Swimming nets have positive effects on populations of the endangered White's seahorse Hippocampus whitei

Habitat degradation and destruction arising from rapidly increasing urbanization represents one of the most significant threats to biodiversity. Human populations are continuing to increase around coastal regions, and as marine habitats are displaced by artificial structures it is important to understand how marine species may be impacted by these changes in habitat availability. The endangered seahorseHippocampus whiteihas been observed inhabiting protective swimming nets in Sydney Harbour, Sydney, Australia, even in the presence of natural habitats. This study tested whether the presence of a swimming net results in increased seahorse numbers at sites around Sydney Harbour, or whether seahorses are attracted away from natural habitats. Density surveys and mark-recapture population estimates were done at sites with pre-existing swimming nets and compared to control sites where only natural habitat was present. A manipulative experiment was conducted in which panels of swimming net material were installed at two sites in Sydney Harbour, with comparisons to control sites over a period of 14 months (April 2018 to June 2019) to test whether the installation of swimming nets would affect seahorses on surrounding natural habitat or increase site abundance. The pre-existing and installed swimming nets were found to support greater densities ofH. whiteias well as some increases in site-scale abundance, with no effects on seahorse density on natural habitats. It is likely that increased seahorse production is occurring on the nets, with no evidence that seahorses are being attracted away from natural habitat; however, effects may vary across survey occasions and sites. Furthermore, swimming nets may serve as a useful replacement habitat in locations where natural habitat has become sparse or absent.