The site is a remote islet in the western Indian Ocean, roughly 480 km north-north-west of Mauritius and 390 km east of the Masoala Peninsula, Madagascar. The islet is low, flat and coralline. The native vegetation is in good condition, composed of Tournefortia argentea thicket and 3–5 species of herb. The island is an unmanned Nature Reserve, but has a meteorological station with four permanent staff. It is frequently affected by cyclones.
Two seabird species nest, both boobies (Sulidae). During 1993–1996, 200–250 pairs of Sula dactylatra nested (with up to 700 birds present, and increasing), alongside 130–180 pairs of S. sula (up to 500 present, declining). These populations comprise less than 1% of the global populations, but the site is considered to be an Important Bird Area for the following reasons: first, Sulidae populations in the western Indian Ocean have declined seriously and these are among the healthiest in existence; second, S. dactylatra is represented in the western Indian Ocean by an endemic race (S. d. melanops), of which Tromelin is one of the strongholds; and third, the S. sula population at Tromelin is the only polymorphic one in the region, indicating uniqueness and biogeographical isolation. Fregata ariel became extinct before 1968 and F. minor in the early 1980s, but up to 120 Fregata spp. use the island for roosting. Resident landbirds are absent.
Non-bird biodiversity: The islet is a nesting site for the sea-turtle Chelonia mydas (EN).
BirdLife International (2022) Important Bird Areas factsheet: Tromelin. Downloaded from http://www.birdlife.org on 16/08/2022.