Heading south: Marine warming to impact local fisheries

It’s not the Great Barrier Reef, or Ningaloo.

It’s the kelp forests of the Great Southern Reef.

Running from Kalbarri, along the coast south and over to Tasmania and New South Wales, the Great Southern Reef covers 71, 000 square kilometers and hosts a kelp forest that contributes $10 billion to the fishing and tourism industries.

Last Thursday researchers published a study that showed the effects of an unprecedented marine heat wave in 2011 on this biodiversity hotspot, and the results didn’t look too good.

*Biodiversity hotspot: an area of extremely high biodiversity, with many species found nowhere else in the world; AND is considered threatened, with at least 30% of habitat lost.

The worst of it involved the complete extinction of 100 km of kelp forest near Kalbarri, which showed no sign of recovery even after five years, and although the event was localised, lead author of the study Thomas Wemberg predicts events like these will continue heading further south in future.

“What we think will happen is that what we’ve seen in the the north will start happening further and further south,” Mr Wemberg said.

He said although Mandurah hadn’t been affected this time around, it could be in future.

The study found the heatwaves and increasingly warm water temperatures were causing “tropicalisation”, which co-author Scott Bennett said increased the abundance of tropical grazing fish, preventing kelp forests from recovering.

“When the water is warm, you get new species shifting south, so one of the things that we’ve seen, and this has been seen down to Mandurah and further south as well, is that catches of fishes that are tropical or sub-tropical have increased substantially in recent times,” Mr Wemberg said.

Tropicalisation may at first seem like a good thing: who doesn’t want a Ningaloo reef off right off our own beaches?

However, this ecosystem shift is not quite so simple.

“The big problem here is that we’re not talking about a temperate marine ecosystem being replaced by a new tropical ecosystem… it’s definitely not what’s happening now,” Mr Wemberg said.

Instead, he said, some of the desirable components of a temperate system are lost, while tropical components are gained.

“You’re really stuck with something in between that’s a lot less desirable in terms of productivity in terms of all of the species and the biodiversity it can sustain.”

This, of course, has implications for that $10 billion industry based on the kelp forests.

“It could have an impact on any type of fishery that is associated with the kelp forest...some of the big fisheries in Australia,” Mr Wemberg said.

Mr Wernberg said like trees in a forest or corals on a coral reef, the kelp forests were the foundations of the ecosystem. 

“Rock lobster and abalone are quite tightly linked to the kelp forest, so they would certainly be impacted if the kelp forest was to disappear.”

Mr Wemberg was also concerned for the future of recreational fisheries.

“Ocean warming is a one-directional process, so the water is going to get warmer and warmer, and that’s not going to chance in the near future, regardless of what we do basically,” he said.

He said that aside from large-scale reduction of carbon emissions that are causing the warming, mitigation measures are possible, to slow the effects of temperature change.

He said locally, we should aim to control sediment loads from dredging, surface runoff, and nutrient runoff.

“By reducing those local stressors we can increase the resilience of the system so that better it can withstand the heating,” Mr Wernberg said.

“Somewhere between 30 and 80% of all species we have here [in the south-west] are not found any where else,” he said. 

“If our kelp forests disappear, a lot of those associated species will disappear, and they will be gone from the planet because they aren’t found anywhere else.”

The full study is published in journal Science and can be read here.