Expedition report: Little and large: surveying and safeguarding coral reefs & whale sharks in the Maldives (September 2023)

Abstract

In September 2023, Biosphere Expeditions ran its 11th annual Reef Check survey expedition to the Maldives. International citizen scientists, supervised by a professional reef biologist, performed Reef Check surveys for one week at North and South Ari atoll at exposed, semi-exposed and sheltered sites. We repeated visits to sites last surveyed in 2018 in this atoll. In the 2018 survey, most of the reefs had not recovered significant coral cover from the 2016 mass coral bleaching event.

Outer reef sites had not changed considerably in their generic coral cover, dominated by Porites, Millepora and Pocillopora corals. Diversity was low, but coral cover was moderate to high (with Bathaalaa, Rasdhoo, and Dhigurah Wall averaging 40% ± 15). The deviation was low at these sites, illustrating the relatively homogeneous nature of outer reef drop off locations (between 5 and 12 m depth).

The two inner reefs differed markedly from each other in response to the bleaching event. Dega thila has become increasingly smothered with corallimorpharians (Discosoma) since the first surveys at the site in 2008, and since 2016. Here, before the 2016 bleaching event, the deeper layers of the reef survey (>5 m) were always dominated by these smothering organisms. These had encroached into the shallow (<3 m survey depth) after the bleaching event, leading to shallow water cover of 3.75% that was almost entirely comprised of Pocillopora damicornis, rather than the historically dominant Acropora table and finger colonies (e.g. A. humilis and A hyacinthus / clathrata).

Kudafalhu had recovered to 43% cover since the 2016 bleaching event. There was no visible recovery two years after the 2016 bleaching event at this site when surveyed in 2018, where coral cover was 2% in shallow waters. However, considerable recruitment of branching and table corals had occurred by September 2023. This is the only site we have visited since the 2016 bleaching event that has shown considerable Acropora recruitment and growth, making it a reef that should be considered for protected area status.

Fish populations continue to reflect the impact of overfishing on the coral ecosystem. There are low numbers of mid-level carnivores at most sites (snappers and sweetlips), with groupers only occasionally spotted at size >30 cm (fewer than one or two fish on average per transect). We also observed that the important grazing parrotfish were not abundant at reefs, with larger densities frequenting shallow transects.

There were only two crown-of-thorns starfish recorded during the entire expedition, both at Bathaalaa Maagaa. There was little coral disease, but a high prevalence of Drupella corallivorous snail outbreaks at offshore sites, with approximately 40 living colonies affected at Bathaalaa Maagaa alone.

Rare and charismatic animals sighted included nurse sharks (Baros training site), a large school of glasseye Heteropriacanthus cruentatus at Dega Thila (over 300 individuals), reef octopus, hawksbill turtle, blacktip and white-tip sharks, and an eagle ray at Rasdhoo. Three bumphead parrotfish and a white-tip shark were seen off-transect at Bathaalaa Maagaa. In addition, a dogtooth tuna was recorded. A hawksbill turtle was recorded off transect at Kudafalhu. A napoleon wrasse, triton shell and hawksbill turtle were recorded at Dhigurah. A banded coral shrimp was recorded at Bathaalaa Maagaa.