Dictyoceratid sponge surveys and experimental sponge farming in Arnhem Land, northern Australia

During late 2003, 8 locations throughout Arnhem Land were surveyed for bath sponges. One sponge species, Coscinoderma, located at Sims Island and near the Warruwi Community on South Goulburn Is was considered to have quality spongin. Two additional sponge species, Ircinia gigantea from Maningrida and a Luffariella species from Sims Is, were also used during the first stages of the project. The three species were experimentally farmed in collaboration with Sea Rangers from the Warruwi and Maningrida communities at Sims Is and Haul Round Island, respectively. Four systems were examined for their suitability for farming bath sponges in Arnhem Land. The dropper-line system farmed sponge pieces or explants on weighted dropper lines that were attached to a subsurface horizontal line stretched between two floats. Survival was highest overall for explants placed inside mesh bags, with some explants doubling in size in a few months. However, the system was difficult to monitor and prone to tangling and thus unsuitable for commercial sponge farming in Arnhem Land. The second system we tested was a long-line system commonly used to farm pearl oysters. Sponges were farmed in monofilament and spider mesh panels that hung down from a suspended horizontal line. For Coscinoderma experimentally farmed at Sims Is, survival was very low with only 7 of the original 108 explants alive after 1 year. These explants were about two-thirds their initial size. Some of the hanging mesh panels became entangled with the rocky substrate. At Haul Round Is at Maningrida, all Coscinoderma translocated from Sims Is died within 2 months. The system was vandalised shortly afterwards. The third system we tested was the horizontal-line system, which farmed explants in mesh bags tied to a line suspended just off the substrate. Unfortunately, the horizontal-line system can tangle easily and is not suitable for farming bath sponges in its present form. The final system we trialled was the riser-line system, which consisted of a single line reaching from the bottom to the surface with explants grown in mesh tubes. The two systems we deployed appeared to be vandalised before we could monitor the farmed sponges. Of the four systems tested, the long-line system appeared to be the most suitable for Arnhem Land. For any commercial success, however, it would have to be deployed in sheltered areas that have shallow, flat substrate. This is rare in Arnhem Land. The farming response of Coscinoderma was disappointing overall, which most explants dying within a few months. The distribution and farming results suggest that Coscinoderma requires very specific environmental conditions to survive and grow. Overall, bath sponge aquaculture in Arnhem Land is not commercially viable at this stage. Exploring the potential of in-situ farming of bath sponges by indigenous communities in Arnhem Land: Baseline species survey and farming method experiments. This project was funded by the Indigenous Land Corporation and the Department of Agriculture Fisheries and Forestry. In-kind support was given by AIMS, Lo Tech Aquaculture, Warruwi and Maningrida Communities.