Global warming is driving tropical fish northward, destroying temperate algae forests

The oceans are warming up and tropical fish are taking advantage. A new study reveals that they’re extending their ranges poleward, chowing down on temperate algal forests and seagrass beds and leaving unproductive barrens… in their wake.

This is bad news for both the local ecosystem and the fishermen, as the local fish population declines due to the loss in habitat and the fishermen suffer the collapse of the fishery. Their impact was most pronounced in southern Japan and the eastern Mediterranean, with invading rabbitfish, surgeonfish, and unicornfish causing the collapse of the abalone fishery in southern Japan (abalone live among the kelp) and a 40% decrease in the number of species in the eastern Mediterranean. The research team consists of researchers from Australia, the United States, Spain, Singapore, the United Kingdom, and Japan.

“The tropicalization of temperate marine areas is a new phenomenon of global significance that has arisen because of climate change,” says study lead author Dr. Adriana Verges from the University of New South Wales. “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.”

“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. 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,” says Dr Verges. A phase shift from an algal-dominated community to a coral-dominated community has already been documented in southern Japan.

Increasing temperatures aren’t just raising water temperatures – they’re changing wind patterns as well. These changing wind patterns are intensifying warm, poleward-flowing western boundary currents (WBCs), and these WBCs are taking reef fish larvae along for the ride. Southern Japan is affected by the Kuroshio current, which flows northward from the Philippines. While these larvae would normally die in the colder waters, the warmer waters allow the larvae to survive and grow.

Rabbitfish were introduced into the eastern Mediterranean Sea when the Suez Canal was opened in 1869, connecting the tropical Info-Pacific Ocean and the temperate Mediterranean Sea. Once established, the rabbitfish removed all the canopy-forming algae, resulting in a 60% decrease in overall benthic biomass. Rabbitfish are now so numerous that they’ve now an important part of the fisheries catch in the Mediterranean. They were previously limited to the eastern Mediterranean because of temperature limits but because of global warming, rabbitfish are now expanding into the western Mediterranean as well. — TJD, GMA News

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