Results described here are based on three sets of surveys of shallow reef biodiversity across all major reef systems in the Coral Sea Marine Park (CSMP). Initial surveys undertaken by Reef Life Survey (RLS) divers in 2012-2015 represent the only Park-wide baseline of reef condition prior to the implementation of the 2018 CSMP management plan and also prior to recent marine heatwaves. Subsequent Park-wide surveys followed two extreme heatwaves with associated bleaching. This report was commissioned by Parks Australia, with a primary focus of using data from these three time periods (a) to assess changes in reef biodiversity in relation to management, including the recently declared Habitat Protection and National Park Zones (2018) as well as IUCN Ia and II zones from the previous Coral Sea Commonwealth Marine Reserve; (b) to understand the ecological impacts of the 2020 heatwave (and associated coral bleaching event) in the context of normal interannual variation and changes resulting from the 2016 heatwave; and (c) to assess overall net change across the marine park over the last decade.
Ecological surveys were conducted by trained RLS divers led by University of Tasmania researchers along 793 50-m transects at 143 sites distributed along the length of the CSMP. Data collected from each site consisted of abundance and size of fishes, abundance of mobile macroinvertebrates and cryptic fishes, and percentage cover of sessile biota.
Total hard coral cover on shallow reefs across the CSMP decreased from 17% of the benthic cover on baseline surveys (2012-2015) to 14% following the 2020 bleaching event (equivalent to a 17% loss of the live hard coral cover observed on baseline surveys). Both 2016 and 2020 heatwave events resulted in similar overall declines (from 17.1% to 15.7% to 14.3% absolute cover), although data for the 2020 heatwave are preliminary as many southern reefs, where greatest impacts are likely, have not been recently assessed by Reef Life Survey. Most coral taxa in the far north region of Ashmore and Boot Reefs recovered from declines associated with the 2016 heatwave by 2021; however, the majority of reefs elsewhere in the CSMP showed overall decline through the last decade, with greatest coral loss occurring in the area extending between Osprey Reef, Bougainville Reef, and reefs in the Willis Islets.
The total biomass of reef fishes also showed widespread and significant decline across the CSMP over the past decade (from mean of 125.42 kg to 118.07 kg per 500 m2). This biomass decrease, which reflects declining populations of large-bodied fish species, did not appear to be influenced by protection status within the CSMP. Further investigation is needed to determine the ultimate magnitude of fish biomass loss and its cause. Possible factors that should be considered are recreational fishing effort expanding in the CSMP, negative heatwave impacts that manifest after a lag, and natural stochastic variability.
A consistent shift in the reef fish community across the CSMP was evident through time, particularly for reefs in the central region after the 2020 heatwave. This heatwave was accompanied by a reduction in regional differences, with increased similarity of north and south fish faunas to the central fauna (i.e. increased homogenisation).
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The mobile macro-invertebrate fauna also showed changing patterns of abundance through time. Species richness generally increased in the early survey years then declined more recently, although patterns were highly patchy between reefs. This patchiness was partly driven by differing taxonomic composition on reefs, with sea urchins predominant in the south and relatively rare further north. Sea urchin densities more than doubled between the first and second survey periods when averaged across all reefs.
Only four species listed as threatened under the EPBC Act were sighted, all turtles; however, a total of 61 species listed as threatened on the IUCN Red List were recorded, primarily sea cucumbers, corals, giant clams, turtles, and large-bodied or small range endemic fishes. None showed consistent population declines over the past decade. Amongst other species of conservation concern, sea snake densities declined in synchrony with the 2016 heatwave, with little apparent subsequent recovery. The most substantial declines in observed sea snake abundances occurred at the two reefs with highest initial densities – Marion and Saumarez.