Climate: Invasive tropical fish already changing ocean ecosystems in eastern Mediterranean Sea

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Rabbitfish are changing ecosystems in the Mediterranean Sea.

Studies document loss of kelp forests and overall biodiversity

Staff Report

FRISCO — Tropical fish are moving north as the global climate warms, in some cases with devastating impacts to ocean ecosystems. In parts of the eastern Mediterranean Sea, rabbitfish have reduced the algae canopy by 65 percent with a cascading effect on ecosystems, including a 40 percent decrease in the total number of plant and animal species.

“The introduction of tropical fish into more temperate regions is troubling and this new study gives a vivid example of what can happen when non-native species occupy a new ecosystem,” said Fiona Tomas Nash, a courtesy professor of fisheries and wildlife at Oregon State University and a co-author on both studies.

“We now know that the arrival of tropical fish into temperate areas is occurring on an increasing basis around the world,” she added. “This is the first attempt to characterize what impacts these fish are having – and the mechanisms driving these impacts.”

Scientists surveyed about  1,000 kilometers of coastline in the eastern Mediterranean Sea, focusing on two distinct areas — one with warmer regions that attract abundant numbers of rabbitfish, and colder regions where they are very rare or completely absent.

“The fear is that if the colder regions warm just a bit through climate change or some other mechanism, rabbitfish will begin moving into those areas as well,” Tomas Nash said.

To learn more about how the rabbitfish changed the ecosystem, the researchers videotaped fish feeding in the Mediterranean off Turkey in two areas – one dominated by tropical rabbitfish and the other dominated by native temperate fish. They were surprised by what they found. Native temperate herbivorous fish actually had higher consumption rates than the tropical rabbitfish. “We did not expect to see that,” Tomas Nash said.

But while native fish targeted only adult macroalgae, the two species of rabbitfish fed complementarily – one targeted the mature kelps while the other fed almost exclusively on emerging algal “recruits,” or juvenile plants.

“The result is that one species denudes the forest and the other prevents it from recovering,” said Tomas Nash, who also has a faculty appointment with the Mediterranean Institute for Advanced Studies in Spain.

A study off Japan by collaborators found that the introduction of tropical species there, including rabbitfish and parrotfish, resulted in the loss of kelp forests and the emergence of non-native corals in as little as 20 years.

Together with other studies, the latest research paints a detailed picture of how tropical herbivorous fish are moving into temperate zones, including South Africa, Brazil, the Gulf of Mexico, Australia and Japan, as well as the Mediterranean. Other areas, including the Pacific Northwest of the United States, have not seen sustained spread of tropical species likely due to prevailing currents and because surface waters are too cold due to seasonal upwelling.

The researchers found algal forests in the waters off Greece had not been severely affected because only the rabbitfish that feeds on adult algae is present and in relatively low densities. They have just begun studies of rabbitfish and chub arrivals in Australia.

“The greatest damage that we documented was off Turkey, which may be serving as the proverbial canary in the coal mine,” Tomas Nash said. “The barrenness of the underwater habitat is unique and quite striking – it is spread over hundreds of kilometers.”

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