Weedy seadragons (Phyllopteryx taeniolatus) are iconic inhabitants of Australia’s Great Southern reef. Our long-term monitoring and sampling research (since 2001) has indicated population fluctuations and some declines in SE Australia over the last two decades, and that there is high spatial genetic and morphological structuring. In February-April 2022, a series of severe climate-related East Coast Lows battered SE Australia, and in April local beach walking citizens reported patchy strandings of over 150 weedy seadragons on shorelines from Gosford to Wollongong. Recently (in March 2025 ), there was a small wash-up event off the NSW coast possibly as an aftereffect of an algal bloom and strong storm surge associated with east coast lows in the aftermath of Cyclone Alfred in late February 2025.
However, the population impacts of these strandings are unclear as seadragons are patchily distributed and difficult to census accurately. Here we document decadal changes in seadragon abundance using citizen science dive data, plus intensive mapping of rocky reef areas that are rarely dived, to estimate total population sizes.