By Bianca Nogrady
FRDC-funded research shows that exposure to seismic surveys:
The efficacy of seismic surveying as an undersea geological exploration tool has been well established since it was first introduced in the 1920s. Less clear cut is the impact it has on those creatures that live on and just above the sea floor.
Research into the effects of seismic noise on marine mammals such as whales and dolphins has led to practices designed to minimise negative impact on the ocean’s larger inhabitants. But relatively little is known about the effects on smaller creatures, particularly fish, shellfish and crustaceans.
Seismic surveying has been a significant concern for fishers and the communities that depend on fisheries for their livelihoods. To date, much of the discussion has been based on anecdotal and circumstantial evidence.
However, two FRDC-funded studies are providing more scientific evidence. Both experiments have examined the effects on two important fisheries species: the Southern Rock Lobster and the Commercial Scallop.
Seismic surveying uses rebounding sound to build up a picture of the sea floor and the geological structures up to 10 kilometres beneath it. The most common method used is an air gun towed behind a ship, which fires a pulse of sound towards the sea floor every eight to 15 seconds. The reflected soundwaves are recorded by rows of hydrophones also pulled on cables behind the ship.
The first of the two FRDC-supported studies is a collaboration between the University of Tasmania’s Institute for Marine and Antarctic Studies and Curtin University’s Centre for Marine Science and Technology.
Researchers Jayson Semmens, Ryan Day, Robert McCauley, Quinn Fitzgibbon and Klaas Hartmann selected two study sites – a 10 to 12-metre-deep sandy site for Scallops and a 10 to 12-metre-deep limestone rock platform for lobsters. They then acquired 380 Southern Rock Lobsters and 560 Scallops from sources including nearby sites and commercial fisheries and stored them briefly before relocating them to lobster pots and enclosures at the sites.
The two species were exposed to differing numbers of passes by the air gun at distances designed to mimic the exposures they would experience in the wild. The experiments were conducted in summer and winter and with different pressures of the air gun. There was also a control group for each species that was not exposed to the air gun at all.
Jayson Semmens and his team saw no evidence of the mass fatalities some fishers had feared. However, their findings suggested the air-gun exposure was far from benign and caused a range of what researches call “sub-lethal” effects.
First, in the summer experiment, they found lobsters exposed to the air gun had an immediate decrease in the simple tail extension reflex, which in some cases lasted for up to two weeks after exposure.
“The effects of stress in lobsters are known to be exacerbated in warm summer conditions, which explains why this response was only observed in the experiment conducted in the summer,” the researchers wrote. “However, the duration of the response indicates that its cause cannot be explained simply by fatigue.”
Second, the lobsters exposed to the noise also took longer to right themselves after being placed on their back, which is a more complex reflex. When the researchers investigated further by examining one of the organs that plays a key role in this reflex – the statocyst, which functions in a similar way to the human inner ear – they found significant damage that was linked directly to slower righting times.
One of the winter trials recorded no difference in righting time between the exposed and control lobsters. Researchers examined the statocyst in both groups and found animals in both had significant damage to the organ.
As the lobsters in this trial were were originally collected from sites with higher levels of human-induced noise, the researchers have suggested this damage may have occurred prior to the trial.
However, the results of this trial also indicate that lobsters might be able to adapt to statocyst damage because they did not display impaired righting reflexes.
Researchers also looked at the biochemistry of the lobsters’ haemolymph – their equivalent of blood – and found a significant decrease in the the number of immune cells that serve as an indicator of general health in exposed lobsters.
“These results raise some concern that exposure may affect the immune system of lobsters over a chronic (months post-exposure) time period, leaving them vulnerable to pathogens,” researchers wrote.
The researchers did not see any negative effects of the air gun exposure on lobster embryos.
In the case of Scallops, there was no evidence of mass mortality, but researchers did see increasing mortality with an increased level of exposure to the air gun.
Unexposed control Scallops had a mortality rate of less than five per cent 120 days after the experiment began. Scallops exposed to one pass from the air gun had a nine to 11 per cent mortality. Those exposed to two passes had mortality rates of 10 to 16 per cent and those exposed to four passes had a mortality rate ranging from 13 to 20 per cent.
The Scallops behaved slightly differently after the air gun had passed, showing less of their usual positioning and swimming behaviour. They also demonstrated a new flinching behaviour that was only seen when the air gun was going off.
As with the lobsters, the exposed Scallops showed lower immune cell counts. The team of scientists also saw “extreme” changes in the biochemistry of the Scallops’ haemolymph, which persisted over the course of the 120-day experiment. “The ecological implications of these extreme physiological changes also warrant further study, as they may have substantial impacts on the ability for Scallops to cope with further stressors (for example, dredging, temperature changes, etc) in the wild following exposure to seismic signals,” the researchers wrote.
This study represents one of the most detailed and controlled experiments worldwide on the impact of air guns on these populations in a natural environment.
In another study – an FRDC joint venture with Geoscience Australia, CSIRO and the University of Tasmania’s Australian Maritime College – Rachel Przeslawski, Lynton Hurt, Alex Forrest and Andrew Carroll set out to explore the effects of a real seismic survey on ‘wild’ populations of Scallops in the Gippsland Basin off the coast of Victoria. In this case, the team used an autonomous underwater vehicle to take snapshots of Scallop beds before and after the seismic surveys, and also dredged some areas to look for evidence of Scallop die-offs.
The researchers saw little impact from the seismic surveys on Scallops in the region, even though sound monitoring at the sites confirmed that the Scallops were exposed to a significant amount of noise.
Rachel Przeslawski and her colleagues said their study was the third of its kind to find no evidence of mass die-offs in Scallops immediately after exposure to seismic surveying. However, they pointed out that there might be other negative and sub-lethal effects they were not able to detect.
“The main recommendation of the current study is to focus efforts on the long-term or physiological effects of seismic surveys on Scallops and other invertebrates, rather than short-term gross effects such as mortality, as well as other factors that may cause mass mortalities (for example, disease and marine heatwaves),” they wrote.
As is often the case, the two studies deliver some answers but raise many more questions in the process. While the studies may go some way towards reassuring fishers and fishery managers that immediate mass die-offs are not the result of seismic surveying, there is still clear evidence that the surveys are having an effect – possibly long-term – on these valuable fishery species.
FRDC Research Codes: 2012-008, 2014-041
Jayson Semmens, firstname.lastname@example.org
Rachel Przeslawski, email@example.com