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Shallow water habitats

A biodiversity hotspot

Lucie Machin
Lucie Machin

Dive Deeper

The shallow seas around Ascension Island contain a mixture of rocky volcanic reef, boulder field, sand and rhodolith bed habitats.

There are no pronounced seasons because of the tropical location, and the temperature of the water at the surface is between 22 and 29oC all year round. That doesn't mean these shallow habitats are tranquil, however. The entire island is exposed to the Atlantic South Equatorial Current and trade winds from the south-east, and, though the north-west coast of the island is slightly more sheltered, all areas experience strong currents and heavy swells. It is these shallow-water habitats that support the highest diversity of species found in Ascension’s MPA. Including a total of 133 coastal fish species, the ancestors of which came from both the east and west coasts of the Atlantic, potentially reaching Ascension as larvae travelling on currents. In addition, species that are typical of pelagic habitats such as tuna and marlin are found very close to the island due to the proximity of deep water. These also play a role in shallow-water ecosystems.

The isolation of the island has resulted in the evolution of 11 endemic fish species, which are found nowhere else on earth. A further 20 fish species are shared only with St Helena and St Peter and Paul’s rocks (Wirtz et al., 2014). Taken together, this means the mixture of fish species found in the shallow waters around Ascension is unique.

Anyone visiting Ascension Island is immediately struck by the abundance of marine life. The number of black triggerfish is particularly astounding. Their combined grazing and scavenging activity shapes inshore habitats by preventing the growth of soft corals or algae and favouring hard coralline algae and rhodolith around Ascension’s coasts (Price & John 1977). The high abundance of predatory fish also encourages nocturnal and cryptic behaviour in potential prey species and leads to the sharp contrast between the species active during the day and at night.

The other striking feature of Ascension’s shallow-water habitats is the presence of high numbers of large, predatory fish that seem to defy conventional wisdom about abundance decreasing up the food chain.

This phenomenon is also seen globally in the small number of other sites where fishing pressure has never been high. What can initially seem a strange excess of large fish may be the natural state for shallow water areas, and Ascension could serve as a model example of how all the worlds oceans should look.