Australia: Destruction of kelp forests by tropical fish shows impact of ocean temperature rises

Deforestation near Coffs Harbour coincided with 0.6C temperature rise, which had ‘catastrophic’ effect of attracting fish
Calla Wahlquist The Guardian 14 Nov 16;

Herbivorous tropical fish have destroyed kelp forests in northern New South Wales, showing that even small increases in ocean temperature can lead to kelp deforestation, an Australian study has found.

The University of NSW study, published in the Proceedings of the National Academy of Sciences on Tuesday, found that the disappearance of kelp from waters near Coffs Harbour coincided with a threefold increase in the number of tropical fish in the region.

The deforestation coincided with an 0.6 degree temperature rise. While that was not enough to directly impact the kelp, lead author Dr Adriana Vergés said it had the “truly catastrophic” effect of attracting hungry fish.

The study examined video footage of 12 sites between 2002 and 2011. In 2002, six of the sites contained kelp. By 2010, all the kelp was gone.

The proportion of kelp showing signs of bite marks increased from less than 10% in 2002 to more than 70% in 2008, before there was no kelp to measure. At the same time the proportion of tropical fish in the ecosystem increased from less than 10% to more than 30%.

Most prolific were surgeonfish, which increased from 9% of the local fish population in 2002 to 33% at the end of the study period in 2011.

Once the kelp had been removed, the ecosystem changed “quite dramatically” to become more tropical, in a trend Vergés said could potentially be seen globally.

“We call it a homogenisation of community,” she said. “It’s a bit like globalisation – everything starts to become the same everywhere.”

Researchers used video originally filmed to catalogue fish populations to track the decline of the kelp forests and also conducted experiments in kelp-free areas to see what fish would appear to snack on a spring of transplanted kelp.

In one of the videos, a school of rabbitfish crowded around a scrap of kelp briefly lifts to let a shark glide past, before descending again.

In another a lamington urchin, a creature that looks like a bald tennis ball that has been cut in half and covered in white spikes, can be seen motoring towards the kelp from some distance away before crawling on to the frond.

Urchins are traditional kelp grazers but this species is tropical.

Vergés said the decision to examine the impact of fish stemmed from a similar study into mass kelp death off the Western Australian coast, where a 100km stretch of kelp forests died in a marine heatwave in 2011.

That study found the kelp was prevented from regenerating by rabbitfish and parrotfish, which ate any regrowth.

The director of the Sydney Institute of Marine Science, Prof Peter Steinberg, who contributed to the UNSW study, said the findings increased the range of temperatures that could be considered to have a catastrophic impact because the tropical fish were triggered by a much smaller increase in temperature.

“It may be in Western Australia the heatwave simply beat the fish to it,” he said.