Warmer waters lure sub-tropical fish to the south

Original story by  Sarah Taillier, ABC News

As ocean temperatures rise off Western Australia, sub-tropical fish are swimming south and many are staying there.

Gary Jackson, the principal research scientist with the Department of Fisheries, has been monitoring changes in fish distribution off the state’s coast.

“We’re definitely seeing warming of the waters off the west coast and marine creatures, including fish, are taking advantage of this and actually moving basically southwards,” he said.

“We’ve got a whole portfolio of reports coming through in the last two to three years of some pretty unusual fish occurring south of Geraldton, off Perth and as far around as the Capes on the south coast.

“So strange things are happening and we’re only really starting to scratch the surface now on what is going on.”

The southern migration of fish and marine creatures is driven by two main factors; a gradual increase in ocean temperatures and the effects of the Leeuwin Current which Dr Jackson describes as a type of “fish highway.”

He says when that current, which runs southwards along the entire coast of WA, flows strongly, a lot of marine life moves with it.

“Small fish actually get pushed along by that strong current and larger fish basically get on it and it’s easier swimming for them and they’ll really go where that plume of warm water takes them,” he said.

Craig White runs a charter boat business out of Jurien Bay and has been fishing in waters off the Mid West coast of WA for about 25 years.

The offshore world champion angler says since a marine heatwave moved through the area in late 2010 and into early 2011, he has noticed new fish species moving in.

He says now there are signs those fish are prospering.

“After that (marine heatwave) happened we could get red throat snapper in a few very isolated areas and quite a random catch whereas up in Leeman you could get quite a lot of them and in Jurien Bay very few,” he said.

“Now we can get them from the back of the reef all the way out.

“They’ve actually become prolific, they’re absolutely everywhere, there’s been a massive influx of them.”

Heatwave results in coral bleaching and fish kills

During the marine heatwave in 2010/2011, ocean temperatures between Kalbarri and Jurien Bay rose by up to five degrees, resulting in coral bleaching, and fish and invertebrate kills in an area considered to be a global biodiversity hotspot.

Marine ecosystems within the area are still recovering from the damaging effects of what is referred to as an “unprecedented extreme warming event.”

Ming Feng is a physical oceanographer and principal research scientist with CSIRO who leads the Integrated Marine Observing System, IMOS, which monitors the long term changes in the Leeuwin Current and the impact of climate change in WA.

Dr Feng says over the last 60 years, ocean temperatures off the west coast of Australia have warmed by up to one degree.

He says ocean temperatures off the state’s coast are expected to continue to rise which means the frequency and severity of ocean heatwaves are likely to intensify.

“So with the warming temperature tend, any future marine heatwave event will be superimposed on that slow rising trend,” he said.

“So if you have the same threshold for marine biota the marine heatwave you experience in the future will have a much higher anomalies than probably 20 or 30 years ago.”

Dr Feng says ocean temperatures off WA are generally determined by the strength of the Leeuwin Current which is influenced by the tropical Pacific climate.

He says climate forecast models for the next few months predict a neutral condition for the tropical Pacific climate.

“So if the tropical Pacific climate is neutral in the coming summer, then it’s very unlikely we will experience another extreme marine heatwave off the WA coast,” he said.

Dr Feng says although extreme marine heatwaves are unlikely over the next few months, other less severe temperature events may occur.

“Marine heatwaves can be triggered by other variabilities of the local winds in the Indian Ocean off this coast but these type of marine heatwaves or temperature anomalies tend to be more moderate,” he said.

He says another extreme heatwave would have a damaging effect on marine environments, especially for ecosystems in areas off WA which are expected to take years to recover from the last extreme heat event in 2010/11.

Sub-tropical fish taking up residence in southern waters

Dr Jackson says certain fish are taking advantage of the warming ocean temperatures.

“Warm water or sub-tropical species have a certain temperature range,” he said.

“And, if those temperature ranges are found a bit further south of where they typically are, then those animals will quite happily move there, live there and stay there, if those conditions are right and obviously if they can find food,” he said.

It appears the conditions are right in many areas as Dr Jackson says fish are changing locations and remaining there.

“They seem to have stayed, survived through the winter – which is normally the telling thing if they can survive through the cooler temperatures – and some of those fish appear to be basically breeding in the region now,” he said.

Meanwhile, Mr White says the warming oceans temperatures have brought benefits.

“I personally think it’s a good thing, we’re getting a lot more species of fish, a lot more variety, and it doesn’t appear to be bothering the local stocks,” he said.

“That’s coming from a typical fisherman’s point of view.

“There’s a saying going around that ‘global warming means rising sea levels…more fishing spots’.”

If you’ve seen any unusual fish or marine life in your area, scientists are asking you to take a photo and upload it to Redmap (<http://www.redmap.org.au/> – a national, citizen science project tracking the changing distribution of marine species.