The tracked birds took between eight and 20 days to reach Australia from the Arctic - over 13,000 kilometres.
The mass death, or "wreck" of shearwaters, extended from southern Queensland right down the east coast to Tasmania. Pic: ABC News: Margaret Paul
30 March 21
Many Australian surfers and beach lovers will remember the many dead and dying seabirds washing up along the East Coast back in 2013, and finally there is an answer to this mystery. From the strange and tragic sea stories file, as reported by the ABC Science:
When millions of dead and dying sea birds washed or dragged themselves ashore on Australia's east coast in 2013, scientists found something unexpected in their stomachs.
The birds were short-tailed shearwaters — migratory ocean-going birds that spend the northern summer in the Arctic, before heading to southern Australia to breed, usually around September.
Necropsies revealed that nearly 90 per cent had eaten pumice stone pebbles — stones created when explosive volcanic lava hits water and cools quickly.
The birds had an average of four to five stones in their stomachs, some with many more.
The question for scientists was, had the birds starved because they had eaten the pumice stones and couldn't digest enough food, or were the pumice stones a symptom of their starvation?
Now they have an answer, and they've published their findings in the journal Marine Ecology Progress Series.
Was an underwater eruption the cause?
In July 2012, a passenger on a commercial airline discovered an erupting underwater volcano out the window of their plane during a flight from Samoa to New Zealand.
The passenger reported what they had seen to scientists, including volcano researcher Scott Bryan from the Queensland University of Technology.
Using the pumice on the sea surface, known as a pumice raft, as their starting point, the researchers were able to pinpoint the location of the eruption to a seamount in New Zealand waters that was not known to be volcanically active.
"Havre [volcano] in the Kermadec Islands ... the summit is about 900 metres below sea level and produced one of the biggest pumice rafts we've seen," Associate Professor Bryan said.
One of Dr Bryan's areas of interest is in how "life interacts with volcanism". A key part of that is how animals use pumice rafts to transport themselves across oceans.
Because the Kermadec Islands eruption produced such a big rafting event, he had taken an interest and was one of a number of scientists who tracked the path of the pumice as it drifted across the Pacific and eventually began washing up on Australia's coast in 2013.
Birds were eating 'anything they could'
Meanwhile, other researchers were tracking the migratory path of the shearwaters, including some fitted with small leg-mounted tracking devices, according to study co-author Lauren Roman from the CSIRO.
"The birds we tracked left [the Arctic] between the 9th September and the 15th October and they were taking between eight and 20 days to get here," Dr Roman said.
"When they start their journey, they travel pretty directly."
By combining their knowledge of the birds' flight path with the track of the pumice raft from the Kermadec Islands, the researchers were able to pinpoint when the birds came across the raft and ate the pumice, Dr Bryan said.
"The advantage we had with the pumice was we knew the source, we knew where it was because we'd been tracking it, and we therefore knew when the shearwaters had eaten it up."
If the birds had eaten the pumice weeks before they began dying, it was possible that the pumice was the cause of their starvation.
But as it turned out, the birds had come across the raft in the few days and hours before reaching the Australian coast, and were therefore already desperate for food.
"They were in such a bad state through starvation that we could conclude that in the final hours and days they were eating anything they could," Dr Bryan said.
"The pumice was quite small. They were very selective in the size they were eating."
Why were the birds starving?
So the questions then became, why were the birds eating the pumice? And if it wasn't the pumice that caused the mass starvation, what was it?
In answer to the first, as the pumice travels across the ocean, it does accumulate some life — things like corals and barnacles.
It's possible that the birds thought they could get some sort of energy from eating the stones, but more likely they were simply acting out of desperation, Dr Bryan said.
"They were so starved that anything that looked like food, they went for it."
This may have important implications for ocean plastics as well, according to Dr Roman.
"With increasing pressures from things like fisheries and climate change, which may be depleting their natural prey, there's more chance that the animal will be eating under duress and ingest something they shouldn't," she said.
"It shows that these interacting threats may create a greater danger to the animals than either of the threats in isolation."
"It's likely something had gone wrong in the north, rather than something going wrong down here," Dr Roman said.
There are a range of reasons why seabird wrecks occur, according to ecologist Mary-Anne Lea from the University of Tasmania.
One of the theories is that fish may out-compete the birds for food.
"Shearwaters and pink salmon eat the same prey and there are big numbers of pink salmon every second year," Associate Professor Lea said.
Other factors include storms that blow the birds off track, and things like overfishing.
"The jury is still out on the patterns and it's partly because there are so many contributing factors," Dr Lea said.
“The Blob” associated with seabird wrecks
A massive heatwave known as "the blob" emerged off the US Pacific coast in 2013, bringing water temperatures up to 6 degrees Celsius above typical maximums.
That in turn caused an upheaval in the food chain, with the number of small fish like sardines and anchovies plummeting.
"The blob" persisted for several years and in 2015-16, around 62,000 common murres — a North Pacific seabird — washed ashore between California and Alaska.
The crash in the small fish abundance was found to be the cause of the birds' starvation.
In 2013, there was also a marine heatwave in the Arctic, Dr Roman said.
"In the Arctic circle in the last few years there have been quite a few marine heatwaves," she said.
"There was a marine heatwave in 2013 and we know they have caused wrecks in other Arctic seabirds, but we [can't say] definitely."
Whatever the cause, research manager Duncan Sutherland from Phillip Island Nature Parks thinks we'll see the frequency of wrecks increase in the future.
But the number of shearwaters have bounced back on Phillip Island since the 2013 wreck, he said.
"We've been doing quite a bit of monitoring on Phillip Island and indications are we've had relatively good breeding seasons since then and numbers have recovered," he said.
"There's some suggestion the frequency of these wrecks has increased over more recent decades.
"I suspect that climate change is going to make this more likely into the future."
- AUTHOR: NICK KILVERT
- SOURCE: ABC SCIENCE