Status of coral, giant clam and sea cucumber communities including CITES listed species on a remote Australian coral reef atoll and the potential impact of illegal fishing

Following a recent dramatic increase in illegal fishing by Indonesian fishing vessels in Australian waters in 2022, we conducted an extensive survey of coral reef communities covering 33,000 m² at Mermaid Reef Marine Park in the Rowley Shoals off north-western Australia in July 2022. Species richness of sea cucumbers was 13 species (three CITES listed) and 6 species of giant clams (all CITES listed). The most abundant sea cucumber species were the low or intermediate value, asexually reproducing species Holothuria atra and H. edulis. Seventy-one percent of all sea cucumbers were found in the deep lagoon habitat despite this habitat comprising only about one-third of total survey effort. Most of the higher value species, such as CITES listed Thelenota ananas, were more abundant in the deep lagoon habitat. The exceptions to this pattern were H. whitmaei (CITES listed) and Actinopyga mauritiana (high value) and Bohadschia marmorata (intermediate value) that were most abundant on the reef flat, reef slope and shallow lagoon habitats respectively. The burrowing giant clam Tridacna crocea was abundant in all habitat types, whereas T. squamosa and Hippopus hippopus were most abundant on the western outer slope, T. gigas on the reef flat and T. derasa in the deep lagoon. The results of previous and subsequent surveys in 2003, 2018 and 2023 were compared with our 2022 survey. High value species that mainly occur in shallow water (e.g., H. whitmaei) were more abundant in 2003 while T. ananas and Stichopus herrmanni were more abundant in the deep lagoon habitat in 2022 and less abundant in the shallow habitats than they were in 2003 while there was no change in the more abundant, low to intermediate value species (e.g., H. atra, H. edulis). There were no significant differences in sea cucumber densities between 2022 and 2018 or 2023, however the comparison was confounded by the small number of 2018 and 2023 survey sites which were spread over multiple habitat types. Survey results were consistent with the expectation that illegal fishing by free-diving occurs predominantly in shallow and intertidal habitats and that sea cucumbers in those habitats are more vulnerable to fishing than those in deep water habitats. We were unable to ascertain levels of catches from Mermaid Reef or the Rowley Shoals as a whole, however there were numerous reports of vessels being intercepted with catches of many tonnes of sea cucumbers. At Mermaid Reef, to remove one tonne of sea cucumbers, from reef flat and shallow lagoon habitats would require harvesting all sea cucumbers from 38 ha (2037 individuals). This represents about 9% of the total area of reef flat and shallow lagoon at Mermaid Reef Marine Park and indicates how rapidly sea cucumber populations can be depleted through illegal fishing. Coral communities at Mermaid Reef were healthy, consistent with previous surveys over two decades with mean hard coral cover highest (40-50 %) on the exposed western outer reef slope and lowest on the reef flat (<10 %). Evidence of damage to coral communities from illegal fishing operations, bleaching or crown-of-thorns sea stars was negligible.