Widely-Used Marine Seismic Gun Negatively Impacts Zooplankton

Zooplankton underpin the health and productivity of global marine ecosystems. Here we present evidence that suggests seismic surveys cause significant mortality to zooplankton populations. Seismic surveys are used extensively to explore for petroleum resources using intense, low-frequency, acoustic impulse signals. Experimental air gun signal exposure decreased zooplankton abundance when compared with controls, as measured by sonar (~3–4 dB drop within 15–30 min) and net tows (median 64% decrease within 1 h), and caused a two- to threefold increase in dead adult and larval zooplankton. Impacts were observed out to the maximum 1.2 km range sampled, which was more than two orders of magnitude greater than the previously assumed impact range of 10 m. Although no adult krill were present, all larval krill were killed after air gun passage. There is a significant and unacknowledged potential for ocean ecosystem function and productivity to be negatively impacted by present seismic technology.

Phytoplankton and their grazers—zooplankton—underpin ocean productivity, therefore significant impacts on plankton by anthropogenic sources have enormous implications for ocean ecosystem structure and health. In addition, a significant component of zooplankton communities comprises the larval stages of many commercial fisheries species. Healthy populations of fish, top predators and marine mammals are not possible without viable planktonic productivity.

Man’s dependence on fossil fuels requires continual exploration for new resources. Deposits of undiscovered oil and gas reserves in the world’s oceans4 are estimated to be substantial, with exploration occurring in most petroleum provinces. In the marine environment, exploration is achieved via an acoustic imaging technique that uses intense, low-frequency impulse signals generated near the sea surface and directed into the seabed (‘seismic surveys’). Spatially distributed arrays of air guns simultaneously release high-pressure air (13.8 MPa or 2,000 psi) into the water to produce the impulse signal. Reflections from sub-sea density discontinuities received by strings of hydrophones enable sub-sea image generation. Commonly, a series of closely spaced parallel tracks are followed to systematically survey large swathes of ocean, each track with a series of acoustic signal locations.

Published details of global seismic survey activity are scarce. As an example of effort, in Australian waters alone during 2014 and early 2015, an average of 15,848 km of petroleum-related marine seismic surveys were completed every three months. Along with petroleum exploration, seismic surveys are also used: (1) to image sub-sea formations likely to be used as ‘traps’ for sequestering CO2; (2) in scientific surveys of the Earth’s geology; 3) (for shallow, engineering-related ‘site’ surveys; or (4) for monitoring petroleum recovery from producing fields. Source: Nature