Invading tropical fish pose a serious threat to temperate waters by overgrazing on marine vegetation, say scientists.
Experts studied the harmful impact of fish migrating away from the tropics as a result of ocean warming.
They found the strongest evidence in the eastern Mediterranean and southern Japanese waters, where there had been dramatic declines in kelp forests.
Other areas that had suffered damage included coastal waters around the US and Australia.
Increases in the number of plant-eating tropical fish can profoundly alter ecosystems and lead to barren reefs
The invaders, which include unicornfish, parrotfish and rabbitfish, are also said to be affecting seagrass meadows.
Australian lead researcher Adriana Verges, from the University of New South Wales, said: “The tropicalisation of temperate marine areas is a new phenomenon of global significance that has arisen because of climate change.
“Increases in the number of plant-eating tropical fish can profoundly alter ecosystems and lead to barren reefs, affecting the biodiversity of these regions, with significant economic and management impacts.”
As the ocean temperatures rise, hotspots are developing in regions where strengthening currents transport warm tropical waters towards the poles, said the scientists.
One example is the increased flow of the East Australian Current, which has caused waters southeast of Australia to warm two to three times faster than the global average.
As a result, tropical fish are now common in Sydney harbour during the summer months. Japan, the US east coast, northern Brazil and southeastern Africa are also strongly influenced by currents transporting warm tropical waters, the researchers point out.
“In tropical regions, a wide diversity of plant-eating fish perform the vital role of keeping reefs free of large seaweeds, allowing corals to flourish,” said Verges.
“But when they intrude into temperate waters they pose a significant threat to these habitats.”
Writing in the journal Proceedings of the Royal Society B, the researchers conclude: “Tropicalisation of temperate marine communities could become a global phenomenon.”
Report findings
■ In southern Japan, more than 40 per cent of the kelp and algal beds have vanished since the 1990s, a phenomenon known in Japan as isoyake. Tropical species including rabbitfish and parrotfish are thought to be chiefly responsible.
With the loss of the marine plants, corals now dominate the ecosystem in many locations. The changes have led to the collapse of the abalone fishery.
■ Tropical fish entered the eastern Mediterranean from the Red Sea after the Suez canal was opened in 1869. Rabbitfish numbers have increased in recent decades, resulting in the deforestation of hundreds of kilometres of seabed and a 40 per cent decrease in the variety of marine species.
■ Warming water has led to a 22-fold rise in the abundance of parrotfish in the Gulf of Mexico, US. The fish are consuming seagrass at five times the rate of native species. Numbers of plant-eating green turtles and manatees have also increased.
■ Emerging evidence suggests that in western Australia increases in the number of tropical fish are preventing the recovery of kelp forest damaged by a heat wave in 2011.
■ In eastern Australia, kelp has disappeared from numerous reefs in the past five years, following intense grazing by tropical fish.