Damage on South African Coral Reefs and an Assessment of Their Sustainable Diving Capacity Using a Fisheries Approach

BULLETIN OF MARINE SCIENCE, 67(3): 1025–1042, 2000 CORAL REEF PAPER

DAMAGE ON SOUTH AFRICAN CORAL REEFS AND AN ASSESSMENT OF THEIR SUSTAINABLE DIVING CAPACITY USING A FISHERIES APPROACH

Michael H. Schleyer and Bruce J. Tomalin

ABSTRACT Coral reefs in a marine reserve at Sodwana Bay (27°30'S) make it a premier dive resort. Corals are at the southern limits of their African distribution on these reefs which are dominated by soft corals. The coastline is exposed and turbulent. An assessment of the degree to which sport diving damages the reefs is needed for their management. This study showed that recognizable diver damage is generally concentrated in heavily dived areas. This damage and that of unknown cause probably attributable to divers exceeded natural damage on the reefs, despite the normally rough seas. Fishing line discarded in angling areas also caused considerable damage by tangling around branching corals which become algal fouled and die. Heaviest damage was caused in isolated areas by a minor crown-of-thorns outbreak. A linear regression indicated that 10% diver damage occurs at 9000 dives per dive site p.a. Taking uncertainty into account, a precautionary limit of 7000 dives per dive site p.a. was recommended. Further recommendations are that the reefs be zoned in terms of their sensitivity to diver damage, depth and use by divers according to qualification, and a ban be placed on the use of diving gloves to reduce handling of the reefs.

The complexity and beauty of coral reefs make them an attractive and valuable resource for ecotourism (Davis and Tisdell, 1995). However, many corals are fragile and susceptible to damage, making an assessment of the sustainable diving capacity of the reefs and the damage caused by sport-diving essential for their management. While the effects of other human activities on coral reefs have been well documented (e.g., coral mining and dynamite fishing; Brown 1986, 1987; Alcala and Gomez, 1987; Wells and Alcala, 1987), relatively few studies have focused on the deleterious effects of sport diving. These were reviewed by Davis and Tisdell (1995), who found that most were focused on spear-fishing and snorkelling, with the least attention being given to SCUBA diving. In situations in which ideal diving conditions are encountered, such studies can be based on observations of diver behavior, the most comprehensive in this regard being those of Rouphael and Inglis (1995, 1997) which were undertaken on the Great Barrier Reef. However, research on the subject is more often based on a comparison between the damage found on heavily dived reefs relative to undived areas (see Davis and Tisdell, 1995), absolute quantification normally being difficult to accomplish. One quantitative study established that fast-growing, branching corals suffer the most damage in the Red Sea (Riegl and Velimirov, 1991). The level of damage is directly related to the susceptibility of the coral species (Riegl and Velimirov, 1991; Hawkins and Roberts, 1992; Rouphael and Inglis, 1995, 1997; Allison, 1996) and diving intensity (Hawkins and Roberts, 1997; this study). Much diver damage on reefs is unnecessary and can be avoided by modifying diver behavior (Rouphael and Inglis, 1995; Allison 1996; Medio et al., 1997; Hawkins and Roberts, 1997), limiting diving to the sustainable diving capacity of the reefs (Davis and Tisdell, 1995; Hawkins

1025 1026 BULLETIN OF MARINE SCIENCE, VOL. 67, NO. 3, 2000 and Roberts, 1997) and zoning them in terms of their sensitivity to damage (Rouphael and Inglis, 1995). Schleyer (1995, 1999, in press) reviewed the structure and limited extent of coral reefs in South Africa. All fall in conservation areas and are only used for recreational game fishing and sport diving. The reefs are considered a valuable yet vulnerable resource but only became the subject of extensive research in the last decade. Sport diving increased six-fold between 1987 and 1996 on the most accessible of the three reef complexes; however only a superficial examination of reef damage was undertaken in this period (Riegl and Cook, 1995) in tandem with the baseline studies. Alarmist articles on the incidence of diver damage appeared in the press with the increase in diving during this period. A holistic approach was recommended by Schleyer (pers. correspond., 1995, 1999, in press) for the management of the South African reefs. This approach incorporated features such as the inclusion of breeding refugia and the restriction of diving on reefs vulnerable to damage, to divers of advanced qualification. However, no information was available on the sustainable diving capacity of the reefs, leaving the management guidelines incomplete. This study was thus undertaken to quantify damage on the accessible reefs and establish their sustainable diving capacity. Conceptually, we believe that diver damage on coral reefs can be considered equivalent to fishing mortality caused in a fishery and hence the techniques of fisheries stock assessment can be used (for an introduction to fisheries science see Hilborn and Walters, 1992). Diver damage will be a function of diving intensity (effort in a fishery) and behavior (selectivity and fishing power). Catch per unit effort in a fishery could be considered equivalent to diver ‘satisfaction’. Average diver satisfaction would decline with increased diving intensity due to perceptions of crowding and obvious diver damage. The equivalent in a fishery is the inevitable decline in catch per unit effort with increasing effort. Note that as fisheries managers have to accept that any level of fishing will result in some decline in stock size, managers of coral reefs will have to accept that if diving is to be allowed, some level of damage to corals will occur. The question in both cases is how much decline in stock size or increase in damage is ‘acceptable’. In fisheries this is ad- dressed by assessing the predicted total yield versus effort function. Total yield in a fishery would be equivalent to total diver satisfaction (the product of the number of dives and the average satisfaction per dive). We would expect satisfaction to reach an asymptote at an intermediate amount of diving, followed by a decline as diving intensity increases (as does total yield in a fishery). As in fisheries, sport diving generates considerable income to the private sector and the state. In this case, the local nature conservation service ob- tains the bulk of its non-state income from the Sodwana resort. A ‘common’ resource is used by visitors to the Sodwana Bay National Park and, if unrestricted access to the coral reefs is allowed, a “tragedy of the commons” could result (Hardin, 1968). Although fisheries science has a long history and deterministic models are well established, recent developments have emphasized the role of uncertainty in this science (summarized in Hilborn and Peterman, 1996). It is recognized that fishing effort may only explain a small proportion of the variability in the size of a harvested stock. Similarly, it is likely that factors other than diving may cause more spatial and temporal variability in damage to corals (e.g., natural predation, storms, other anthropogenic effects). However, in common with fisheries managers, managers of coral reefs have very little control over factors other than diving intensity (fishing effort) and therefore our analysis focuses on the single factor which is controllable. The other factors can be considered to cause un- SCHLEYER AND TOMALIN: DAMAGE ON SOUTH AFRICAN CORAL REEFS 1027 certainty around the estimation of the effect of diving intensity and we therefore considered it more important to incorporate uncertainty into our analysis than to produce a complex model of the effect of diving intensity. In summary, three fisheries stock assessment concepts were adopted for the analysis of the effects of diving on the reefs: the precautionary approach (FAO, 1995) incorporating the use of limit reference points (Caddy and Mahon, 1995), quantification of trade-offs and the incorporation of uncertainty into risk analysis (Hilborn and Walters, 1992). In this case, a trade-off was considered between maximizing the number of dives per site and minimizing the risk of ‘significant’ damage due to human activity. We did not at- tempt to quantify ecological damage, but considered diver damage to be significant when it was noticed by sports divers. This level was then used as a precautionary limit reference point (FAO, 1995). As the matter is an aesthetic issue, we emphasize the proportion (%) of total damage caused by divers and not the absolute amount (damage per unit area). The trade-off between diving intensity and the amount of diver damage was quantified using cumulative risk curves because the relationship was uncertain.

MATERIALS AND METHODS

Visual estimates of reef damage were made in the central of three reef complexes in northern KwaZulu-Natal which is accessible from tourism facilities and a launch site at Sodwana Bay (Fig. 1). The northern complex is relatively inaccessible while the one to the south falls in a sanctuary not open to the public. The central complex is thus a focus of marine recreational activity. Records of the total number of dives and boat launches made in this complex are kept by the local conservation authority, the KwaZulu-Natal Nature Conservation Service (KZNNCS). The damage estimates were made by SCUBA divers undertaking drift dives on the reefs between September 1994 and November 1995. A three decimal GPS (Navstar XR4G) fix was taken on entry and termination of all but the shortest dives and a record was kept on underwater slates of damage to any sessile organism in a 2 m path as the dive progressed. The length of shorter dives was obtained from direct measurements. The observations were thus made within a transect belt of approximately known area. Twenty three drift transects were undertaken on all of the reefs in the central reef complex. They ranged in length from 20 m (on a heavily damaged COTS site) to 770 m (mean 208 m, Table 1), the divers working within 1 m of the reef surface. Most attention was given to Two-mile Reef (TMR), being the largest and most heavily dived reef, while the relatively undived Nine-mile Reef (NMR) was used as a control. The full range of reef depths was surveyed. The survey included multiple transects on each reef and at each dive site. The latter constituted discreet areas with underwater features regularly visited by sport divers. The life form categories and codes developed by English et al. (1994) were used in recording the damage data. A detailed record was kept of the damage encountered and included a visual estimate of the percentage cover affected. A similar percentage of damage on the surface of a small or a large organism was thus given the same weighting. Identi- fiable causes of the damage were also noted. The experience of years of diving on the reefs was used in this regard and included observations made on, e.g., fish-feeding, predation by molluscs and the damage caused by sport divers. The feeding scars of corallivorous fish were quite distinc- tive on both soft and stony corals. Predation by the gastropods Drupella spp. and Ovulis ovum was usually detected by their proximity to the site of damage. Gross damage to branching, digitate or plate corals could be attributed to divers when diving parties were observed causing damage in the field or when forensic evidence of their passage was found on damaged coral (e.g., paint scrapings or fabric threads). Discarded or lost fishing line appeared to entangle readily on branching and digitate corals and was accompanied by progressive algal fouling (Fig. 2) which was recorded as being due to fishing line. Only in its later stages when the coral had died and it was accreted into an 1028 BULLETIN OF MARINE SCIENCE, VOL. 67, NO. 3, 2000

Figure 1. The major coral reefs in South Africa are found in marine reserves (shaded areas), which include sanctuary areas (dark shading), in northern KwaZulu-Natal. amorphous lump by coralline algae was this cause difficult to diagnose. Algal fouling of no identifiable cause was also found on some corals, probably at old injury sites. The feeding scars of crown-of-thorns starfish (COTS; Acanthaster planci) were readily identifiable. Anchor damage was not observed as anchoring of boats is prohibited on the reefs. Reef trampling was also not encountered as the shallowest diving sites at Sodwana Bay are 10–12 m deep. The observations were summarized as the sum of percentage damage per cause and per organism type for each transect divided by the transect area. This provided a quantitative index of damage for each transect that was standardized per unit area. The resulting indices of damage were subjected to regression analysis against diving intensity at each site, the latter being obtained from KZNNCS records of total dives and detailed site-specific logs kept by professional dive operators. Regres- SCHLEYER AND TOMALIN: DAMAGE ON SOUTH AFRICAN CORAL REEFS 1029

Table 1. Details of the reef damage transects undertaken in the central reef complex at Sodwana Bay (September 1994−November 1995). FMR = Four-mile Reef, NMR = Nine-mile Reef, SMR = Seven-mile Reef, TMR = Two-mile Reef, COTS = crown-of-thorns starfish. The mean transect length was 208 m. Dive Transect length Mean depth Max depth Diving intensity Reef number (m) (m) (m) (%, see Table 4) 1RTM 1564 102. 1 2RTM 1564 102. 1 3RTM 2000 105. 2 4RNM 5012 155. 166. 5RTM 3066 102. 188. 6RTM 7070 102. 115. 1 7RSM 1024 195. 8RFM 2057 187. 9STMR-COT 400 205. 217. 1R0 TM 4002 155. 138. 1R1 TM 701 152. 104. 1 1R2 TM 1055 152. 124. 1 1R3 TM 505 152. 124. 1 1R4 TM 1005 152. 144. 1 1R5 TM 1006 152. 144. 1 1R6 TM 2004 105. 106. 1 1S7 TMR-COT 200 252. 214. 1R8 TM 3023 101. 133. 1R9 NM 803 152. 117. 2R0 NM 1059 152. 157. 0. 2R1 NM 901 152. 127. 0. 2R2 NM 2014 152. 127. 0. 2R3 NM 1092 152. 157. 0.1 sions were computed for indices of damage per 200 m2 and the percentage damage attributable to natural causes, divers, COTS, fishing line and unidentifiable causes. The regression for percent diving damage was re-calculated using diving intensity as the dependent variable and percent diving damage as the independent variable (i.e., inverting the previous set of regressions) in order to predict the mean number of dives (with standard errors) which would result in an increase of 5, 10 and 20% in reef damage due to divers. The variance about the mean for each increase was obtained using the standard equation for linear regressions (Draper and Smith, 1981: 28) and the results used as parameters for the calculation of normal probability distribution functions from which cumulative probability curves of diving intensities causing specified increases in reef damage were plotted. The results of each transect provided an individual datum point in the above regressions. Transects which occurred at the same dive site were attributed the same dive intensity and this may be con- strued as pseudo-replication (Hurlbert, 1984). Therefore, the regressions were re-analysed using a mean for each dive site as a single datum point. Characteristics of dive sites other than dive intensity will also influence the amount of damage caused by divers, e.g., the area of reef, proportion of hard and soft corals, proportion of branching corals, the proportion of sand, etc. These were not considered in the regressions because (1) we wanted to describe the inherent variability between sites and (2) results dependent on the nature of the site would have been more difficult to interpret from a management point of view. However, we did exclude one site from the regressions but not from the graphs where it is clearly marked. This is Anton’s reef which is a small isolated reef used for training. Divers are dropped onto the surround- ing sand and swim around the reef, not over it. Therefore, little diving actually occurs on this reef. 1030 BULLETIN OF MARINE SCIENCE, VOL. 67, NO. 3, 2000

Figure 2. Fishing line discarded at sea tangles mainly in branching and digitate corals where it becomes algal fouled as illustrated on this Acropora (top). The fouling eventually overgrows and kills the coral (center), becoming an amorphous lump once accreted by coralline algae (bottom). SCHLEYER AND TOMALIN: DAMAGE ON SOUTH AFRICAN CORAL REEFS 1031

Figure 3. KZN-NCS record of annual boat launches at Sodwana Bay.

RESULTS

MARINE RECREATION AT SODWANA BAY.—KZNNCS records show that the launching of boats increased considerably in the period 1991 (7903 launches) to 1996 (17,692 launches). The greatest increase occurred in 1995 when the number of launches jumped by 65% from 10,740 to 17,614 (Fig. 3); further substantial increases are unlikely as the launch site at Sodwana Bay can probably only handle this level of traffic. Table 2 provides the breakdown of activities for which boats were launched in 1992 and 1996, these being years in which more detailed records were kept. The figures indicate that nearly equal effort was directed at game fishing and diving in 1992, but the former had more than halved by 1996 and the latter had increased by a factor of 3.8. Recreational diving is clearly becoming the most important activity at Sodwana Bay (Fig. 4), having increased from 20,070 dives in 1987 to 118,389 in 1996. Again these figures must reach a plateau (Fig. 4) as the limitations of the launch site are being realized.

Table 2. KZN-NCS records of boats launched at Sodwana Bay for different activities in 1992 and 1996, these being recent years in which comprehensive records were kept.

A2ctivity 1699 199 lsaunches launche S6ki-boat angling 3),97 (21,448 anglers 1,66 S8pearfishing 2)1 (1693 spearfishermen 6 C8oncession diving 2),71 (92,268 divers 9,96 N8on-concession diving 10,47 6,00 82,390 17,69 1032 BULLETIN OF MARINE SCIENCE, VOL. 67, NO. 3, 2000

Figure 4. KZN-NCS record of the annual number of dives undertaken in the central reef complex located at Sodwana Bay.

Most of the diving takes place on Two-mile Reef which is closest to the launch site; Four-, Seven-and Nine-mile Reefs receive relatively little attention (Table 3). The detailed site-by-site information of the diving intensities used in the data analysis are listed in Table 4. Learner divers account for approximately 20% of this activity and the qualify- ing dives take place in the vicinity of sites known as Anton’s Reef (60%) and Four-buoy (40%; S. Bailey, pers. comm.). OBSERVATIONS ON DIVER BEHAVIOR.—Only limited observations could be made on diver behavior. These showed, firstly, that the strong currents and swell-induced surges which generally prevail on the reefs (Schleyer, 1995, 1999, in press a) caused divers to readily attain a horizontal posture once they had achieved neutral buoyancy. As a result, their fins did not often make contact with the reef and relatively little fin damage was found. Mas- sive sponges were the most commonly affected organism. Divers who did not rapidly compensate their buoyancy caused fairly severe damage if they bounced on the reef. However, most diver damage was attributable to turbulence under rough conditions when divers used their gloved hands to fend themselves from

Table 3. KZN-NCS records of the distribution of diving activities in the central reef complex at Sodwana Bay. Data are presented for recent years in which comprehensive records were kept. The 1992 figures provide a better reflection of the distribution of dives at Sodwana Bay as the record of launches in the other years does not take the number of divers per boat into account.

Tfwo-mile Reef Ffour-mile Ree Sfeven-mile Ree Nine-mile Ree 1%992 Dives 9%0.1 2%.6 3%.4 3.9 1%995 Dive launches 8%6.9 3%.3 6%.2 2.3 1%996 Dive launches 8%5.5 3%.5 7%.7 2.8 SCHLEYER AND TOMALIN: DAMAGE ON SOUTH AFRICAN CORAL REEFS 1033

Table 4. Record of the diving intensity at the different sites in the central reef complex at Sodwana Bay during a peak summer season (December 1994−January 1995).

Reeef Dsive sit Nlumber of Dive % Tota Tfwo-mile Reef: Q1uarter-mile Ree 136 2. S9tringer Reef 666 9. C1hain Reef 233 3. Z1ambezi Alley 570. A5nton's Reef 18,38 19. C8aves & Overhangs 975 13. P1innacle 290 2. S5ponge Reef 178 2. C0oral Gardens 168 2. F6our-buoy 609 10. U3nspecified 552 7. F7our-mile Reef 584 7. S8even-mile Reef 569 8. N8ine-mile Reef 444 6. O1ther 136 2. T4otal 6,99 contact with the reefs, or to grab onto a suitable handhold if they wished to remain sta- tionary. Underwater photographers were particularly culpable of this. Limited souvenir collecting was observed. Despite the fact that some of the survey dives were undertaken after storms in which swells of up to 6 m were generated, little identifiable storm damage was ever encountered. Results of Regression Analyses.—The results of all correlations are summarized in Table 5. There was no significant difference between regressions using all transect data (Table 5A) and those using the mean at each dive site (Table 5B). Only the former are thus discussed further. The key finding is that the index of damage attributed to diving is significantly correlated to diving intensity (Table 5A; P < 0.05, r2 = 0.21, slope = 0.0032, SE = 0.001). In addition, the indices of total reef damage (natural + human + unknown) per 200 m2 were positively correlated with diving intensity (P = 0.03, r2 = 0.19; Fig. 5A). Damage caused by COTS and fishing line were excluded from this regression because, although infre- quently encountered, their consequences were severe and masked the effects of diving damage (Fig. 5E,F). The absolute amount of diver damage was significantly correlated to diving intensity (Tables 5a and b). Relatively little damage was found at a TMR dive site known as Anton’s Reef despite it being subjected to the highest diving intensity in the area (Fig. 5C), including that of learner divers. This site was excluded from the regression because, although it is listed by operators as a dive site, divers generally swim around its periphery and not over it (see methods and discussion). It was plotted on the graphs for illustrative purposes. As was to be expected, the component of total damage attributable to natural causes was not correlated with diving intensity (Fig. 5B). It is of note that the index of mean natural damage per 200 m2 (130.4, 95% confidence interval: 89.2–171.5) corresponded 1034 BULLETIN OF MARINE SCIENCE, VOL. 67, NO. 3, 2000

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Figure 5. Levels of damage vs. diving intensity on KwaZulu-Natal reefs in the central reef complex at Sodwana Bay (a, total; and, as percentages of total, b, natural; c, diver; d, unidentifiable; e, fishing line; and f, crown-of-thorns damage). Data for COTS and fishing line damage were omitted from the regression for total damage, as were data for Anton’s Reef in the regressions included in (a) and (c) for reasons given in the text. Values for the COTS sites are given in the top left corner in (a). Points marked 4-B in (a) and (c) refer to the site known as Four-buoy. According to the regression, divers increase reef damage by 10% at the diving intensity indicated by the arrow in (c). 1036 BULLETIN OF MARINE SCIENCE, VOL. 67, NO. 3, 2000

Table 6. Distribution (%) of the causes of damage among the morphological categories of organism.

Tmype of damage Morphological category of organis Sstony corals Ssoft coral Tsunicate Sponge Σ Egncrusting Beranchin Meassiv Plat Natural damage 393930407010 Diver damage 03874700010 Fishing line damage <01 801<911<0010 Unknown causes 1733827222010 All damage 254621131<4010

very closely with the expected value of total damage when diving intensity was nil (i.e., the Y-intercept in Fig. 5A, 127.5), increasing our confidence in the regression. The cause of considerable reef damage was unknown (Fig. 5D). This must have included a measure of unidentifiable diver damage as it increased with diving intensity. However, unidentifiable natural damage was also present and masked this trend so that the correlation with diving intensity was not significant. Algal fouling of unidentifiable cause is also included in Figure 5D; it comprised 15% of the total damage but was not correlated with diving intensity. Damage caused by discarded fishing line was encountered in the localities where game fishing is allowed in the central reef complex. Only a few such stations were surveyed as fishing is permitted in these areas because they are so little used for diving, but the level of this damage was high (Fig. 5E). COTS were also found at the deeper margins of TMR not used by divers and a few COTS stations were surveyed (Table 1). The level of damage at these was very high (Fig. 5F and COTS sites in Fig. 5A). Table 6 lists the degree to which the categories of organisms were affected by the different causes of damage. Stony branching corals were more susceptible to human-induced damage (divers and fishing line) than other morphological forms whereas natural and unidentifiable damage was more evenly distributed. Recognizable diver damage, expressed as a percentage of total damage, also increased with the diving intensity (P = 0.01, r2 = 0.28; Fig. 5C). Note that this is also the case whether or not COTS and fishing line damage is included in the total damage. The data for Anton’s Reef is again excluded from the regression provided in this figure. The level at which diver damage increased the total reef damage by 10% is marked with an arrow; sport divers complained about reef damage at the sites where this level was exceeded (pers. observ.). We considered that the percentage of total damage caused by diving is the key index which management should focus on and hence used it for the further management-based analyses below. The probability of three damage limits (expressed as percentage diver damage) being exceeded at different diving intensities is presented in Figure 6. This provides the probability of a given diving intensity resulting in a specified level of percentage diver damage. For example, at a diving intensity of 7000 dives per site per year, the probabilities of reef damage being increased by >20%, >10% or >5% are 25%, 40% or 50%, respectively (the last is the expected value). In contrast, 10,000 dives per site per year would result in a 45% probability of a >20% increase in reef damage, or a 70% probability of this rising by >10%. Thus managers of the resource can use these graphs to determine target diving intensities once they have selected critical levels of diver damage and the risk they are SCHLEYER AND TOMALIN: DAMAGE ON SOUTH AFRICAN CORAL REEFS 1037

Figure 6. Probability of selected increments in reef damage being exceeded at different diving intensities. Curves for three diver induced increments in reef damage are presented. prepared to take of that critical level being exceeded. A more conservative approach would clearly be needed in the case of a reef inhabited by a preponderance of fragile corals than one dominated by corals resilient to diver damage.

DISCUSSION

Cyclonic storms occasionally occur in summer along the east coast of southern Africa and these generate swells of up to 10 m (Hunter, 1988). The swells cause extensive damage on the reefs (Ramsay, 1996), relative to which the damage resulting from human activity is commonly considered inconsequential (Ramsay, pers. comm.). However, while cyclonic storms are periodic natural events, human damage on the reefs is persistent and the upper levels of diver damage encompassed in Figure 5A,C,D clearly exceeded natural reef damage. The present study has thus shown it to be significant in magnitude at inten- sively used localities despite the exposed nature of the reefs. There is a marked contrast between two Two-mile Reef stations known as Anton’s Reef and Four-buoy (Table 3). Anton’s Reef is closest to the launch site and is used extensively for the training of novice divers. It is thus subjected to the highest diving intensity at Sodwana Bay (Table 4), yet it manifested a relatively low level of diver damage (Fig. 5B,C). The reason for this is that it comprises a small promontory of the reef surrounded by sand; dive parties travel around its perimeter over the sand and need not come in contact with it. This makes it eminently suitable for diver training and the bulk of such activity at Sodwana Bay takes place here. Instructors keep the divers off the reef, providing the reason for the exclusion of the data for Anton’s Reef in computing the regressions 1038 BULLETIN OF MARINE SCIENCE, VOL. 67, NO. 3, 2000

presented in Figure 5A,C. Four-buoy is also traditionally used as a training area. Dives commence here on a patch of sand in a hollow on the reef; diving instructors intend their novices to attain neutral buoyancy on the sand before proceeding onto the reef. However, this is not often achieved and high levels of diver damage were observed here relative to the diving intensity (Fig. 5A,C). As the dives were executed on the reef, the Four-buoy data were included in the regressions in Figure 5A,C. Species of Acropora proved to be the most susceptible to damage in earlier studies undertaken at Sodwana Bay (Riegl and Cook, 1995) and in the Red Sea (Riegl and Velimirov, 1991). In the present study, this proved to be true of the damage caused both by divers and fishing line (Table 5), in spite of the fact that soft corals of the genera Lobophytum and Sinularia are more abundant than Acropora species and other stony corals on the Sodwana reefs (Riegl et al., 1995). It is also of note that stony tabular corals were fairly susceptible to damage (Table 5) but the causes for this were difficult to determine. A. clathrata and A. hyacinthus were the most abundant in this category and are not particularly fragile. The few observations made on diver behavior were of interest. Turbulence on the South African reefs mitigates against the preponderance of fin damage encountered in other areas where divers more readily remain vertical in the water (e.g., Rouphael and Inglis, 1995; Allison, 1996). Conversely, the turbulent conditions in South Africa undoubtedly increased the degree of ‘holding’ described by Rouphael and Inglis (1995). This was observed particularly among underwater photographers and their models who the latter authors found to be the most destructive of divers in their study. Diving gloves are worn to protect the hands from injury on the reefs at Sodwana Bay and a prohibition on their use has thus been recommended to reduce this type of damage. A number of minor or ‘spot’ COTS outbreaks were encountered during the study at the deep periphery of Two-mile Reef (Schleyer, 1998) and a few COTS stations were included in this study, providing the highest levels of damage encountered (Fig. 5F). Details of the damage are given by Schleyer (1998) and it is of note that both soft and stony corals were affected. A detailed investigation of the outbreaks has commenced. The damage caused by disposed fishing line can be alleviated to some extent by the education of the angling community, an action which has been advocated to the conservation authorities. Note that bottom fishing is banned in this reserve. However, this is a localized problem which has become overshadowed by that caused by the burgeoning diving industry. Game fishing diminished simultaneously with the increase in diving at Sodwana Bay (Table 1) when many ski-boat anglers and spearfishermen again started visiting the less fished waters of Mozambique at the end of the civil war. The degree to which recreational diving has increased at Sodwana Bay raises the question as to whether the present diving intensity is sustainable or acceptable in terms of reef damage. The local sport divers complain of diver damage at sites at which it exceeds the natural background level by more than 10% (pers. observ., the arrow in Fig. 5C). The predicted diving intensity at which this occurs is 9200 dives p.a. This provides an aesthetic limit to the sustainable diving capacity of the reefs but gives no indication as to the diving intensity which will be sustainable. Relatively few of the injuries observed in a study such as this probably result in death and the aesthetic limit is usually chosen to prevent a protected area from being ‘loved to death’ (Davis and Tisdell, 1995). Nevertheless, coral growth and reproduction may be reduced after damage (Allison, 1996) and the frequency (Rouphael and Inglis, 1995) and SCHLEYER AND TOMALIN: DAMAGE ON SOUTH AFRICAN CORAL REEFS 1039 extent (Meesters et al., 1996) of damage will affect colony survival. Application of the aesthetic limit, in such a case, constitutes a conservative approach (well developed in fisheries science as the precautionary approach, FAO, 1995) and should probably be adopted for the limited coral communities in South Africa at the limits of their African distribution. The inverse correlations in Figure 6 provide managers with a means of determining the risk of various levels of diving intensity causing diver damage above levels they consider acceptable. For example, it predicts a 41% probability of a diving intensity of 7000 dives p.a. causing >10% increase in the damage on Sodwana reefs, but this probability is reduced to 27% at 5000 dives p.a. Obviously more diving can be allowed if a greater percentage diver damage is considered tolerable. We are of the opinion that a diving intensity of 7000 dives p.a. would probably be acceptable and sustainable at sites not vulnerable to damage for two reasons: On average, it would increase reef damage by >10% (Fig. 6) and it is close to the level of 5000–6000 dives p.a. that Hawkins and Roberts (1997) found caused little damage at sites in the Red Sea and Dutch Antilles. Regardless of the damage and diving limit selected, the reefs should be surveyed at regular intervals to monitor the damage levels, refine the estimates presented in this study and adjust the diving limits to the sustainable diving capacity accordingly. In addition, the distribution and intensity of diving effort should be monitored and a record kept of diver satisfaction (equivalent to catch per unit effort) in a manner analogous to fisheries monitoring. We believe that this is the first time that concepts of fisheries assessment have been applied in this field. However, our analysis here is very simple compared to most fisheries models: (1) only linear models were considered while non-linear models are routinely applied in fisheries analyses; (2) the data were not transformed by log transformation which is routine in stock assessments; (3) allowances were not made for factors such as the proportion of novices using the different dive sites, equivalent to standardizing effort in fisheries (e.g., weighting fishing effort by vessel horsepower); (4) we took only limit reference points and not target reference points into consideration, the latter allowing optimization of diver satisfaction subject to damage constraints; and (5) we used an equi- librium model, dynamic models being the current norm in fisheries analyses. The last- mentioned was unavoidable as we do not know how long damage remains visible. At this stage in the development of predictive models on diver-coral interactions, we consider simple models to be more robust and more likely to be convincing to managers and users. One recent concept in fisheries management that we have applied here is that of the precautionary approach. We believe that, as in fisheries, the users should bear the burden of proving that they do not have a deleterious effect, i.e., the question is not whether the slope of a regression of damage against dives is significant but rather what is the magnitude of the slope. This is equivalent, in fisheries management, to assuming that there is a relationship between fishing and a decline in stock or between recruitment and stock size (Garcia, 1995). Ignoring the latter leads to overly optimistic predictions of sustainable yield (Punt, 1997). We thus feel justified in excluding COTS and fishing line damage as well as data for Anton’s reef from the regressions. We have also explicitly included uncertainty in our results, albeit in a statistically simplistic manner, as is the norm in modern fisheries assessment (Smith et al., 1993). We believe our findings are comparable with those of Hawkins and Roberts (1997), despite the study areas being so different. They found that 5000–6000 dives per site p.a. caused little damage in the Red Sea and Dutch Antilles. Our regression predicts that this 1040 BULLETIN OF MARINE SCIENCE, VOL. 67, NO. 3, 2000 diving intensity would, on average, increase reef damage by less than 5%, and that there would be a 25% chance of the damage being increased by 10% (Fig. 6). This similarity in results is probably attributable to a dive ‘site’ being a unit of fairly uniform size; the activities of divers are usually focused around moorings and the dive duration and dis- tance are limited by SCUBA cylinder capacity. While mooring is not permitted at Sodwana Bay, diving generally centres around the first boat to arrive in a popular area. The appar- ent capacity of the Sodwana reefs to tolerate a slightly higher diving intensity might be attributable to their atypical community structure; the flexibility of the predominant soft corals must make them fairly resilient to diver damage. Four-mile Reef exemplifies an exception to the sustainable diving capacity generally recommended for the Sodwana reefs. Although large (Fig. 1) and presently subjected to a low diving intensity (Table 2), it is relatively deep (Table 4) and typified by more colonies of branching Acropora than the other Sodwana reefs (Riegl et al., 1995; Schleyer, 1995, 1999, in press). The management authority (KZNNCS) has thus been advised to permit only more advanced divers onto this reef, prohibiting access to divers not qualified to operate at its depth and thus limiting damage to its corals (as well as the possibility of litigation against the KZNNCS in the case of a deep diving accident). A lower limit may have to be imposed on the number of dives at popular sites on this reef in view of its community structure; monitoring for reef damage will be essential as the diving intensity increases. This is likely as the proposed diving limits will necessitate a redistribution of the diving load at Sodwana Bay, reducing the heavy intensity of this activity on Two-mile Reef (Tables 2,3). Similar recommendations have been made concerning a modification of the sustainable diving capacity for the other reefs in terms of their depth and vulner- ability to damage. The level of qualification of a diver does not necessarily provide a true reflection of a diver’s ability; dive school standards vary as well as the regularity with which a diver practices the sport (Davis and Tisdell, 1995). However, Rouphael and Inglis (1995) found that most diver damage is caused by relatively few individuals and suggested that pre- dive briefings, both during and after training, would do much to alleviate this. The ben- efits of this approach were confirmed by the findings of Hawkins and Roberts (1997). These factors will have to be taken into account in formulating the reef management policy and a system of divemaster accreditation is recommended. Proposals have thus been made to ameliorate human damage on the Sodwana reefs by (1) modifying the behavior of users and (2) restricting their activities to an aesthetically sustainable level which takes the fragility of the different reef biota into consideration. Implementation of the proposals should be accompanied by their refinement through further monitoring of reef damage, site-specific diving intensities and sport diver perceptions of reef damage.

ACKNOWLEDGMENTS

We are grateful for the support which staff of the Oceanographic Research Institute and volun- teer divers provided in the field. Special mention must be made in this regard of T. Kay for his professional seamanship in operating the research craft, often under difficult conditions, and for his back-up and friendship over the years. The KwaZulu-Natal Conservation Service provided accom- SCHLEYER AND TOMALIN: DAMAGE ON SOUTH AFRICAN CORAL REEFS 1041 modation for the field work and participated in funding the study with the South African Associa- tion for Marine Biological Research. BP (Southern Africa) kindly sponsored the research craft used for the study and the Mazda Wildlife Fund sponsored a 4 × 4 vehicle.

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DATE SUBMITTED: January 28, 1999. DATE ACCEPTED: May 15, 2000.

ADDRESS: Oceanographic Research Institute, P.O. Box 10712, Marine Parade, Durban 4056, South Africa. Corresponding author: (M.H.S.) E-mail: .