Records of rainfall are extremely important for all aspects of science and conservation on Aldabra. Complete rainfall records can help explain population cycles, animal behavioural patterns and many other dynamics that occur on Aldabra. Nearly all aspects of Aldabra are somehow affected by the amount of precipitation, making the regular recording of rain gauge readings a top monitoring priority.
The status of the tortoise monitoring programme was reviewed during the tortoise population study conducted by ERGO (Environmental Research Group Oxford) at the end of 1997. The review concluded that the basic method of transect sampling is the most appropriate means of monitoring the various sub-populations of tortoise on Aldabra.
A total of twelve transects are monitored on a monthly basis. Three of these transects were established in the late 1970s in the Cinq Cases region, and the other nine were established in 1995 at various sites on Grande Terre, Malabar and Picard.
The primary objective of the tortoise monitoring programme is to provide information required for better understanding, management and conservation of Aldabra’s giant tortoise population. Regular assessments of the tortoise population, including the sub-populations on Grande Terre, Malabar and Picard, together with maintenance of long-term records and a computer database, will allow for identification and examination of major changes and trends in population size and structure. This in turn allows periodic review of management implications and options.
The nesting of Green Turtles (Chelonia mydas) was monitored sporadically on Aldabra between 1968 and 1981, and more consistently using standard methodologies for morning beach surveys and nightly turtle tagging since 1981 (Mortimer 1988). Studies of growth rates and migrations in foraging populations of immature hawksbill turtles (Eretmochelys imbricata) have been ongoing since 1987.
Three types of turtle monitoring are conducted on Aldabra: a) morning beach surveys to count turtle tracks; b) nightly tagging of turtles; and c) tagging and measuring of immature foraging populations within the lagoon. Beach surveys give us an overview of various aspects of nesting activity on Aldabra including: its spatial distribution around the atoll; the seasonality of nesting within the calendar year; and how nesting activity fluctuates from one year to another. With these data we can better understand the status of the nesting population - whether it is increasing, decreasing or stable.
By tagging nesting females we learn about aspects of turtle behaviour that include: nesting site fidelity; nesting frequency within a nesting season (how many clutches laid and how many days between nestings); how many years separate the nesting seasons of individual turtles; and locations of distant foraging grounds for the population. International tag returns provide an index of the rates at which turtles are slaughtered at these foraging grounds.
Monitoring the foraging populations of immature turtles in the lagoon reveals information on the size and distribution of the population, growth rates, and migrations.
The coccid (mealy bug), Icerya seychellarum, was introduced accidentally sometime in the 1960s and impacted the vegetation extensively, threatening the survival of some woody species. Surveys and research were conducted during the late 1970s by Newbery and Hill, and a monitoring programme was set up in 1980. In the late 1980s a coccinelid (ladybird) Rodolia chermesina was introduced as a biological control agent.
The main purpose of the coccid monitoring program has been to assess the level of infestation of Icerya on various woody plant species and to reveal any coccid population fluctuations which might occur. It also aimed to assess the effectiveness of Rodolia as a biological control agent on Aldabra.
These assessments were needed for management purposes. Scientifically it has also been an unusual study of an insect pest invasion in a relatively undisturbed environment where other factors are also being monitored. The ongoing has been curtailed and the accumulated data now require full analysis.
White-throated rail monitoring
The white-throated rail (Dryolimnas cuvieri aldabranus) is the last “flightless” avian species in the Indian Ocean. Populations currently occur on Malabar and Polymnie islands, as well as a newly introduced population on Picard. In 1995 a monitoring programme was set up by Augeri and Pierce to census some of the main sub-populations. The collected data formed a useful baseline assessment of the rail population on Aldabra prior to a full scale scientific study of the rail which resulted in the successful reintroduction of the bird on Picard.
The monitoring of vegetation is important because so many terrestrial organisms depend on plants and their productivity, either directly or indirectly. “The dominant factors explaining the species composition of Aldabra’s vegetation are the degree of influence from salty ground water and the degree of shelter from the salt laden south-east trade winds, which blow for a significant proportion of the year” (ERGO 1997). Vegetation transects were established by Gibson and Phillipson (1983) in the Cinq Cases area, but they represent only a small sub-set of the mixed scrub vegetation in that part of the atoll. Some of these transects were re-sampled in 1988 by Scoones et al. (1989) and again in 1997 by the ERGO group (1997). Although the 1983 field studies were carried out in the rainy season, the 1988 and 1997 surveys were undertaken in the dry season, when many ground flora species are not detectable and some woody species lose their leaves and are difficult to identify.
There is a need, therefore, to repeat the four transects which were surveyed in 1997 during the rainy season. Vegetation monitoring generally only needs to be undertaken every five to ten years, preferably in the wet season.
Subsistence fishing monitoring
The purpose of the fish monitoring programme is to obtain information on the types, numbers and weight of fish caught around Aldabra for consumption purposes. Petrol consumption, the number of people fishing and the number of hours spent fishing are recorded so that an indication of “fishing effort” can be gained for different sites and different months of the year.
The results can also be analysed to reveal spatial and temporal trends in, for example, relative densities and size ranges of different edible fish species. Such information can be used for management purposes (for example, reducing the pressure on certain target species) as well as providing useful information for the scientific understanding of edible fish populations.
Beach erosion and accretion
A new programme was established by Augeri and Pierce in 1995 to quantify beach loss and accretion. The study began by focusing primarily on key turtle nesting beaches to help researchers quantify any correlation or causal relationships with seasonal and long-term shifts in nesting activity around the atoll.
In addition to quantitative measurements one of the best methods for evaluating and monitoring beach loss is through photographic benchmarks.
The main purpose of monitoring of beach erosion and accretion is:
- To help quantify any long-term changes in coastal zone habitats.
- To quantify beach dynamics relative to critical turtle nesting habitat.
- Photographic benchmarks will provide reliable references for both management actions and scientific studies.