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Tropical fish head south for a feast

By Linden Ashcroft

7 August 2014

New research from The University of New South Wales has found that tropical fish are migrating further south, posing a serious threat to the cooler marine environments.

Bright colourful fish from the tropics such as parrotfish, rabbitfish and unicornfish are moving towards the poles as ocean temperatures warm.

While they are bright and beautiful, these plant-eating fish increase pressure on the kelp forests and other cool-water marine ecosystems. Overgrazing from these fish can lead to barren areas, affecting biodiversity, fisheries and coastal management.

Tropical fish that eat plants play an important role in coral reefs by keeping seaweed and algae under control, giving coral a chance to thrive. Warmer ocean temperatures mean that these fish are now moving into areas where algae and seaweed are key habitats, and they are gorging themselves.

The new international study, published in the Proceedings of the Royal Society, found that tropical fish are expanding into temperate regions in southern Japan, across the eastern Mediterranean, and in the Gulf of Mexico.

A map from the study showing ocean currents, the shift in plant-eating fish and the recorded impact that these fish are having on temperature marine plants. Red arrows mark the warming currents that are allowing the fish to migrate.  Kelp species covered with a cross mean a decline in population. Algae species with a plus (e.g. along the Gulf Stream) mean that fish migration has increased the plant population.

In Australia, tropical fish migration has been linked to the disappearance of common kelp (Ecklonia radiata) along the eastern Australian coast. The East Australian Current, which moves warm water from the Coral Sea down Australia’s east coast, has extending 350km south since the 1940s, warming southeastern waters at more than twice the global average.

Western Australian kelp forests damaged by the 2011 marine heatwave are also struggling to recover because of the increase of tropical fish.

It is important to monitor kelp population in these areas, to assess the impact that tropical fish are having on the cool-ocean ecosystem. You can help by recording your kelp and algae sightings for ClimateWatch online or using our smartphone app.

The full scientific article can be found here.