The Tasman Sea off southeast Australia exhibited its longest and most intense marine heatwave ever recorded in 2015/16. Here we report on several inter-related aspects of this event: observed characteristics, physical drivers, ecological impacts and the role of climate change. This marine heatwave lasted for 251 days reaching a maximum intensity of 2.9 °C above climatology. The anomalous warming is dominated by anomalous convergence of heat linked to the southward flowing East Australian Current. Ecosystem impacts range from new disease outbreaks in farmed shellfish, mortality of wild molluscs and out-of-range species observations. Global climate models indicate it is very likely to be that the occurrence of an extreme warming event of this duration or intensity in this region is respectively ≥330 times and ≥6.8 times as likely to be due to the influence of anthropogenic climate change. Climate projections indicate that event likelihoods will increase in the future, due to increasing anthropogenic influences.

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This study was undertaken to compare fertilization rates of the sea star Acanthaster planci that were predicted using sperm diffusion models with those that were determined under natural conditions in the field. During experimentally induced spawnings, measured fertilization rates for broadcast eggs were high. More than 70% of the eggs were fertilized at distances as great as 8 m downstream from a single spawning male starfish, and more than 20% were fertilized at separations of more than 60 m. Fertilization was still measurable, at 5.8%, 100 m downstream. Lateral diffusion of sperm away from the axis of flow produced fertilization rates of 13.8% at 8 m normal to the flow and 32 m downstream. The large volumes of sperm released by male A. planci are the primary cause of high rates of fertilization for eggs derived from widely spaced individuals. Models of sperm diffusion using high sperm release rates such as those found in this starfish accurately confirmed the fertilization rates measured in situ for two populations of A. planci with widely differing rates of sperm release. We observed some changes in starfish density and degree of aggregation in the study population for spawning periods during two spawning seasons, though these were not striking. High levels of aggregation may not be necessary for fertilization success in this starfish, due to the potential for long-distance fertilization and the probability that, for any spawning starfish, the total number of zygotes formed will be greater at some distance from the point of spawning. Although fertilization rates in areas distant from the sperm source were relatively low, the total area for potential gamete encounters is much greater and may make a large contribution to net fertilization. We predict that other behaviors, such as migration to shallow water, commonly associated with spawning in A. planci and other marine invertebrates will have measurable impacts on fertilization success. The potential for high levels of fertilization in A. planci was realized during natural spawnings. Fertilization rates as high as 99% were recorded when levels of spawning synchrony were high.