SIMS

Every summer, snorkelers along the coast of southern NSW are treated to a colourful show as a variety of tropical fishes begin to appear among the rocks and weeds. This is the endpoint for a suite of vagrant coral reef fish larva - individuals can be transported or 'swim' great distances (1000s of kilometres), often winding up in areas well outside their normal range. Hutchins identified 75 species of tropical fishes at Montague Island (36°15' S) and 50 at Merimbula (37°S) in the early 1990s. Our own work has demonstrated this is truly a coastwide phenomenon, documenting the regular occurrence of tropical fishes in south-eastern (SE) Australia to Bittangabee near the Victorian Border (37°S). The expatriation (removal from 'typical' biogeographic range) of tropical fish larvae into temperate waters is also seen in other areas of the world. Tropical fish have been consistently observed since at least 1964 along the east coast of Japan, along the east coast of North America, and along the west coast of Australia. In all cases, a strong, poleward flowing boundary current is the suspected vector for this long distance transport.

Juvenile Dusky Butterfly Fish © W. Figueira

EAC

A survey and sampling program along the coast of NSW, conducted by Prof David Booth (UTS) and Dr Will Figueira (UTS), has reinforced the previously assumed role of the East Australian Current (EAC) in this process and has clarified a host of other smaller-scale physical and biological processes which impact the timing and strength of tropical fish recruitment. For instance, pulses of arriving fishes very often correspond with coastal contact by tongues of the EAC eddies, which commonly occur south of 32°. However, the local increases in water temperatures typically associated with these tongues don't always bring with them pulses of settling tropical larvae. Our research has revealed some interesting patterns and generated intriguing questions. There is a rough correlation with the distance down the coast a species is found and the time that species typically spends in the pelagic phase, known as the pelagic larval duration (PLD). This would suggest that perhaps most tropical species, especially the butterfly fishes with longer PLDs (~45 days), are coming from the southern end of the Great Barrier Reef (GBR). Yet this is insufficient to explain the occurrence of several species of damselfish (especially the sergeant majors, genus Abudefduf) which occur as far south as Merimbula yet have only a 20-22 day PLD. It is possible that these fish are actually sourced from patches of reef in and around the Solitary Islands (SI) where at least one species (A. vaigiensis) is known to breed.

In addition, preliminary analysis of actual PLDs from the ear stones (otoliths) of butterfly fish which settled along the coast indicates that PLDs in Merimbula are actually less than for individuals found further to the north, rather than the opposite.

This could imply there is either another source for these fish located well to the south of the GBR (they are not known to reproduce in the SI), or there may be some vagaries of EAC flow that allow some fish to progress to southern NSW quite rapidly, while others are entrained in eddies and arrive at sites to the north somewhat later.

OCEAN TEMPERATURE

An extremely important factor nested within these patterns is ocean temperature. Water temperature is a large factor determining range extents for many marine organisms and an increase in temperature due to global climate change has been shown to affect species distributions. Recent work based upon the metabolic theory of ecology (MTE) - relating temperature to growth via metabolism - has demonstrated that due to the positive relationship between temperature and growth, there is a very robust negative exponential relationship between temperature and PLD for many taxa of marine organisms. Thus trends for PLD to increase with latitude for many of these tropical species are likely due to a combination of distant sources and metabolic tradeoffs. As these processes will affect all dispersing marine species, any understanding of them can greatly aid our ability to predict changes which may occur in response to global sea warming (e.g. CSIRO reports of EAC strengthening off SE Australia in response to global warming.).

Additionally, we have recently configured winter water temperature changes across SE Australia over the last 140 years which shows not only a steady rise since the early 1900s but also an increased frequency of 'unusually warm winters'. For these winters, above ~18.5ºC, a suite of tropical species overwinter, which may lead to population persistence and eventually range shifts. This raises the intriguing possibility of a change in NSW fish assemblages towards more tropical species at a loss of the cool temperate assemblages, which tend to contain more commercially important species. It now appears that the SE Australian coast is a global 'hotspot' for climate change effects as a result of the EAC strength and duration. We hope that our ongoing studies will provide managers with tools for predicting consequences of climate change to the biotic makeup of estuarine and coastal marine fauna.