Seahorse declines in the Derwent estuary, Tasmania in the absence of fishing pressure

Seahorses have the potential to be flagship species for estuarine conservation around the world and considerable concern has arisen over their declining abundance in the face of exploitation. All seahorses were recently listed on CITES Appendix II in recognition of threats posed by unsustainable trade. However, the efficacy of this measure and the development of other effective conservation solutions will require an understanding of natural seahorse population dynamics at relevant ecological scales. In this study, we provide quantitative data on seahorse populations over nested spatial and temporal scales up to 10 km and 3.5 years. Unexploited populations of the big-bellied seahorse (Hippocampus abdominalis) in the Derwent estuary, Tasmania, Australia consisted largely of mature adults with highly female-biased sex ratios at all sites. While big-bellied seahorses had an aggregated distribution at the spatial scale of our transects (200 m2), most individuals were found alone. When they occurred in groups, the distribution of individuals was random with respect to sex in contrast to the pair bonds found in other seahorse species. All three monitored populations showed significant declines in abundance of 79-98% over the period 2001-2004. Estuarine physicochemical conditions did not change markedly during this period. Possible causes of seahorse declines are interactions with invasive species, disease or reproductive limitation through Allee effects. The magnitude of the declines in the absence of direct exploitation demonstrates that seahorses face multiple threats. Robust fisheries-independent monitoring will be required to demonstrate that conservation actions such as CITES listing are effective.