UTS Faculty of Science

It might look pretty in an aquarium, but an invasive seaweed native to the tropics is posing a threat to fish diversity in NSW estuaries.

 

Pipefish and seahorses, soon to be listed as threatened species in NSW, are potentially most affected by the spread of the green alga Caulerpa taxifolia according to a new study by environmental science researcher Paul York from the UTS Faculty of Science.

Caulerpa taxifolia is native to tropical and subtropical areas of the world, including northern Australia, but in 2000 it was discovered in central NSW, well south of its normal limit. It has since spread to at least nine estuaries in this region, where it covers large areas and also grows amongst seagrass beds.

"Caulerpa is a popular aquarium plant and discarded water from household aquaria is the most likely source of NSW infestations," York said. "It is unlikely to have spread south naturally because there is no evidence of it between Moreton Bay in southern Queensland and Lake Macquarie not far north of Sydney.

"There are concerns that Caulerpa is a threat to biodiversity and could out-compete native seagrasses that support a diverse array of fish and are the nursery grounds of many economically important species. However, there have been few Australian scientific studies to support these concerns."

For his honours research, supported by a grant from NSW Fisheries, York set out to discover if fish populations in areas infested by Caulerpa were different in number and composition to those around native seagrass beds. The study was carried out over five months in Botany Bay and Port Hacking.

Several species of fish that were commonly found in seagrass beds were found to be either rare or absent in Caulerpa taxifolia. These included fishes from the family Syngnathidae (pipefishes and seahorses) and those from the family Monacanthidae (leatherjackets).

"Pipefish are specially adapted for seagrass and spend their lifecycle within seagrass beds," York said. "They look like seagrass leaf blades, which both protects them from predators and provides cover to catch their prey. Where there is lots of Caulerpa there are no pipefish.

"With leatherjackets it has to do with food sources. A large part of their diet is made up of a kind of alga that grows on the leaf blades of seagrass. This alga wasn't found growing on Caulerpa during the study."

York said there also were indications that Caulerpa was not as effective as seagrass as a nursery ground for several fish species of economic value - including bream, blackfish and blue groper - but further study was needed to confirm this.

York said the research, undertaken under the supervision of Dr Bruce Pease and Dr Tim Glasby of NSW Fisheries and Associate Professor David Booth of UTS, is part of a wide-ranging effort to pin down the danger posed by Caulerpa and determine how best to control it.