Tropical fish usually found at Ningaloo Reef move south to feed on kelp forests

By Bonnie Christian ABC News

A major climatic event has caused tropical fish that are usually found off Western Australia’s Ningaloo Reef to swim south to tackle seaweed forests off the Mid West coast, marine scientists say.

The fish are in such numbers and eating the kelp with a voracity not seen before anywhere else in the world.

A team of researchers from the University of Western Australia’s Oceans Institute has been studying the effects of a marine heatwave in 2011 on the temperate water ecosystem off Port Gregory.


Tropical fish devouring kelp forests as they swim south in WA (ABC News)

“This is really quite novel globally,” Scott Bennett said.

“Never have we seen to this scale fishes just overgraze the seaweed forests like this so this is quite unique.”

The researchers translocated some kelp to a reef off Port Gregory during the study and then filmed how the fish responded.

Mr Bennett said the kelp was consumed within hours and at a rate that was on average three times higher than had previously been observed around the world.



He said there were a couple of factors contributing to the high numbers of typically tropical fish species being found further down the WA coast and the devastation to the kelp.

“Ever since 2011, when we had a really warm summer and marine heatwave off the Western Australian coast, we found that the kelps have disappeared from quite a few reefs around Port Gregory,” he said.

“At the same time as that we’ve had a lot of tropical fishes that eat seaweed come down the coast by the strong Leeuwin current.

“They’re now just grazing flat out on the reefs around Port Gregory and they’re stopping any kelp recovery so we’re seeing complete loss of kelp forests.”

Mr Bennett said the research is showing how extreme climatic events can effect marine ecosystems.

“The warming on its own, the kelp could possibly have recovered but the combination of the warming and the interaction between the kelp and the tropical fish from completely different ecosystems means we’ve got a fundamental shift in the nature of these ecosystems and it’s really difficult for them to recover now,” he said.

The UWA research has been tracking the interactions between the fish and the kelp for a number of years to monitor recovery and changes in the fish.

“Over the past three years we’ve had a really warm phase, each summer since 2011 has been some of the warmest on record,” he said.

“Now that’s sort of swinging back and we’re going into a bit of a cooler phase and the El Nino cycle begins again and we’ll see if this gives an opportunity for the kelp to recover.

“At the moment as long as those fish stick around, it looks like it’ll be quite difficult for the kelp forests.”