The answer unfortunately or fortunately if you’re a Nemo fan is sooner than you may expect. In a recent seminar Dr Will Figueira from the University of Sydney explained that tropical fish are being found further and further south along the East coast of Australia. So just why are we finding increasing numbers of tropical fish in temperate waters? The simple answer is due to increased water temperatures thanks to climate change1,2, as a relevant example the waters in Sydney have increased by 1 degree since 1920. Now if you’re still skeptical about climate change (yes there are plenty of people out there) or just want a good laugh check out this video (NSFW) from John Oliver.
Now onto the serious stuff, there is strong evidence for range shifts in fish down the East coast of Australia and they are very consistent with climate change and the large temperature anomalies seen in South East Australian waters. Range shifts in the marine environment can occur at up to 70km a decade, but the question is just how are these fish getting here? Funnily enough it seems Disney had the right idea in Nemo, that’s right the East Australian Current also known as the EAC3, dude.
The main difference between what actually happens and what you just watched is that instead of fish just “riding” the EAC what we find is that it is driven largely by fish larvae that hitch a ride down south. Fish larvae are perfect for the long journey down the EAC as they are highly mobile, resilient4 and in many cases fish can stay as larvae for well over a month, giving them plenty of time to make the journey.
The EAC has been churning away for a very long time, so it is safe to assume that fish larvae have always been brought down the coast. Why we are only seeing tropical fish down south in recent years is because now they are able to survive the conditions they are met with. As mentioned before the primary condition for survival is water temperature. In previous decades in the summer South East Australian waters reached temperatures that tropical fish could survive in, however the winter temperatures were too low and fish died off. Now we are seeing dramatically increased winter and spring water temperatures that allow these tropical fish to survive the winters and establish a populations1, the hypothetical graph I created helps explain.
The pattern of southward range expansions by tropical fishes is increasing thanks to temperature, but why is temperature the major driver of survival chances? The answer to this is all about metabolic rates, basically how efficiently a living thing (in our case a fish) can function. Temperature dictates how efficient metabolic rates are, if it is too cold or too hot the rate is low and the organism cannot function properly. When the temperature is just right, the peak rate is reached and this is known as the optimal metabolic rate. Tropical fish require higher temperatures for effective metabolic function which is why it is only in the last decade or so that they have been increasingly found further south.
It must be noted that metabolic rate is not the only driving factor that allows tropical fish to successfully survive in temperate waters. Characteristics such as current range boundaries, fecundity, larval duration, investments in parental care, swimming ability, settlement size (how big they are once they leave the larval stage) and of course the ability to successfully reproduce all play an important role in determining whether or not a tropical fish population can survive in temperate waters.
Finally, while this blog has been primarily focused on the south east coast of Australia the occurrence of tropical fish being found in temperate waters is a problem that is happening globally, from Brazil, to America, to Africa and even to Japan3,5. It is a problem because it means that temperate fish species are forced to either compete with tropical fish for resources or migrate further south themselves. Increased competition on resources and/or migration may not be a feasible option for many tropical and temperate fish and there is a high possibility that as we see water temperatures increase there is a dramatic loss of fish species as the result.
So what does this mean for Nemo? Well the good or bad news (depending how you look at it) is that as winter water temperatures increase there will be more and more tropical fish that can survive in temperate waters. In the long term unless water temperature increase is slowed, there will be increasing numbers of tropical fish establishing populations further South on the East coast of Australia. Who knows one day Nemo may stumble upon 42 Wallaby Way, Sydney.
Nemo: Coming soon so a Sydney beach near you!
1. Poloczanska. E et al. 2013. Global imprint of climate change on marine life. Nature Climate Change Volume 3, October 2013.
2. Nakamura. Y et al. 2013. Tropical fishes dominate temperature reef fish communities within western Japan. Plos One. 8(12): e81107.
3. Vergés. A et al. 2014. The tropicalization of temperate marine ecosystems: climate-mediated changes in herbivory and community phase shifts. Proceedings of the Royal Society B. 281: 20140846.
4. Figueira. W and Booth. D. 2010. Increasing ocean temperatures allow tropical fishes to survive overwinter in temperate waters. Global Change Biology. 16: 506-516.
5. Vergés. A et al. 2014. Tropical rabbitfish and the deforestation of a warming temperature sea. Journal of Ecology. 102: 1518-1527.