Tropical fish are invading temperate waters

The last few years have seen a growing trend of tropical fish species moving closer to the poles and invading temperate aquatic ecosystems. As the world’s oceans continue to warm, tropical fish species are now able to make a home in temperate oceans which were previously unsuited to their needs. As a result, native kelp forests and sea grass meadows are being wiped out by the invaders.

This is a phenomenon known as “tropicalization”. The southern coast of Japan is one of the most striking examples of this phenomenon as 40% of the areas famous kelp and algal beds have disappeared in just the last twenty years or so. As the kelp forests disappear, so too do the local fish. This is incredibly damaging to the fishing industry in the area which is the livelihood of thousands of families.

“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.”

While the entire ocean is warming, there are a few “hot spots” around the world where the damage is especially bad. In these areas, climate change is causing currents that transport tropical waters to the poles to become stronger, and therefore, increasing the warming rate of these areas to two to three times that of the global average.

One such hot spot is in Australia, where the strengthening of the East Australian Current has contributed to a growing number of tropical fish species in Sydney Harbor every summer for the last few years. Unicornfish, parrotfish, and rabbitfish are some of the tropical species which have most effectively infiltrated temperate waters.

The influx of plant eating fish has a profound impact on local ecosystems and can lead to barren reefs, thus affecting the biodiversity of the area. Experts say that the most effective way to combat the influx of tropical fish is to eat them. This is a delicate issue, however, as these fish are vital to their own ecosystems.

Read more about the story at National Geographic.

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