It’s what every snorkeler diving into warm equatorial water hopes to see — a prismatic array of tropical fish darting around coral reefs. But as the world’s oceans have warmed, technicolor schools of fish have become unwelcome visitors in what used to be temperate aquatic ecosystems. And as the fish invade waters closer to the poles, they are wiping out native kelp forests and seagrass meadows — radically changing life on the sea floor.
One of the most striking examples of this phenomenon, known as “tropicalization,” has taken place off the southern coast of Japan, where 40 percent of the famous kelp and algal beds have disappeared in just the last two decades. The kelp forests have taken the lucrative abalone fishing industry with them, affecting livelihoods onshore. The destruction of kelp forests has similar knock-on effects to clear-cutting a terrestrial forest.
“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 Dr. Adriana Verges of University of New South Wales, lead author on a new paper outlining the problem. “But when they intrude into temperate waters they pose a significant threat to these habitats. They can directly overgraze algal forests as well as prevent the recovery of algae that have been damaged for other reasons.”
The research highlights regions around the world threatened by tropicalization. While the entire ocean is warming, there are certain “hot spots” around the world where climate change is causing currents that transport tropical waters toward the poles to strengthen, leading to rates of warming that are two to three times the global average.
In Australia, the intensification in the flow of the East Australian Current — made famous in “Finding Nemo” — has led to tropical fish in Sydney Harbor every summer in recent years. Kelp on reefs has also been declining steadily for the past five years. Off the coast of Western Australia, the researchers say that the influx of tropical fish is stopping kelp forests damaged by a heat wave in 2011 from recovering.
Unicornfish, parrotfish, and rabbitfish are some of the tropical species which have most effectively infiltrated temperate waters.
Rabbitfish in the Eastern Mediterranean have deforested hundreds of miles of kelp, resulting in a 40 percent decrease in marine species biodiversity.
“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,” said Verges in a release.
The study authors recommend eating the invading fish as the most effective way to immediately combat their numbers. It will be a delicate balance, however, as many of the species causing the problems in temperate waters are so vital in tropical ecosystems. A recent United Nations Environment Program report stressed the need to protect parrotfish to ensure the health of coral reefs in the Caribbean, which are even more directly threatened by the changing climate.