5 m plot, only 84 were swept out of the plot; a mean distance of 5.4 m and with a maximum movement of 17 m. At the other sites most fragments not attached to lines were entirely swept away and lost. Several coral fragments were recovered at the Laurel and Media Luna sites up to 35 m from their original positions. The rubble treatment was discontinued subsequent to the hurricane because meaningful comparisons became questionable due to the effects caused by severe abrasion and breakage of secondary branches. Table 1. Within-colony origin as a factor in the mortality of 8-12 cm coral fragments on back reef rubble at 6 mo: numbers dead out of 10 fragments per replicate given. Sites: A. cervicornis, back reef apical branch >10– 15 cm A. cervicornis, reef front apical branch >10– 15 cm A. prolifera, back reef

apical branch >10– 15 cm

Within-colony data for complete reps (n = 10)

apical branches >10– 15 cm below

ML 0 3 1 6 0 1 1 10

L 4 8 0 5 ( 4.) lost 4 13

SC 5 4 1 2 2 3 8 9

LI ( 0.) lost 0 5 4 8 4 13

mean 2.3 5.0 0.5 4.5 2.5 4.0 4.25 11.25

(SD) (2.6) (2.7) (0.6) (1.7) (1.9) (3.6) (2.87) (2.06)


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Figure 3. Site specific percent mortality of coral fragments on reef flat rubble at 6 mo, pooled means of all three Acropora varieties (n=30 branches per size per site).

MORTALITY IN RESPONSE TO VARYING SUBSTRATES AND DEGREES OF SAND CONTACT.—All of the 8–12 cm fragments placed directly onto sand died within the first few weeks of the experiment. By 3 mo the mean fragment mortality for fragments on rubble remained significantly lower (ANOVA P < 0.001) than fragments placed on sand (Bowden-Kerby, 1997). The last complete data set for mortality with all four contact treatments at 6 mo excludes the site lost to the hurricane (Fig. 5). Percent mortality for the sand contact and above sand treatments on wire frames at one year was analyzed by ANOVA using arcsin transformed percentage data in a split-plot factoral design (n = 30, 3 coral types ¥ 2 substrates ¥ 5 replicates). The results show statistically significant differences between coral variety (P < 0.025), degree of sand contact (P < 0.001), and a significant interaction between the two (P < 0.01). No significant difference was found between the back reef sites at 1 yr.

Figure 4. Cummulative site-specific mortality of Acropora fragments on rubble at 2, 3, 6, and 7 mo (pooled data, n=90 per site). Hurricane and post-hurricane mortality ocurred at 6 and 7 mo.

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Figure 5. Mean percent mortality at 6 mo for three Acropora varieties under four substrate treatments (n=48,40: 4 sites ¥ 12 fragments per treatment per site; 10 per rubble treatment).

CORAL GROWTH.—The corals from the reef front continued to maintain a distinctly more robust morphology in the calmer growth conditions of the back reef, even after a full year. This was measured in hind-sight by calipers at points 1 cm above the bases of secondary branches, means of three branches per colony (t-test = P < 0.0001, n = 19), indicating a genetic component to Acropora morphology related to the original growth environment of the source population. Hurricane damage prevented obtaining meaningful growth data from the reef flat experiments, however the back reef sites came through the hurricane surprisingly intact. There was breakage and mortality on all frames at all sites throughout the year, mostly unrelated to the hurricane. Breakage, although apparent when fresh, became difficult to detect after several months of re-growth. Mortality and breakage were considered nuisance variables to the question of the relative effect of sand contact on growth. To discount breakage and mortality, mean relative growth (total finishing length ∏ starting length) was calculated as the mean of the three largest fragments (3 out of 12) per treatment per frame (Table 2). The maximum single-branch growth values for each variety and species: relative growth, sum total of all new branch lengths, and maximum one-dimensional linear growth, as well as total overgrowth onto the wire frames are given in Table 3, indicating the maximum growth potential for this lagoonal environment. FRAGMENT ATTACHMENT AND OVERGROWTH ONTO WIRE MESH FRAMES.—For coral fragments suspended above the sand in the sites, firm adherence and overgrowth onto the frames occurred within 2 mo for 88.3% of back reef A. cervicornis fragments, 80.0% of reef front A. cervicornis fragments, and 73.3% of back reef A. prolifera fragments. For most of the fragments that remained unattached at 3 mo, the fragment portions in contact with the frame had died so that self-attachment and overgrowth onto the frame were no longer possible. The majority of branches growing on the frames experienced partial death to the older main axis at 1 yr, perhaps related to senescence. Bioerosion of the dead basal areas caused weakening of attachment points and many branches began detaching from the frames at 1 yr, attachment rates decreasing to 32.9, 59.7 and 27.1%. At 1 yr only 30, 33 and 15% of


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Table 2. Relative growth (total finishing length ∏ starting length) of Acropora branches grown together on frames in the back reef for 1 yr. Means were calculated from the three largest fragments (n = 3 out of 12) per treatment per frame, to exclude breakage and mortality. Sites: A. cervicornis, back reef type above sand mean sand contact mean A. cervicornis, reef front type above sand mean sand contact mean A. prolifera, back reef type above sand mean sand contact mean A. prolifera, reef front type above sand mean sand contact mean

ML

L

SC

LI

mean

(SD)

10.5 7.1

11.9 11.3

9.0 9.9

15.9 9.3

11.8 9.4

(3.0) (1.7)

16.3 10.1

23.2 12.2

14.6 8.4

22.0 10.7

19.0 10.4

(4.2) (1.6)

10.4 7.9

14.1 9.0

4.8 3.6

23.9 6.4

13.3 6.7

(8.0) (2.3)

19.6 20.7

23.5 13.2

– –

– –

21.5 17.0

(2.8) (5.3)

branches were completely alive in the basal region for each variety. Basal portions of reef front A. cervicornis fared the best, the dead areas being an average of 14.6% smaller than for the back reef variety (paired t-test, P = 0.01). Means of frame overgrowth based on the three sites with a data set for all four coral varieties, calculated from the means of the three maximum overgrowths per variety per site (Table 3), showed a significant difference between coral varieties by ANOVA (P < 0.01) but no significant differences between sites (P < 0.10: n = 12; 4 varieties ¥ 3 sites). Looking at A. cervicornis data from all five sites, the reef front variety overgrew the frames significantly greater than did the back reef morphotype (P < 0.05 by ANOVA, n = 10; 2 varieties ¥ 5 sites). Table 3. Maximum annual growth and frame overgrowth values for two varieties of two Acropora species (greatest single branch out of 96 original 6-12 cm branches; 48 for reef front A. prolifera). Relative growth = total finishing length ∏ starting length; 3-D growth = sum of all new branch lengths; Linear growth = maximum growth along the major axis; Overgrowth = sum of all overgrowths from the point of contact with the wire mesh frame; Mean overgrowth = mean of means for maximum three overgrowths per frame per variety per site (n = 4,4,4,2).

A. cervicornis back reef reef front A. prolifera back reef reef front

(SD) Maximum Mean overgrowth overgrowth

Relative growth

3-D growth

Linear growth

17.0x 25.3x

221.4cm 281.7cm

16.0cm 21.8cm

10.7cm 31.3cm

7.5cm 13.2cm

(0.4) (6.7)

28.9x 37.6x

349.0cm 494.0cm

15.4cm 15.3cm

12.7cm 34.0cm

6.9cm 21.3cm

(2.6) (3.7)

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DISCUSSION CORAL GROWTH IN THE EXPERIMENTAL SITES.—On the wire frames, corals grew at exceedingly fast rates, among the maximum rates ever recorded for corals. A possible underlying factor for why the corals grew so rapidly in this study is that they started out as moderately sized juvenile corals rather than as very small juvenile or larger adult colonies, assuming conformation to the logistic growth model. Measurement of growth in branching corals by most researchers has either been for short time periods, or has measured linear growth in only one dimension (Shinn, 1966; Lewis et al., 1968; Buddemeier and Kinzie, 1976; Gladfelter et al., 1978; Crossland, 1981; Yap and Gomez, 1981; Charuchinda and Hylleberg, 1984; Gomez et al., 1985). Simple linear measurement along a single growth axis seriously underestimates growth as a three-dimensional process in branching corals. Measuring all new branches was quite time-consuming and tedious, especially when carried out in the field (several hours per frame). However, the fact that significant results were obtained with low replication testifies to the utility of the method, especially when enough subsamples are included to allow for the exclusion of nuisance variables such as breakage and mortality when growth is of primary interest. SENESCENCE AND ACROPORA ECOLOGY.—Meesters and Bak (1995) record an exponential decrease in the ability of A. palmata to heal lesions inflicted along the proximal-distal growth axis. These researchers suggest that senescence may be adaptive in A. palmata, promoting fragmentation and asexual propagation within this species as older portions of the colony die, helping to ensure a large fragment size and a subsequently higher survival for the resulting fragments. A similar adaptive value for senescence could potentially apply to A. cervicornis and A. prolifera. EVIDENCE FOR ADAPTATION IN ACROPORA POPULATIONS.—There are several indications that the reef front variety of A. cervicornis is adapted to higher energy conditions of the reef front, investing more energy in maintaining healthy basal tissues and in self-attachment even when planted in the back reef. Thicker branch morphology, greater attachment ability, and maintaining living tissues on lower colony portions would all have adaptive value in remaining attached to the substrate and in resisting wave and current action. METHODOLOGICAL LESSONS LEARNED FROM HURRICANE HORTENSE.—Observational comparisons in the field between corals tied on lines and those unsecured on rubble substrate indicate that securing fragments to lines tied to a stake results in considerably more abrasion than when fragments are allowed to tumble freely across the rubble zone. However severe the abrasion to coral fragments was, abrasion appeared to have less serious consequences than being thrown onto back reef sand, where death to small one-dimensional fragments is likely. In areas where storm currents are periodically strong enough to move fragments from a site before they can attach themselves securely to the substrate, lines can be used effectively as a methodology for reef restoration (Lindahl, 1998). Securing both ends of the line to stakes might add stability to the lines and prevent some of the abrasion. Corals secured to the wire frames surprising showed no obvious signs of damage from the storm, especially considering the heavy damage to reef flat corals just meters away. Culturing coral fragments on weighted wire frames thus appears to be an effective and storm-resistant methodology for growing small coral fragments quickly into large colonies in sheltered back reef areas.


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On Media Luna and San Cristobal reefs, I had scattered numerous A. cervicornis fragments onto rubble bars some 2 yrs prior to the hurricane in preliminary experiments (Bowden-Kerby, 1997). These coral fragments had grown into numerous multi-branched coral colonies of large size by the time the storm hit. Three months before the storm, 23 colonies sampled on Media Luna Reef had a mean height of 26.7 cm, while 29 colonies sampled on San Cristobal Reef, with a mean height of 24.6 cm. Currents generated by Hurricane Hortense swept most of these coral colonies away, transporting them into the sandy lagoon, some as distant as 60–70 m away. Colonies apparently rolling head-overheels like tumbleweeds to their final resting-places, with only the branch tips suffered damage during transport. Larger colonies traveled the furthest; the mean height of the five farthest colonies (>50 m from the reef flat) at San Cristobal was 30.4 cm, compared to a mean height of only 19.2 cm for the five colonies deposited nearest to the reef flat (