Chagos is home to at least 784 species of fish that stay near to the shores of the islands including the endemic Chagos clownfish (Amphiprion chagosensis) and many of the larger wrasse and grouper that have already been lost from over-fishing in other reefs in the region.
As well as the healthy communities of reef fish there are significant populations of pelagic (open ocean) fish such as manta rays (Manta birostris), sharks and tuna. Sadly, shark numbers have dramatically declined as a result of illegal fishing boats that seek to remove their fins and also as accidental by-catch in the two tuna fisheries that used to operate seasonally in the Chagos. Prior to the declaration of the Chagos Marine Reserve and the subsequent ban on fishing, skipjack (Katsuwonus pelamis) and yellowfin tuna (Thunnus albacares) were caught for about two months of the year as their year-long migratory route took them through Chagos waters. The international fleet that followed the schooling fish took 95,590 – 24,784 tonnes of target species (i.e., this figure does not include bycatch) per year while in Chagos waters. By establishing the strictly no-take Chagos Marine Reserve, populations of all these species have been freed from fishing pressures as long as they were within the reserve’s protective boundaries.
Bigeye tuna (Thunnus obesus) are deep water fish, occupying almost all the world’s oceans within about 45º of the equator. With a diet of fish, cephalopods and crustaceans, and a migratory nature, the adults roam deeper waters whilst juveniles often associate with floating objects acting as fish aggregation devices. As one of the larger tuna-fishes they can reach 2.5m in length with up to 200kg of high performance muscle. Tunas have an unusual amount of red, blood-rich sprinting muscle (hence the colour of sashimi) which makes them some of the fastest fish in the oceans. Bigeye tuna are one of the most sought after and commercially important species of tuna, and as such, the IUCN lists them as Vulnerable and many stocks globally show signs of depletion.
The silky shark (Carcharhinus falciformis) has a cosmopolitan distribution and can be found around the world in tropical waters. It is most often found near the edges of continental shelves, from the surface to a depth of 50 m. It can grow to a length of 2.5 m (8.2 ft) and has a metallic bronze-gray colour on its upper body. It is a mobile predator that feeds on bony fishes and cephalopods, and aggregations of silky sharks are often found trailing schools of tuna. As a consequence it is one of the most common shark species caught as bycatch in tuna longline and purse seine fisheries operating throughout its range. This species is hunted for its fins, with much of the product being sold at the Hong Kong shark fin market. Data now suggest that silky shark numbers are declining in many areas, and the IUCN assessed the global silky shark population as being Near Threatened.