Have you ever wondered why you find tropical fish when diving in Sydney? It's because they're riding the EAC dude...

Prof David Booth and Dr Will Figueira (UTS) have been surveying tropical fish that hitchhike the East Australian Current (EAC) every summer since 2003. The most popular tropical fish species to spend their summers in Sydney include four species of Sergeants; the Bengal, Scissortail, Indo-Pacific and Whitely's. Other species include the neon damselfish, the threadfin butterflyfish and the dusky butterflyfish.

Most people know of the East Australian Current from the film Finding Nemo, where families of  boudaceous Sea Turtles hitchhike from the tropical waters of the Great Barrier Reef to the more temperate waters of Sydney and beyond.

Many tropical fish have life stages, usually larval, that are spent in the open ocean, known as a pelagic phase. When fish larvae drift from their home reef where they were spawned from, they enter the EAC which stretches from the Coral Sea in the North to Bass Strait in the South. These larvae swim thousands of kilometers in their first weeks of life, sometimes without ever having a meal.

From Booth's and Figueira's reseach, this phenomenon is a regular occurance of tropical fishes. But where are their home reefs ?

Fish have ear stones which are quite similar to the rings of tree trunks, and are known as Otoliths. The rings on a fishes ear stone are laid down daily and can be used by scientists like David and Will to not only age the fish but also determine how well the fish was growing. Trace elements in the Otolith indicate approximately where the fish was at the time that individual ring was laid down. This is done by correlating the levels of the trace elements with those found in the environment.

David and Will's reseach has shown that the amount of time a fish spends in the pelagic phase is correlated to the distance down the coast it is found. Their work shows that tropical fish found further south in New South Wales have not yet neccessarily travelled more than fish that were found in Northern NSW. In one case, Butterfly Fish found in Merimbula had a travel time less than the fish found in Sydney. These particular fish could have source reefs scattered further south of the Great Barrier Reef than first thought. Another theory is these fish have accessed warmer eddies within the EAC that act like a turbo-boost which shortens travel time down the coast, whilst their friends are stuck in EAC traffic.

How will global climate change impact upon the migratory patterns of these fish that use the EAC?

Oceanic temperatures affect species distribution of fish throughout the world. The metabolic theory of Ecology indicates a positive relationship between temperature and growth in fish. Fish found in warmer waters usually grow bigger and reach sexual maturity faster than fellow species in cooler waters that take longer to grow and mature. Fish living in warmer waters have a herbivorous diet, as they have a warmer environment to support the digestion of plant material. Fish in temperate waters are more likey to be omnivores, where they eat not only plants, but also plankton and other fish too.

Fortunately, temperature records for the past 140 years have been kept, which indicate an increase in oceanic temperature in Sydney since 1900. From David's and Will's research and general time spent underwater, they are noticing that tropical fishes are overstaying summer in Syndey and wintering in the Harbours warmer waters. There have been increasingly successive winters where sea temperatures have been 18° or warmer. The holidaying period of these tropical fish can cause a downwards shift in populations and assemblages of their temperate Sydney cousins. The demise of Sydneys temperate fish populations and assemblages due to the overstaying of tropical fish will not only affect marine habitats but also impact upon some of the most valuable fisheries in Australia.

Future Research

South-Eastern Australia's coastline as well as the EAC is proving to be a valuable area for further climate change research for scientists such as David and Will, who are already witnessing the affects of a warming ocean on fish assemblages up and down the coast.