Roundup: Warmer oceans could ruin global fish populations: Aust'n expert

Climate change and the unabated rise of water temperatures could have catastrophic consequences for the global fish population, says an Australian research expert.

As a member of a Swedish-led research project, Dr Timothy Clark from the University of Tasmania has tested the effects of temperature on European perch in the Baltic Sea.

In their study published in online journal Nature Communications on Tuesday, the team found the species of fish had only limited ability to adjust its metabolic rate, which regulates body temperature, in hotter environments - meaning the fish's tolerance would eventually peak.

"There's been a lot of speculation about what fish populations will look like in 100 years, but this is the best evidence to date that fish are not going to be able to adjust indefinitely - which is why we have to manage greenhouse gas emissions and try to limit global temperature increases," Clark said in a statement on Tuesday.

Using the University of Gothenburg's experimental 'Biotest' lake, the group discovered that while fish can modify their resting physiological functions to cope with incremental rises in temperature, they presently weren't adapting quick enough.

"The fish can increase their lethal temperature (threshold) by a certain amount, but they can't keep up with the current rate of global water temperature increases," Clark, a research fellow with the university, said.

For more than 30 years, researchers have subjected the experimental perch to elevated water temperatures - between five to 10 degrees Celsius warmer than usual - in the man-made testing sanctuary.

Over the most recent testing period between 2012 and 2013, they compared the test perch with 'reference' perch taken from outside the lake, finding extreme biological changes in the experimental breed.

"When we warmed the reference perch quickly, their resting metabolic rate rose dramatically, whereas the Biotest fishes' resting metabolic rates were significantly lower at the same temperature, showing they've adjusted to temperature increases over time," Clark said.

But the perch's maximum oxygen uptake - a key factor in keeping core temperature stable long-term - remained unchanged, despite the time spent in the warmer enclosure.

Without tangible reduction of water temperatures, Clark suggested fish would begin to disappear from the world's oceans and, subsequently, the dinner table.

"A lot of the world's population relies on fish for protein, for example the Pacific Island nations, so if we do start losing fish populations, it will impact on humans quite dramatically," he said.

Clark also feared heatwaves, like the one currently wreaking havoc on Australia's Great Barrier Reef, would become more frequent, and destroy water sanctuaries closer to his home state.

"There's already issues facing aquaculture, for example growing salmon (in Tasmania), where the animals are not doing as well in the summer months," he said.

"So the research doesn't just speak to wild fish populations, it is also relevant to aquaculture, which is a huge industry here in Tasmania."

Earlier this year, Australia's largest salmon farmer, Tassal, had to withdraw its bid for supply tenders with a major supermarket chain, citing warmer water temperatures impacting fish development.

On the other side of the country, hundreds of dead fish - from mixed species including cod, sea perch, north-west snapper and coral trout - washed ashore near the Western Australian city of Broome in March.

WA Fisheries regional manager Peter Godfrey told the Australian Broadcasting Corporation it was "likely" warmer than average water temperatures were to blame for the mass fish kill.

Next year, Clarke will return to Sweden to continue testing on the perch populations. Endit