Cliona sponges burrow into shell causing damage and occasionally death, rendering the half-shell, and occasionally the pearl, unsaleable. Estimates of cost to the industry from Cliona sp. run into the millions of dollars per year (Moase et al. 1999) and preliminary studies have identified the species responsible and some aspects of their reproductive cycle relevant to pest management and control (Rawlinson 2000, Fromont et al.submitted). However, there is need to understand, and then control, the recruitment/ infection rate of Cliona, and the project shall provide this knowledge. The specific knowledge sought is an understanding of the process of recruitment or transmission of sponges onto pearl oysters, and an assessment of the source of bioeroding sponges in the environment. Key areas of investigation are the distribution and abundance of Cliona sp relative to size and age of oysters, and in the fishing ground habitats, and key biological questions are: is the incidence of Cliona increasing in wild stocks over time, and what effect is the biomass of bioeroding sponges in large, unfished pearl shell having on the fished proportion of the population? Effective management of these sponges in the pearling industry requires that the recruitment issue be resolved, so remedial action can be focused correctly. Management outcomes from the project may include changes to wild-shell fishing practices, shell treatment and shell discard processes, all of which will ameliorate the economic impact of Cliona sp on the industry and maintain the health of the wild stocks.
Final report
Data on bioeroding sponge species that excavate shells of the pearl oyster, Pinctada maxima, was collected from the main fishing grounds in North Western Australia. Estimates on infestation rates across size/age classes of shell are provided from the main fishing grounds and over three consecutive fishing seasons. Minimal infestation occurred in smaller shells therefore lowering the minimum size provides one approach to minimizing the impact of these sponges. Estimates of infestation rates in calcareous substrates of the fishing ground are also provided and compared on an area basis with shell infestation rates, including shell density and calcareous substrate density estimates of the fishing grounds. Techniques were developed to estimate the volume of bioeroding sponge erosions in pearl oyster shells and results are compared between different sponge species and fishing grounds. A field guide was developed to demonstrate the most common visual appearances of bioeroding sponge infestations. Industry and research personnel can use this field guide for bioeroding sponges in general and as a guideline to determine grades of infestation during field surveys.
