Biology and ecology of deployed shellfish habitats in the Swan-Canning Estuary

Charles J. Maus

Large extents of shellfish reefs have become degraded around the world as a result of anthropogenic activities to the point where such reefs are functionally extinct in some regions. Due to the ecosystem services provided by these biogenic habitats, there has recently been a concerted effort to restore shellfish reefs, particularly in Australia. While oysters have traditionally been used as a candidate species, the Mediterranean Mussel Mytilus galloprovincialis is gaining popularity and provides a similar suite of ecosystem services to oysters. As part of a pilot program, shellfish habitats, each comprising translocated M. galloprovincialis seeded onto 100 wooden stakes, were deployed at three sites in Melville Water in the Swan-Canning Estuary (Western Australia). The aims of this study where to; 1) investigate the mortality, body condition and growth of the translocated M. galloprovincialis; 2) compare the characteristics of fish fauna at the shellfish reef habitats and nearby unstructured (control) habitats and 3) determine the benthic macroinvertebrate and tunicate species associated with the shellfish habitat. The mortality of M. galloprovincialis was high at all three sites. This was attributed to poor environmental conditions in offshore waters of Melville Water, compounded by stress associated with translocation, their spawning activity, and fouling by ascidians. Seasonally adjusted von Bertalanffy growth models best explained the growth of M. galloprovincialis and growth was rapid, with individuals attaining ~50 mm within their first year. This is likely due to the high phytoplankton availability in the Swan-Canning Estuary. The shellfish habitats harboured a significantly different fish faunal composition compared to nearby unstructured habitats (sandy areas), with many species observed only at shellfish sites or in greater densities. The increased abundances of zoobenthivores on the shellfish habitats suggest they are utilising the invertebrate prey communities associated with the structure as a food source. The invertebrate community varied spatially among the three sites and over time. A suite of non-native ascidians rapidly colonised the stakes along with the mussels, which, in turn, supported many small crustaceans. Given the importance of shellfish restoration globally, and the aim to undertake large-scale projects to provide such habitats in south-western Australian estuaries, the results of this study will increase the understanding of the biology of M. galloprovincialis and help elucidate how faunal communities respond and utilise shellfish habitats. The results of this pilot study will assist in the planning of future mussel reef restoration projects, in particularly those under development in southern Australia.

Biology and ecology of deployed shellfish habitats in the Swan-Canning Estuary

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