Seismic surveys use airguns that emit low frequency high magnitude sound to detect subsea resources and to map seabed geology. Such surveys are often conducted in areas that support high-value fisheries and sometimes within or near the spawning grounds of commercially or ecologically important species, creating spatial and resource use conflicts between stakeholders. Although underreported, the extent of seismic surveys is significant. For example, the Norwegian Petroleum Directorate estimates that the extent of seismic surveys in the Norwegian Exclusive Economic Zone is about 133 000 vessel kilometres per year.
Very few studies have assessed the possible effect of seismic airgun blasts on invertebrates and there are even fewer on planktonic crustaceans. However, a recent field experiment (McCauley et al., 2017) reported high mortality in zooplankton populations at distances up to 1.2 km from the airguns and modelled scenarios based on that finding suggest a mortality of 14% in zooplankton at a distance of 15 km from the seismic blast.
In the North Atlantic, a region in which there is significant seismic survey activity, the Calanus spp. complex is the dominant copepod genus. Both juveniles and adults of many commercially important fish stocks (Atlantic cod, herring, capelin, mackerel, blue whiting) depend on the production of lipid-rich adult Calanus spp. During spring and summer, and interannual variability of Calanus spp. production is linked to recruitment in these species. Additionally, Calanus spp. are important prey for larger zooplankton such as euphausiids and for the North Atlantic Right Whale.
There is no published information on the effect of seismic airgun blasts on Calanus spp. To fill this knowledge gap, a team of researchers from the Institute of Marine Research in Norway and the Bigelow Laboratory for Ocean Science in the USA conducted a field and laboratory study to assess if exposure to blasts from airguns used in seismic surveys affects mortality, predator escape response, or gene expression of Calanus finmarchicus.
Immediate mortality of copepods was significantly different from controls at distances of 5 m or less from the airguns. Mortality one week after the airgun blast was significantly higher—by 9% relative to controls—in the copepods placed 10 m from the airgun blast but was not significantly different from the controls at a distance of 20 m from the airgun blast. The increase in mortality—relative to controls—did not exceed 30% at any distance from the airgun blast. Only two genes changed in response to the airgun blast; however, their function is unknown. There were no sublethal effects of the seismic blasts on the escape performance or the sensory threshold needed to initiate an escape response at any of the distances from the airgun blast that were tested.
The observations made in this study are inconsistent with the recent report concluding that airgun blasts produced high mortality in zooplankton at distances up to 1.2 km: exposure to airgun blasts in this study did not result in immediate mortality at distances >5m from the airgun. As a result, model assessments of the broader impacts of seismic surveys on zooplankton will have to be revisited.
Reference cited: McCauley, R. D., Day, R. D., Swadling, K. M., Fitzgibbon, Q. P., and Watson, R. A. 2017. Widely used marine seismic survey air gun operations, negatively impact zooplankton. Nature Ecology and Evolution, 1: 1–8.
As the favourite food of several important fish stocks, Calanus finmarchicus is a species of zooplankton that plays a key role in the ecosystem in the Norwegian Sea. Image: David Fields.
Authors: David M. Fields, Nils Olav Handegard, John Dalen, Christiane Eichner, Ketil Malde, Ørjan Karlsen, Anne Berit Skiftesvik, Caroline M. F. Durif, Howard I. Browman.