The University of Tasmania’s Institute for Marine and Antarctic Studies and Curtin University’s Centre for Marine Science and Technology has investigated the effects of seismic survey acoustics on adult rock lobsters and scallops.
Seismic surveys are used to produce detailed images of local geology to determine the location and size of possible oil and gas reservoirs. Sound waves are bounced off underground rock formations and the waves that reflect back to the surface are captured by recording sensors for analysis later.
Important fishing grounds and areas of interest for oil and gas exploration overlap in south east Australia. In 2010, fishermen in Bass Strait reported the death of 24,000 tonnes of scallops worth $70 million after seismic testing carried out by the Victorian Government. The fishing industry is concerned about the potential of intense low frequency acoustic signals produced during these surveys to disturb, harm or even kill fisheries species.
The study used an industry standard seismic air gun in lobster and scallop habitat off Tasmania to simulate seismic testing undertaken by the oil and gas industry. Following exposure, 302 lobsters, the majority of which were berried (egg bearing) females, and 560 scallops were sampled. The lobster habitat used was a hard limestone reef which increased sound transmission losses compared to the sandy scallop site.
Seismic exposure did not result in any immediate lobster deaths but a range of other effects were observed. Lobsters exposed to the replicated seismic survey in summer showed a 32% decrease in the ability to extend their tails compared to control lobsters that were not exposed. This response persisted for 14 days after exposure, when exposed lobsters had a 23% decrease in the ability to maintain tail extension. The effects of stress in lobsters are known to be worse in warm water, which explains why this response was only observed in the experiment conducted in the summer. However, the duration of the inability to tail extend indicates that its cause cannot be explained simply by fatigue and the cause is more complex. A lobster who cannot extend their tail may be less able to escape from a predator.
The second reflex affected in lobsters was the righting response – the time lobsters took to right themselves after being placed on their back. Depending on season, righting time increased by 80-157%. Most concerning was that in one experiment righting was slowed for 365 days. Lobsters moult annually so the study concluded that the damage caused by seismic surveys may be permanent. The study investigated an organ called the statocyst; a pair of fluid-filled sacs found at the base of the lobsters’ antennas. These organs are similar to the human inner ear, and are filled with sensory hair cells that detect gravity and body position. After exposure to a seismic survey hair cells showed significant damage. Statistical analysis showed that this damage was correlated to slower righting time, with greater damage resulting in slower righting.
To evaluate whether exposure affected the development of lobster embryos following exposure early in embryonic development, the berried (egg-bearing) female lobsters were maintained until the eggs hatched. Hatched larvae were found to be unaffected in terms of egg development and the number of hatch larvae.
There was no immediate mortality of scallops exposed to simulated seismic surveys. However, repeated exposure to air gun passes were found to significantly increase mortality. Compared to control scallops, which were found to have a total mortality rate of 5% at day 120 post-exposure, exposed scallops showed mortality rates of 9%-11% for 1-pass treatment, 11%-16% for 2-pass treatment and 15%-20% for the 4-pass treatment.
Scallop behaviour was also altered by exposure to the seismic survey with a decrease in classic behaviours including positioning, mantle irrigation and swimming and the observation of a flinching behaviour.
As with lobsters the righting reflex was also significantly slower in exposed scallops. The ecological implications of these changes in behaviour and reflexes require further study, as they may have substantial impacts on the ability for scallops to cope with predators in the wild.
Given the compromised physiological condition of the exposed scallops following seismic surveys the study concluded it likely that scallops would have reduced tolerance to subsequent environmental, nutritional and pathological stressors. Further, that this impairment would result in increased mortality in time frames beyond those examined in the study.