Photo by David Clode

Forensic approach tracks species on the move

Scientists have developed a method combining environmental DNA (eDNA) analysis with traditional visual surveys to track tropical fish species migrating into cooler waters, detecting species movements that conventional monitoring overlooks.

The research, published in Diversity and Distributions, surveyed fish communities along 2,000 kilometres of Australia’s east coast, from the Great Barrier Reef to New South Wales kelp forests. Climate change has already caused more than 12,000 species globally to shift their ranges, with tropical fish moving into temperate reefs along Australia’s east coast—one of the fastest-warming marine regions on Earth.

DNA ‘fingerprints’ in seawater

Fish shed mucus, scales and waste, all containing DNA that can be extracted from seawater samples. Chloe Hayes, lead author from the University of Adelaide, explained: “Every organism leaves behind traces of itself in its environment — fish shed mucus, scales and waste, all of which contain DNA. By collecting and filtering samples of seawater, we can extract this DNA and identify the species that have lived in a particular area.”

Professor Ivan Nagelkerken, project leader, said: “Just as detectives solve crimes by analysing fingerprints or hair left at a scene, ecologists can build a picture of marine life from the genetic fingerprints floating invisibly in the ocean.”

Detecting cryptic tropical arrivals

Researchers collected seawater samples alongside traditional visual surveys, where divers swim transect belts recording every fish observed. A few litres of water holds invisible genetic fingerprints of hundreds of species.

The eDNA detected tropical species in temperate ecosystems that had never been recorded there before, including the lined surgeonfish, striated surgeonfish, common parrotfish, black-blotched porcupinefish, silver sweeper, and speckled squirrelfish—cryptic species that hide in caves or only emerge at night.

These are exactly the kinds of fish divers are most likely to miss.

Complementary methods needed

For temperate species, divers were often better at detecting them than eDNA was, showing that eDNA is not a replacement for traditional visual surveys, but a powerful complement. By combining the two approaches, researchers can better track species on the move.

Professor David Booth from University of Technology Sydney stated: “New coral and fish species arrive in Sydney’s oceans every year, and this is expected to increase with future climate change.”

These migrations are not unique to Australia. Species worldwide are shifting their ranges as climate change alters temperatures, ocean currents and habitats, making improved detection methods crucial for understanding how climate change is transforming marine ecosystems.