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Mauritius is a volcanic island in the Indian Ocean approximately 2,000 km off the coast of Africa. The island is surrounded by 240.4 km2 of coral reefs (Turner & Klaus 2007). The study area is focused on the coastal waters along the edge of the coral reef to a maximum of four km offshore (> 1000m depth). Two week-long surveys were conducted around the entire island in addition to more concentrated effort in three specific areas: Trou aux Biches (between Grand Bay and Port Louis), the North (between the mainland and two offshore islands: Gabriel and Flat Island) and Mahebourg lagoon in southeast Mauritius (Fig. 1). The current study was part of a larger project focused on cetacean populations, so the specific areas were chosen based on previous cetacean sightings.
Figure 1. Map of Mauritius indicating sites and distribution of survey effort and turtle sightings.
Line transect surveys were conducted in 2013 and 2014. Observations were made from a number of different vessels including a 14-m catamaran, a 5-m dual engine pirogue (traditional fishing boat) and speed boats. The average speed of surveys was kept relatively consistent despite using different boats, and varied between 9.5 km h-1 and 13.5 km hh-1. A transect was defined as each ‘on-effort’ track. Therefore, on each day there could be several transects of varying length. As the primary focus of the study was cetaceans, the transects depended on marine mammal sightings that occurred. In addition, surveys were abandoned if sea conditions and/or visibility deteriorated, as such no transects were repeated.
In general surveys focused on the area behind the reef out to the 50 m contour although deviations were sometimes necessary into deeper water to avoid fishing activities (fish traps with buoys). However, when time and sea conditions allowed, surveys were extended out as far as the 2,000 m depth contour. In general, between two and six observers scanned the ocean for megafauna. Environmental variables that could affect detectability, including Beaufort sea state, swell and cloud cover (oktas) were recorded every 15 minutes and for all sightings. When possible, the depth at the boat was recorded for each sighting. The vessel was not stopped or diverted for turtle sightings to avoid double counts.
The sampling effort differed at the three locations. More surveys were done at the North area compared to the other two locations since this area is known to have more cetacean and boating activity and likely to have more impacts on cetacean and turtle populations. The angle and distance from the boat to the sea turtle were estimated. The turtle sighting locations were recorded using a global positioning system (GPS) device. For each sample location, a density estimation of sea turtle populations was created using standard distance sampling in DISTANCE 6.0 (Thomas et al. 2010). The perpendicular distance was calculated from the angle and the distance to the boat. Right truncation was used to delete unreliable data and outliers that are hard to model; this included the largest 5% of the distances. Only one observation in Mahebourg lagoon at a distance of 148 m was removed from the analysis. The data were grouped into six intervals of 10-m distances for a better fit of the model. The detection probability function was modeled for the whole data set as we expect the same detection probability at all sites given the average sea conditions. Densities were then calculated for each location. Half-normal key function with cosine series expansion was used in the model (Fig. 2).
Figure 2. Detection probability function.
In total, 115 on-effort transects were conducted opportunistically, covering an area of over 400 km2. This included 36 transects covering the circumference of the island; 15 transects in Mahebourg lagoon, 47 transects in the North area, and 17 transects at Trou aux Biches. During the survey, 75 sightings of turtles were recorded (Fig. 1). On average for all surveys and sites sea conditions were good (Beaufort < 3, mean = 2) and swell was less than one meter. Cloud cover averaged 3.6 oktas during the study period. Depth was recorded for 76% (n = 57) of sightings. Turtles were seen at depths ranging from 2 m to more than 60 m (n = 5) with an overall mean of 19.2 m (n = 52). In 57% (n = 43) of cases, the turtle species was not identified due to distance and the short surfacing time. The remaining sightings included 27 green turtles and 5 hawksbill turtles; 45% of the records (34 sightings) were from the surveys around the whole island. Trou aux Biches had the greatest number of sightings with 21 turtles observed, while there were 16 in the North area and four at Mahebourg.
Table 1. Density/abundance of sea turtles per site (% CV = coefficient of variation, d.f. = degrees of freedom, CI = confidence interval). Density measured as turtles per km2; abundance = number of turtles estimated per location.
According to the DISTANCE results, the highest density of sea turtles was in Trou aux Biches with 0.87 turtles per km2, while the lowest density was at Mahebourg with 0.20 turtles per km2. The results from the island perimeter surveys and the North area indicate densities of 0.47 and 0.40 turtles per km2, respectively (Table 1).
The study was designed to provide preliminary results for the sea turtle density in Mauritius. We were not able to provide separate density estimates for the two species of marine turtles as it is difficult to identify species at sea even for well trained and experienced observers. According to the results of this study the green turtle was the species more often recorded. However, this result may be related to the bigger size of the species, greater abundance and higher detection probability due to behavioral differences between the species (National Research Council 2010).
The results demonstrate that Trou aux Biches had the highest turtle density. This may be associated with food availability and shelter in the area. Coral ecosystems at this site have remained relatively stable with little change in coral cover (Ahamada et al. 2002). The area includes the Balaclava Marine Park, which was gazetted in 1997. Based on the results, we can assume that the area is an important site for sea turtle aggregation. However, more detailed and systematic research, that includes other important bay areas like Black River Bay (Webster 2013), is needed for more reliable results. In the North area, the most turtle sightings were recorded around the two offshore islands: Gabriel and Flat. The aggregation of sea turtles in the area may be related to a possible nesting site. Gabriel Island is uninhabited and has suitable nesting beaches while Flat has suitable beaches and a small number of Coast Guard personnel are the only permanent human presence on the island. There are however, daily boat trips taking guests to a restaurant and barbeque facilities on both islands. During the course of routine monitoring trips MMCS staff have observed evidence of turtle nesting activity on these islands in the form of body pits. There have also been similar body pit observations made by other wildlife conservation organizations and reports of hatchlings from locals and the Coast Guard.
As with most marine animals, sea turtles can be difficult to survey, especially in Mauritius where little or no nesting activity is observed and the use of sampling methods like female nesting counts is impossible. The density estimation results presented here are the minimum (surface) densities of the species. We were not able to sample all the turtles on the transect line as the submerged animals were missed. For future analyses we recommend estimating a multiplier for the DISTANCE analyses as a proportion of time spent above and below the surface for each species (Beavers & Ramsey 1998). Furthermore, the sampling effort at the different locations were not standardized, which may also be a source of bias for the density estimations. Transects were conducted opportunistically according to the weather conditions and availability of resources. Moreover, the surveys were part of a broader Biodiversity Project and sometimes the research vessel followed marine mammals in order to observe their behavior. It is difficult to predict how well an in-water survey represents the true density of marine species. Using integrated methods like line transect surveys and satellite telemetry may provide a better understanding of habitat use and provide linkages between surface densities and the characteristics of the study areas (National Research Council 2010).
Within the region, there is a lack of knowledge from Mauritius and the isolated islands of Rodrigues, Agalega and St. Brandon with regards to nesting, and the importance of these areas, their status, and their relation to the known regional management units. In addition, there is limited knowledge on recruitment, site preference and selection within Mauritius as well as spatial dynamics of oceanic migrations within the region. However, the current preliminary study is providing the first estimates of sea turtle densities in Mauritius, which can be used as a baseline for future research and conservation efforts. A broader survey on marine turtles is essential for a better understanding of the conservation status in the country and adequate protection of the species. Identification of important turtle sites is needed in order to provide more adequate protection of the species as well as for the coral reef. Sea turtles play an essential role in sea grass communities and coral reef ecosystems (Pandolfi et al. 2003). Their decline can lead to dramatic changes in food webs and the entire marine ecosystem (Bjorndal & Jackson 2003). The Mauritius Marine Conservation Society, along with other local marine NGOs, is working to raise the public awareness regarding the importance of the sea turtles and to develop further research (Webster 2013). MMCS is actively working toward an adequate and effective sea turtle conservation management in Mauritius.
Acknowledgments. We thank all the volunteers of the Mauritius Marine Conservation Society who contributed to the data collection. Thanks to the Global Environment Facility - Small Grants Programme for providing funding, and local partners for both Corporate Social Responsibility contributions and logistical support. Also, thanks to the skippers/boat owners who provided access to boats in the different areas. Thank you to Albion Fisheries Research Centre Government of Mauritius for approving the Biodiversity Project “Study on the biodiversity and abundance of cetaceans around Mauritius” and providing the necessary permit (F 20/2/5).
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