The future of seabirds and turtles in St Brandon

By Nik Cole

At the end of October I had the opportunity to accompany two researchers of the North-West University, Potchefstroom, South Africa on an eight day expedition to St Brandon. The expedition was in conjunction with the University of Mauritius to investigate levels of persistent organic pollutants (POPs), such as dioxins, PCBs and DDT in seabird eggs.

The islands of St Brandon, also known as the Cargados Carajos Shoals, are situated approximately 430 km north-east of Mauritius.

Currently, there are at least 35 islands and sand bars ranging in size from an estimated <0.1 to 138 ha, but the size and number of the smaller sand bars are not static and change during storms. Only three of the main islands are inhabited with a rotating population of 40-50 people that are based mostly on Ile Raphael. The main activity is fishing, such that the majority of the islands are relatively undisturbed and represent some of the most important islands for seabird and green turtle nesting grounds in the Republic of Mauritius.

Although the main objective of the expedition was to investigate POPs, we also conducted marine debris surveys, seabird and shorebird population surveys, turtle track counts and surveyed the islands for invasive vertebrates. A total of 23 islands were accessed and a further eight small islands were surveyed from the boat.

From surveys made in 1997, Red-Footed Booby and Wedge-tailed Shearwater were thought to have disappeared. Greater and Lesser Frigate birds, Roseate Terns and Masked Boobies were also thought to be in decline. The cause of these losses and declines were mostly attributed to poaching and disturbance by fishermen. In the last decade Red-footed Booby have been rediscovered and during our expedition we recorded a new island nesting site. We have also found that the Roseate Tern population has increased since 1997, but we found that the number of Lesser Frigate birds had declined and saw no Greater Frigate birds. In 1997, it was feared that continued poaching of turtles and nests in the region would cause continual declines. Fortunately, it appears that turtle and seabird poaching has declined to a level that it is undetectable, as we found no recent evidence of persecution and on undisturbed islands the number of turtle nesting attempts has increased since counts were made in 1997.

The 1997 surveys recorded the presence of chickens, rabbits, rats, mice, cats and geckos, which had been introduced. We also recorded the presence of these introduced species on a number of islands, but fortunately it appeared that cats are no longer present. Whilst the ubiquitous house gecko (Hemidactylus frenatus) is almost certainly introduced we also confirmed the presence of a second species, the mourning gecko (Lepidodactylus lugubris), which may be native, as it is on Rodrigues.

On some islands the decline and absence of nesting turtles and seabirds appears to be catastrophic and we believe that the cause of this is predation by the introduced black rat. The introduction of rats is one of the most significant causes of species loss from islands around the world and their impact is no different in St Brandon.

In 1997 rats were reported on the 5 ha island Poulailler, which had a population of approximately 10,000 Sooty Terns, considerably lower than the 40,000 reported in 1975. During our survey we only saw three visiting shorebirds and numerous rats; the island was effectively dead. We found the same situation on another four rat-invaded islands that should have been teaming with bird life. Furthermore, turtle activity on these islands was barely evident.

In 1997, the Sooty Tern population on the 38 ha island Puits a Eau was estimated at 30,000 and only three turtle tracks were recorded. Today, with poaching no longer evident, the Sooty Tern colony appears to have increased and 72 turtle tracks were counted. It was therefore very disappointing to discover the footprints of rats on the island.

It is of no great surprise that rats are present on the main inhabited island, Ile Raphael, but here the fishermen control the rats to enhance the productivity of their chickens. Consequently, the island still supports Fairy Terns, Common Noddies and Lesser Noddies. Ile Raphael is the main port and from where most of the other islands are accessed by boat. The risk of rats reaching other islands is therefore only a matter of time. For the protection of these important seabird and turtle nesting grounds the eradication of rats from St Brandon must be considered.