Early life history of the introduced seastar Asterias amurensis in the Derwent estuary, Tasmania: The potential for ecology-based management

The early life-history of the introduced Northern Pacific seastar (Asterias amurensis) in the Derwent Estuary, Tasmania, is investigated with an aim to identify opportunities for improved management. A discrete non-equilibrium model of gamete dispersal and fertilization success is developed. The model is parameterized with species-specific gamete traits measured in the estuary population, and is validated using empirically determined estimates of fertilization success, which required containing gametes in collection flasks. In situ measurements of the proportion of eggs fertilized ranged from 0.80 ± 0.08 (SE) adjacent to the spawning male, to 0.16 ± 0.04 (SE) for separation distances of 16 m. The model is then adapted to simulate dispersal of gametes without the experimental artifact of containing gametes in flasks. There is good concordance between empirical estimates and model predictions when the model simulates the experimental procedure. However, when simulations allow for dispersal and fertilization without confinement of gametes in the collection device, predicted fertilization success is substantially lower at 0.05 adjacent to the spawning male and 0.03 at 16 m. The model is extended to predict the reproductive potential of discrete populations of the seastar in the Derwent Estuary. The effects of population density, group size, spawning synchrony, sex ratio, and water depth on the total number of eggs fertilized and the proportion of eggs fertilized is predicted. Within the range of parameters tested, group size, density and water depth had the most significant effects on fertilization success. The model predicted a 300% increase in fertilization success when density is increased from 0.025 to 0.2 individuals m-2. Spatial variability in the reproductive potential of populations in the estuary is also assessed based on gonad indices of seastars determined at 9 sites in the estuary. Gonad indices of starfish at yacht clubs were 3 times higher than 'control' and wharf sites. Given results of the gamete dispersal model, and spatial variability in density and gonad indices, it is likely that discrete populations in the Derwent Estuary contribute differentially to larval production, and particular populations might potentially be targeted for management. Dispersal of larvae of the introduced seastar Asterias amurensis in the Derwent Estuary, and advection of larvae out of the estuary, is predicted using an inverse transport model incorporating the behavioural responses of larvae at different stages of development to salinity and light. In laboratory conditions larvae demonstrate reverse diel migration, and do not swim into water 99%) of late-stage larvae are advected out of the estuary. Substrata that induce settlement and metamorphosis of laboratory reared larvae were determined by introducing competent brachiolaria into wells containing substrata commonly available in SE Tasmania. Larvae settled at high rates when exposed to non-geniculate coralline algae (0.98 ± 0.02 SE after 2 days) and at moderate rates on mud and bare rock (0.37 ± 0.06 and 0.44 ± 0.06 respectively after 7 days). On day 7, larval settlement on sand and in the control (filtered seawater) were low (0.01 ± 0.01 and 0.05 ± 0.02 respectively). Based on the distribution of these substrata in the Derwent Estuary, these results suggest that larvae potentially settle at low rates in the lower estuary where the benthos is largely sand, and at moderate rates in the mid-estuary where fine sediments dominate. Settlement at high rates might potentially occur on fringing reefs in the mid and lower estuary, but this habitat comprises a small portion of the estuary.