Moving south may be the best protection for marine fish in the face of rising ocean temperatures, but how far south can they go?
The last two decades have seen marine fish communities along the southwestern coast of Western Australia (WA) changing their makeup in response to climate change. These changes include warm water species that have moved south into higher latitudes and cooler waters, and resident temperate species in turn being pushed further south. But at what point do marine species run out of coastline in Australia, and which of these fish are more successful in their new environments?
The coastline of WA is a highly unique environment, boasting the second highest proportion of marine endemic species in the world. However, recent climate change trends, including temperature increases and a marine heatwave in WA circa 2011, may further influence this. This effect was best observed in wrasses and parrotfishes (family Labridae), both members of the second most diverse marine fish family. In WA, their sizes range from a couple of centimetres to over 1.5 metres (as is the case of the Western Blue Groper, which can also live up to a very respectable 75 years).
For parrotfish and wrasse species, there are many success stories. Since 2015, the tropical parrotfish species, Scarus ghobban, has thrived to such a degree that they are now the defining species of the fish community in the warmest parts of WA. This is in contrast to 2006, when these tropical parrotfish were counted and only rarely observed. Sub-tropical species have also been particularly successful in more recent years; species such as the Western King Wrasse (Coris auricularis) have increased from a few individuals in 2006 to over 2,000 individuals, in just one of the 125m2 areas that we studied! In 2015, we found 10 species that were not present in 2006 - all of these newcomers were of tropical or sub-tropical origin. However, while most wrasse species increased in number, some of the resident temperate species have declined, including the long-lived Western Foxfish (Bodianus frenchii) and Western Blue Groper (Achoerodus gouldii).
Our recent publication in Diversity and Distributions discovered that fish composition in southern areas of the coastline in WA (e.g. Albany, Bremer Bay, and Esperance), which were once thought to be isolated from the effects of the gradual ocean warming and the significant temperature spike caused by the WA heatwave in 2011, was also affected. In 2006, tropical wrasses were only observed from Geraldton to Flinders Bay (WA’s most southern point) but nine years later, when we re-surveyed the area, we found that tropical species were observed from Geraldton all the way to Esperance. That is a range shift of nearly 1000 km to the south-east.
The continued movement of species further south poses a real problem in Australia. Indeed, once a species moves to Flinders Bay, they have no more coastline left to shelter from the rising temperatures – given that the next available land mass is Antarctica. This means that species are left with no option but to adapt to the rising temperatures or face local extinction.
However, the biggest changes in marine species populations occurred in warmer waters, where the diversity of tropical species has increased to a point where they are the most important representatives of wrasses in their new environment. Importantly, the normally cooler and “stable” marine populations along the southern reaches of Australia are starting to see warm water species successfully moving in.
Continued monitoring of these marine populations is essential to determine if the trends seen in 2015 will continue. Indeed, will warm water species continue to shift their range limits further south, pushing temperate endemic species further towards the poles? Investigating this question is more important than ever, with another sustained heatwave currently spreading from Karratha in north western WA all the way down to South Australia (SA).
Jack Parker, Research Associate, Curtin University
Dr Joseph DiBattista, Scientific Officer, Curator Ichthyology, Australian Museum Research Institute
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