Original story by Bianca Nogrady, ABC Science
Plant-eating tropical fish species are causing serious damage to algae and kelp forests in sub-tropical and temperate regions around the world, an international team of experts warn.
The findings come from a review published today in the Proceedings of the Royal Society B, which suggests that climate change is leading to ‘tropicalisation’ – the movement of tropical species towards the poles – as waters get warmer and ocean currents strengthen.
It reveals how algae and kelp-eating tropical fish such as rabbitfish have already led to the collapse of kelp forests – and their associated abalone fisheries – in Japan, and decimated the canopy-forming algae forests in the Mediterranean.
Two herbivorous tropical species – rabbitfish and drummer fish – have also been implicated in the loss of kelp forests on both the east and west coasts of Australia, says lead author of the study Dr Adriana Verges, marine ecologist at the University of New South Wales.
Overgrazing of algae and kelp by fish hampers recovery of the ecosystem from events such as heatwaves.
‘The west coast of Australia had a really bad heat wave that wiped out the kelp, and then because it was warmer, a whole lot of other species came in that prevented the kelp from coming back,’ says Verges.
Verges likens the loss of algae and kelp forests to the clear-felling of terrestrial forests.
‘Once the algae forests disappear, everything that goes with them goes, so we lose fish, we lose biodiversity, and we lose biomass,’ she says.
This also impacts on fisheries, as shown by the collapse of abalone fisheries in Japan following the loss of the kelp forests, the result of overgrazing by rabbitfish, parrotfish and drummer fish.
‘Crayfish and abalone are probably some of the most commercially-appreciated species that are supported by algae,’ says Verges.
Evidence from the United States suggests that turtles and dugongs are becoming more common in sub-tropical regions of the Gulf of Mexico, and are having a negative impact on seagrass beds, and on the fish and shellfish that use these areas as nurseries.
Verges says one solution may be to target the tropical fish species for food, particularly rabbitfish, which are considered a top eating fish in many parts of the world.
‘Maybe if we start to target our fisheries towards some of the species that are shifting and that are causing harm, perhaps we can at least slow the process down,’ she says.
However evidence from oceanographic modelling suggests this tropicalisation is happening much faster than expected.
Verges says Sydney Harbour can expect consistently tropical conditions, in which the winter temperatures exceed 18°C and the summer temperatures start exceeding 25°C by between, 2030 and 2060.