Changes to the global climate, as well as ocean warming, are strengthening the currents that transport warm tropical water towards the poles. This leads to the formation of ‘hotspots’, where water temperatures are rising at rates two to three times the global average, allowing tropical fish such as rabbitfish (above) to expand their ranges into temperate areas.
More than 40 per cent of the kelp and algal beds in southern Japan have disappeared since the 1990s due to overgrazing of rabbitfish and parrotfish. Although these fish have been present in the area for some time, their grazing rates have increased as ocean temperatures in winter have risen. Similar effects have been seen in the eastern Mediterranean, the Gulf of Mexico and Australia.
‘In tropical regions, plant-eating fish perform the vital role of keeping reefs free of large seaweeds, allowing corals to flourish,’ said the study’s lead author, Adriana Verges of the University of New South Wales. ‘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.’