Weedy seadragon long-term abundance changes in Sydney, Australia: using reliable citizen divers to compile temporal patterns

The weedy seadragon Phyllopteryx taeniolatus (family Syngnathidae) is an iconic fish endemic to Australia’s southern coast. There are indirect indications of population declines over the last decade, with large numbers of individuals washed up in 2022, and the impacts of such events are exacerbated by limited dispersal and spatial differences in genetics. Given the limited data from scientific surveys, we compiled citizen science data for south-east Australia over 3 decades (1990–2025) from long-term divers who regularly record the seadragons they encounter (seadragons are popular targets for divers, especially for photography). These data suggest a peak of seadragon abundance (measured as maximum number and number seen per dive) in 2000–2003, followed by a decline, with a possible increase from 2021. A recent stranding event on the 100 km of coastline around Sydney and in South Australia adds to concerns of population declines in these regions, despite some sites having high densities. We highlight both the efficacy and shortcomings of using data from long-term citizen scientists to assist in population assessments of marine species.