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Each summer larval (baby) tropical fishes are carried by the 'East Australian Current' from north-eastern Australia southwards providing areas such as Sydney Harbour with an exotic sprinkling of tropical fish.
The East Australian Current is a warm tropical current which flows from the hot equatorial top end of eastern Australia south to the southern regions of New South Wales and occasionally as far as Tasmania. Rapid changes in water temperature are often experienced along the New South Wales coast in early summer. These are caused when warm eddies being swept towards the shore from the East Australian Current as it travels down the coast displace the local cooler water. The cool waters are then replaced by upwelling near the coast during the northerly winds.
Many tropical fishes such as surgeonfishes, Barramundi Cod Chromileptes altivelis, Rainford's Butterflyfish Chaetodon rainfordi, Moorish Idol Zanclus cornutus, damsel fishes and wrasses can be seen on the temperate reefs of Sydney Harbour during the summer and autumn. Most of these 'vagrant species' start to disappear by about May. In winter when the harbour is dominated by much cooler currents from the south the tropical residents becomes less active and much more susceptible to predation. While many adult tropical fishes do survive Sydney winters, the cooler waters prevent them from breeding.
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