The rapid decline of an Endangered temperate soft coral species

Abstract

Dendronephthya australis, an Endangered soft coral primarily found in the central coast region of New South Wales (NSW), Australia, is in rapid decline. The largest known aggregations are located within the Port Stephens estuary, where the populations have been mapped in detail via towed video at four-year intervals since 2011. Since the distribution was initially mapped in 2011, the areal extent decreased by 67.5%, and is now 9300 m2 compared to 28,600 m2 in 2011. Forest-based Species Distribution Models (SDMs) determined that the primary variables associated with the presence of D. australis (including depth, current velocity, slope, and the distance from the estuary mouth and the shoreline) remained largely unchanged suggesting an alternative cause of declines. General Linear Models (GLMs) identified that losses were associated with distance from shore, and change in depth and current velocity from 2007 to 2018. As these three variables are linked to sediment mobilisation within the estuary, this process is likely to be a key contributing factor to the decline of D. australis – this is supported by in situ observations over the same time period. As Port Stephens is a hydrodynamically complex environment, and has a wide variety of both commercial and recreational uses, other, as yet unidentified, processes may also be contributing to soft coral loss. The only remaining aggregations are found in depths of 4–11 m, which makes them highly vulnerable to fishing and boating impacts, as well as severe storm events.