The Effect of Physical and Human Factors on Beach Selection by Green Turtles (Chelonia mydas) at Ras Al-Hadd Reserve, Oman
ABSTRACT
Beaches at Ras Al-Hadd Reserve, Oman, share common physical features ideal for nesting green turtles (Chelonia mydas). However, human activities related to commercial fishing and coastal development impact nesting. Beaches with hills as a backdrop and with minimal human activities were the primary nesting sites at the reserve. During peak nesting season (monsoon/wet period), the mean number of excavation attempts was equal between oviposited and nonoviposited turtles. During nonpeak season (dry period), the mean number of excavation attempts was significantly higher in nonoviposited turtles than in oviposited turtles, and insufficient sand moisture frequently resulted in one or several nest collapses and oviposition failure.
Sea turtles are facing a continuous destruction of major nesting grounds in the world due to steady increase in human activities such as sand mining, dredging, beach armoring, fishing activities, beach erosion, pollution, and excessive use of artificial light (for review, see Lutcavage et al. 1997). Steinitz et al. (1998) reported that in Florida, beach renourishment creates inferior nesting habitats compared to natural nesting beaches. In addition, beach renourishment can cause the development of steep berms just above the surf zone, which prevents turtles from reaching their nesting sites (Steinitz et al. 1998).
During the last 5 years, steady increases have occurred in the human population of Ras Al-Hadd as well as in tourism. Tourism has almost doubled during the last 3 years according to records from the reserve office. Moreover, fishing activity has increased, particularly during the last 3 years. This involves offshore trawling, primarily for tuna. The steady increase in human activities is the main threat to the nesting beaches at the reserve.
Studies on beach selection in sea turtles suggest that, in addition to human factors, nesting frequency is influenced by physical and biotic factors. The physical factors include sand texture and density (Stancyk and Ross 1978; Horrocks and Scott 1990; Mortimer 1990; Kikukawa et al. 1999), offshore accessibility to the beach (Hughes 1974; Mortimer 1982; Mrosovsky 1983), beach dimensions and geomorphology (Mortimer 1982; Johannes and Rimmer 1984), and biotic factors (Al-Kindi et al. 2001).
Ras Al-Hadd Reserve is located on the Gulf of Oman and Arabian Sea, which hosts one of the largest concentrations of nesting green turtles in the world (Ross 1979). According to Salm and Salm (1991), at least 275 beaches are present along the entire coast of the Gulf of Oman and the Arabian Sea, with 90% of nesting in the area found on the Arabian Sea coast. Thousands of sea turtles nest year-round, predominantly in discontinuous isolated sandy beaches, backed and separated by rocky hills that give complete isolation to each beach. Generally, nesting and feeding grounds follow the same pattern of distribution (Ross and Barwani 1982; Salm and Salm 1991). However, more study is needed, especially with a mark–recapture program, which is already underway at the reserve.
The purpose of this study was to evaluate nesting distribution and frequency by green turtles at Ras Al-Hadd relative to physical and biotic factors—including human activities—during peak and nonpeak nesting periods. This research is part of a comprehensive ongoing project, which includes nesting behavior, hatchling and emergence success, population dynamics, migration, and reproductive physiology.
Methods
Ras Al-Hadd Reserve is a 45-km coastal area on the Gulf of Oman and the Arabian Sea with over 20 major beaches that extend from Al-Jarama Bay to Ras Ar Ruays (Fig. 1). Most beaches are isolated at the back and sides by rocky hills, with a crescent shape configuration. As a result, each beach is separate from adjacent beaches. The dimensions of the major beaches range between 1.0 and 7.0 km shoreline length and 50–150 m in width, while the minor beaches have dimensions of 50–100 by 10–40 m. The beaches have deep, open sandy offshore approaches, typically free of submerged rocks. The main beaches consist of sand with medium-size particles and gentle slopes (0.5–5.0 m), and are free of rocks or major barriers, with sparse or no vegetation. The beaches are exposed to moderate- and high-energy wave action, especially during monsoon season.
Citation: Chelonian Conservation and Biology 5, 2; 10.2744/1071-8443(2006)5[289:TEOPAH]2.0.CO;2
The total area of each nesting site is based on the total shore length and width of the beach where the turtles dig their nests. The depth of the nesting area in all sites ranged between 50 and 60 m from the high tide zone. Nesting density was based on the number of nests or tracks per 10002 m of nesting area.
Site 1 (Ras Al-Jinz) is surrounded by rocky hills at the back and sides of the beach. The back hills are about 100–200 m from the tide zone. The site is about 1 km of shoreline divided by rocky hills (20–50 m height) in the middle, separating the site into north and south sectors. Only 50 and 60 m of the beach width is used for nesting. Commercial fishing is limited to the southern sector and only from November to April.
Site 2 (Saih Al-Marai), like nesting Site 1, is well isolated by hills; however, the hills are farther away (300–400 m) from the tide zone. The width of the nesting area is 50 m with shoreline length of 1 km. Commercial fishing occurs year-round.
Sites 3 and 4 have unsheltered beaches, without any hills in the back or sides of the beaches. The beaches slope gently from the shorelines, and in certain areas, there are steep slopes toward the landsides, obscuring the sea horizon completely from the nesting turtles' view. The shoreline length of both sites extends for 6.9 km with an average nesting area width of 50 m. In this study, the data from Sites 3 and 4 were combined because of the sparse nesting density, similarities in their physical properties, and the level of human activities.
In addition to the four primary sites, other beaches (n = 11) close by in the reserve also were observed for nesting frequency. Population density is based on 1000-m2 area for each study site and was calculated by multiplying shore length × nesting beach width. The beach width is defined as the area where nesting takes place, and this can be recognized by the presence of abandoned body pits. Data on nesting activity relative to shoreline length were based on selecting 11 different nesting beaches. These 11 beaches are located south of Site 1 and between Sites 1 and 2. They have the same main physical characteristics found in the other study beaches relative to the offshore approach, beach slope, and sand texture. The number of complete nests was then counted per 1000-m2 area so a uniform density count could be compared between the 11 beaches. Observations on narrow beaches (< 8 m wide) for the presence of nesting turtles or their tracks were based on 35 observations at 7 different beaches located south of Site 1. In these areas, rocky hills were very close to the shoreline.
According to Ross (1979), the most important factors that control nesting activities in Oman are the southwest (SW) and northeast (NE) monsoons. The SW monsoons (May–September) bring high winds to the coastal region and cause a large clockwise flow along the coast of the Arabian Sea, which breaks down into a series of unstable gyres. The large water surface movement causes upwelling along the Somalia and Arabian coast where water temperature falls to 13°–21°C and inorganic levels increase significantly, which consequently increases food materials (Ross 1979). SW monsoons also bring misty air and add substantial moisture to the sand, making nest digging easier during this period. During NE monsoons (October–April), a reversal flow takes place into less intense counterclockwise gyres. Water temperature rises to 27°C, and the nutrient levels fall. At this time, the wind direction changes from north to south, causing high-energy waves with dry air, and nest digging becomes very difficult because of the dry sand.
Commercial fishing in the reserve is year-round and extensive, particularly near Sites 2, 3, and 4 (Fig. 1). Fishermen's shacks, nets, and boats in most areas are only 50–100 m from nesting sites. Fishing involves offshore trawling, which primarily targets tuna. The fishing activity occurs predominantly during morning (0600 to 1200 hours) and occasionally during late afternoon.
Assessment of human activities on the nesting beaches was based mostly on number of fishermen on the beaches and occasionally on tourists and other visitors. Counts took place from 0600 to 1200 hours and from 1600 to 2000 hours, which are the times of greatest human activity on the nesting beaches in the reserve. Each survey involved visiting the study beaches the same day. Thirty-four surveys were conducted during 1998–2000.
Between 1995 and 2000, the four sites were visited daily by the Omani government patrols who work for the Ministry of Regional Municipalities, Environment and Water Resources, Sultanate of Oman. Our research team was involved in part of the survey with the patrols, especially from 1998 to 2000. Tracks of turtles that either laid their eggs (crawls) or failed to lay their eggs (false crawls) were counted daily during early morning while the tracks were still fresh and undisturbed. Fresh nests that were associated with crawls were also counted daily.
Green turtles often made several nesting attempts, digging several body chambers. Some tracks were hard to trace—especially during peak nesting—and were not counted, therefore, in these cases we relied on fresh nests only. Turtles on the beach were tagged for other studies, and tagging was always done after the turtles finished nesting and were returning to the sea. Two patrols were employed to count tracks and nests daily during early morning. The present data on track or nest counts probably underestimate the actual number of turtles, but is the best estimate available for nesting frequency and density for each site, which reflects pattern of beach selection at the reserve.
The number of trials to excavate a body and nest chamber at different locations was monitored in reference to oviposition and nonoviposition during peak and nonpeak periods, which was based on nests of 62 turtles for each period. The number of abandoned body and nest chambers was counted for each turtle. The number of these excavated chambers reflects the number of nesting attempts a turtle makes at different locations.
A nonparametric method was used to determine the correlation between different nesting densities and the beach lengths. Independent and paired sample t-tests were used wherever appropriate, to compare differences in the means. Differences with p values less than 0.05 were considered statistically significant.
Results
Generally, the density of nesting (number of turtles per 1000 m2), as inferred from tracks or nest counts, was not uniform on the four study beaches. These differences have been consistent for the entire 5-year study period (Fig. 2).
Citation: Chelonian Conservation and Biology 5, 2; 10.2744/1071-8443(2006)5[289:TEOPAH]2.0.CO;2
Nesting density varied significantly and consistently between the four main sites. These differences were correlated with physical form of the beach, particularly the presence or absence of background and side hills, and also with the degree of human disturbance (Table 1). Site 1 with low human activities and hills had significantly higher nesting density (p< 0.001) than Site 2, which had high human activities and hills. Sites 1 and 2 also had significantly higher nesting density (p < 0.001) than Sites 3 and 4, which were associated with high human activities and flat shoreline topography (Fig. 2). Site 1 had the lowest human activities and also had significantly higher population density than the others (p < 0.001, Fig. 2).
The width of the nesting area in each of the 4 sites ranged between 40–50 m and started 1–2 m from the high tide zone; turtles seldom nested beyond this nesting strip in both sheltered and unsheltered beaches. There was no nesting on narrow beaches < 8 m in width (based on 35 observations on 7 beaches). Data obtained from 11 beaches with different shoreline lengths revealed that nesting density did not correlate with shoreline lengths during high or low density nesting periods (Table 2). In addition, nesting on the 11 minor beaches also revealed that beaches with surrounding hills and low human activities had a positive effect on beach selectivity. There was no monthly correlation between shoreline length and number of nesting turtles (Table 2) during peak and nonpeak periods.
During peak nesting, the percentages of oviposited versus nonoviposited turtles were not significantly different. In contrast, during nonpeak nesting, the differences were significant (p < 0.01, Table 3). In relation to oviposition, the number of unsuccessful attempts to excavate a nest was significantly higher in turtles during nonpeak over turtles during peak nesting periods (p < 0.03, Table 3). The peak period, which coincides with the SW monsoon, contains enough moisture to reduce the rate of nest collapse during the digging process and, consequently, fewer nest abandonments. In contrast, nest collapse and abandonments are significantly higher during nonpeak periods because of the dry northern wind, which reduces sand moisture.
Table 4 illustrates the pattern of human activities expressed in number of people observed on the beach. Almost all beaches had high human activities except Site 1 of the major sites and Site 10 from the 11 minor sites.
Discussion
The frequency at which green turtles select specific beaches depends on interaction between biological and physical factors. All beaches at Ras Al-Hadd share common physical features favorable for nesting. These are deep, sandy offshore approach without submerged rocks, gentle slopes without rocks, and medium and uncompacted sand texture. These features were also found to be conducive to nesting for green turtles at Ascension Island (Mortimer 1998). Most beaches at Ras Al-Hadd are bordered by rocky hills in the back and on their sides, and, thus, each beach is completely separate from adjacent beaches. This seems to be the most positive physical feature that attracts thousands of nesting green turtles to specific beaches at Ras Al-Hadd. At Sites 3 and 4, the beaches are unsheltered, with flat surrounding topography, which may be one of the reasons for low nesting frequency, particularly during high-density nesting periods. The presence of barriers such as hills or tree lines in the background may be preferable because it gives adults and hatchlings a clear view of the sea horizon (Mrosovsky 1983).
Excessive human activities and lack of beach isolation are the most negative factors that deter turtles from nesting. The nesting activity in Sites 3 and 4 was significantly lower than in Sites 1 and 2, and this may be related to gradual increases in human activities, especially during the last 5 years. Coastal development and commercial fishing are on the rise in all sites, but significantly so at Sites 2, 3, and 4. Fishing is year-round, except in the southern sector of Site 1, where it is limited from November to April. Furthermore, the expansion of Ras Al-Hadd village toward the nesting beaches in Site 2 has a harmful effect on nesting activity. There is substantial use of artificial lighting, littering, human traffic, and tourism, particularly in Sites 2, 3, and 4, and especially near the villages of Ras Al-Hadd.
With the uniform physical features of the beaches, it appears that the two main variables in beach selection at Ras Al-Hadd are the high human activities and surrounding topography (sheltered or unsheltered beaches). Overall, when shelter condition and high human activities were compared relative to nesting densities at the four sites, the results indicate that shelter condition had a greater impact on nesting density than high human activities.
Data obtained from 11 beaches with different shoreline lengths revealed that nesting density did not correlate with shoreline lengths during high and low density nesting periods. In addition, nesting on the 11 beaches also showed that sheltered beaches with low human activities were visited more frequently by green turtles. This suggests that beach length has no influence on beach selection in sea turtles as long as favorable physical features are present. Similarly, beach length had no influence on beach selection in loggerhead turtles (Caretta caretta) nesting in Japan (Kikukawa et al. 1999).
The difference in nesting density between Site 1 and Site 2 was not significant during peak nesting despite the high impact of fishing activity. However, during nonpeak periods, nesting density was higher at Site 1. In addition, lack of topography and high fishing activity year-round at Sites 3 and 4 apparently deterred the turtles from nesting on these beaches.
Peak nesting period in Oman coincides with the SW monsoon season, which brings moist air conditions to coastal shores of the Arabian Sea (Ross 1979). The sand at this time has enough moisture to significantly reduce the rate of nest collapse and nest abandonment. During nonpeak nesting periods, the sand contains low moisture because of dry air; thus, nest collapses and nest abandonments were significantly higher during nonpeak periods. Based on the lower rate of nest abandonment during peak periods, the percentage of successful nesting was significantly higher than during nonpeak periods. During nest chamber excavation, there must be adequate sand moisture to make the sand firm and to prevent repeated nest collapses and consequently repeated nest abandonments (Al-Kindi et al. 2003).
It is unclear why nonoviposited turtles during peak nesting had fewer trial attempts for nest excavation than during the nonpeak period. At Ras Al-Hadd, there are large numbers of nesting turtles present on the beach during peak period (200–400 per night) compared to 10–20 per night during nonpeak periods (Al-Kindi et al. 2003). Thus, during peak periods, the crowded conditions may cause unsuccessful turtles to return to sea with fewer attempts than during the nonpeak periods. More study is needed to clarify this point.
Ras Al-Hadd Reserve hosts one of the largest nesting populations of green turtles in the world (Salm and Salm 1991). More investigation is urgently needed on population structure and distribution of green turtles on the shores of the Arabian Sea. Moreover, because green turtles at Ras Al-Hadd nest year-round in large numbers on undisturbed beaches, comprehensive management and conservation programs must be established to minimize human influence, particularly during peak nesting periods.
Map of Ras Al-Hadd Reserve, Oman.
Monthly tracks or nest counts (per 1000 m2) between 1995 and 2000 at Ras Al-Hadd Reserve.
