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Seismic Blasts Used to Find Oil Can Blow Massive Holes in Plankton Populations

"We were quite gobsmacked."

MIKE MCRAE
23 JUN 2017
 

Using bursts of sound to explore the ocean floor for signs of oil deposits has been shown to kill microscopic plankton more than a kilometre away.

The new research expands estimates on how far the lethal impact of the blasts could reach, and adds yet another concern to the growing list of risks seismic surveys poses on marine life.

 

Since the late 1960s, surveyors have used bursts of compressed air to map the geology of the ocean floor, bouncing a series of sound-waves up to 255 decibels in volume between a boat and the rock below in search of underground resources such as oil deposits.

The practice has long been controversial, with mixed evidence on the extent of the impact such loud noise has on fisheries, marine mammal migration, and other components of the surrounding ecosystems.

At its worst, the blasts could be causing significant distress to whales and dolphins up to 70 kilometres (44 miles) away, who rely on sound to communicate over long distances.

There is some speculation that the use of air guns could even be behind a number of beach strandings, and evidence of damage to fish up to several kilometres away, and potentially be causing harm to turtles and large marine invertebrates such as squid.

Oddly, little research has been done on the physical damage being done to the lower rungs of the marine food chain – the zooplankton.

"It could be that our focus has kind of been blinkered because it's been on whales," lead researcher Jayson Semmens from University of Tasmania told Jeff Tollefson at Nature.

 

In 2015, Semmens and a team of marine biologists from several Australian research institutions conducted a study off the Tasmanian coast, using nets and sonar to measure the numbers and diversity of living and dead zooplankton prior to and following their use of an air gun.

The single gun they used was similar in size to those used by the oil exploration industry.

The researchers found that the absolute numbers of zooplankton had dropped by 64 percent within an hour of the survey, with two to three times the relative number of dead individuals being found after exposure to the air gun blasts.

Earlier attempts to model the impact of the acoustics on zooplankton within just 10 metres (30 feet) of the blast suggested the noise would be insignificant compared with the naturally high turnover of the populations.

The study found the actual impact was 100 times greater than thought, and up to 1.2 kilometres (0.75 miles) away from the air blasts.

"We were quite gobsmacked," says Semmens.

In effect, each blast from the air gun seems to be blowing massive hole through the very foundations of the food web, one which took hours, if not days to close again.

 

The full ecological ramifications of this destruction aren't yet clear, nor do the researchers know exactly how the sounds are harming the microscopic animals physically.

One possibility is the sounds destroy the delicate hairs the animals use to orient themselves, rather than killing them outright.

Oil exploration is a political explosive topic across the globe, with a pause in US surveys off the Pacific and Atlantic coasts expected to resume in the near future.

That isn't to say seismic surveys will be used in the search of new deposits, or how the National Oceanic and Atmospheric Administration would permit surveys in light of the evidence of impact on marine life.

In Australian waters alone during 2014 and early 2015, seismic surveys covered a distance of just under 16,000 kilometres (about 10,000 miles) in search of oil every three months.

Research such as this suggests the full effect of seismic surveys on marine ecosystems is far from understood, with a need to now put air blasts under the microscope.

This research was published in Nature Ecology & Evolution.

 

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