Why are tropical fish from Queensland moving to the Mid North Coast

While a rise in sea temperature might make a morning swim easier, it’s not necessarily good news. In fact, experts warn the “tropicalisation” of the ocean is leading to a dramatic shift in the health of our underwater ecosystem.

❓What’s happening: From Woolgoolga to Tea Gardens, kelp forests along areas of the Mid North Coast are being over-grazed by seaweed loving species moving from the tropics.

• Global ocean temperatures are rising as climate change, caused by the burning of fossil fuels releasing carbon dioxide into the atmosphere, has an ongoing impact on our environment. Warmer waters, together with the eastern Australian current getting stronger, has been bringing new species into the region.

The introduced species include herbivorous fish, or vegetarian fish, like rabbitfish, surgeonfish and drummerfish, that like to eat kelp forests – a little too much.

🧠 Tell me more: The Mid North Coast is included in the Great Southern Reef (GSR) – an extensive and interconnected system of temperate reefs predominantly characterised by kelp forests.

• Kelp is a type of underwater seaweed that thrives in cold water environments.

• The GSR spans 8,000 km of coastline across the entire southern part of Australia and hosts a range of habitats – providing food and shelter.

🗣️ Adriana Vergés, Professor in marine ecology at UNSW Sydney and the Sydney Institute of Marine Science (Australia), explained how one impact of climate change has been the “tropicalisation” of ecosystems. In short: It’s bad news for our kelp forests and the species that rely on them.

ℹ️ Quick refresh: Tropical fish species are expanding south because they can. Like humans, marine life has a preferred temperature.

• “We're seeing tropical species that belong in places like the Great Barrier Reef, that are starting to expand their distribution southward following the East Australian current,” Vergés said.

The temperature of water that was once only found in the tropics is now being found further south, like on the Mid North Coast, and tropical fish have realised they can survive and thrive along the coastline.

The east Australian current getting stronger means species that wouldn't normally be able to get to a certain location can now travel with ease, introducing them to a new area.

🪸 An imperative underwater feature: Kelp forests cover a quarter of the world’s coastlines. Vergés said these incredibly productive (and beautiful) ecosystems play the same role as trees on land.

• More than 70 percent of species found in the Great Southern Reef are unique to that area – “certain fish, invertebrates and seaweeds are only found here.”

• “When we lose our kelp forests, and for species that are very closely associated with them, we're kind of losing them from planet Earth,” said Vergés.

📉 A disappearing habitat: Some of Verge’s work has focused on the Solitary Islands, off Coffs Harbour. Here, she said the kelp forests were in decline. 

• “The kelp itself is what's giving the habitat, it's what's creating the habitat. So if it gets eaten, then the entire habitat is gone,” Verges said.

• “When these herbivorous fish come in and they consume and over-graze the kelp, they're creating an unvegetated alternative habitat, which then has a consequence for all the species that would normally associate with the kelp forest.”

🌏 The bigger picture: Tropicalisation and damage to kelp forest habitats is not just a problem on the east coast of Australia – it’s an issue across the country.

• “We're definitely feeling the impacts of climate change in colder water ecosystems,” Verges said. “So just like the Great Barrier Reef is bleaching, we are also seeing impacts in higher latitude reefs, which in some cases are really kind of dramatic”. 

In 2011, an extreme marine heat wave led to the loss of kelp forest along 150 kilometres of coastline – the equivalent of the distance between Port Macquarie and Coffs Harbour.

• “We haven’t seen anything as dramatic on this side of the country, yet. But it could happen,” Verges said.

🌊 Tassie trip for underwater life: Currently, warm water species are even making it all the way down to Tasmania, causing havoc in kelp forests in the area. 

• “Species that don't normally exist in places are shifting their range so that sea urchins, which would never be in Tasmania, are now finding their way there and causing absolute devastation in that area and eating the kelp beds, whereas previously currents stopped them being able to spread that far,” said marine ecologist, Dr Lissa Schindler.

🗣️”Once you get past mainland Australia, you see that the sea floor starts to gradually get deeper and deeper and there's nowhere for these communities to move,” said Schindler. “Things are happening so quickly, more quicker than it's ever been before, that some of these animals and plants are not going to be able to move and adapt at the rate that climate change is happening.”

This phenomenon is happening across the world. Verges said the decline of kelp forest she witnessed in the Solitary Islands was not unlike what's happening in South East Japan.

🔎 Searching for solutions: Facing the impacts of rising ocean temperatures, Schindler said Australia would need to cut emissions to maintain the natural balance of the ocean.

• “The ocean and everything is in really succinct balance. And what we're seeing is that as temperatures are warming, it's starting to put things off kilter,” said Schindler.

• “The governments need to play a leading role, especially Australia, in cutting greenhouse gas emissions domestically and phasing out the export of fossil fuels like coal, because this is what's driving the heat in the system is the mining and burning of fossil fuels.”

• “We need to start looking at how to boost the health of the ocean by making sure that we're tackling things like water pollution, making sure that fisheries are sustainable, and looking at ways to reduce every other pressure that these marine species have on them, so that when we do get increases in temperature every summer that it's not going to be a whole range of  impacts.”

🤝 How to help: The Range Extension Database & Mapping Project, known as Redmap, allows Australians to “spot, log and map” marine species that are uncommon in the country, or along particular parts of our coasts.

• “Fishers, snorkelers, scuba divers, whenever they see something that they think is out of range, they can upload that observation onto the website and then it gets validated by experts, and then that information gets used by scientists to understand what species are moving, how fast, and what are the potential implications,” said Professor Verges.

Thumbnail: Unsplash.