Risk and reward for invasive seastar feeding aggregations at Victorian shellfish farms

Invasive species often benefit from impacts of human activities. Coastal aquaculture may be one such example, where spilled feed or dead animals can provide a significant local food subsidy for any wild animals that are... [ view full abstract ]

Invasive species often benefit from impacts of human activities. Coastal aquaculture may be one such example, where spilled feed or dead animals can provide a significant local food subsidy for any wild animals that are willing to utilise this novel resource. In Port Phillip Bay, the invasive northern Pacific seastar Asterias amurensis and the larger native eleven-arm seastar Coscinasterias muricata co-occur at piers and shellfish aquaculture fisheries reserves (AFRs) where they feed on mussels and other prey species. If per capita food availability is high and predation events are rare, then these artificial structures are likely to act as population sources for Asterias. However, if Asterias are attracted to these sites but then suffer high predation rates or severe competition for food, the benefit of these artificial structures for invasive species will be lessened and may even be reversed, potentially forming an ‘ecological trap’ or attractive population sink. The function of such a trap might even be improved by harvesting Asterias at these sites of high density, or preventing seastars from accessing the food resource, which would have the added benefit of opening up the seabed for scallop farming by reducing seastar predation on scallops. This project examines seastar population density, body condition and reproductive investment at two mussel farms (Clifton Springs and Grassy Point) in Port Phillip Bay to assess the likely role of a novel habitat in the maintenance of an invasive seastar population. Findings to date reveal that both seastar species were several times more abundant under shellfish farms relative to comparable ‘natural’ habitats during the spawning season. Individuals of both species were also significantly larger on average within the farm boundaries (observations of predation by larger seastars inside the farm boundaries and may explain this trend), and in equal or better condition in terms of pyloric caeca and gonad development. Together, these findings indicate that farms attract large densities of seastars of both species, and that the food availability is high enough to maintain breeding condition in both species, although predation makes this a risky resource for Asterias.