Noise Dose for Aquatic Animals: Preliminary Estimates for Two Seismic Surveys

Two major air-gun exposure experiments have been conducted in Norwegian waters: The Nordkappbank experiment in 1992 [Can. J. Fish. Res. Aq. Sci., 1996, Vol. 53, pp. 2238-2249] and the Vesterålen experiment in 2009 [Fisken og Havet, 2010, Nr. 2, 76 pp. Inst. of Mar. Res.]. Although changes in catch rates and distribution of fish were observed in both cases, the responses were higher in the first experiment. Simple metrics such as number of exposures by time and area reveal large differences between the experiments. This is further detailed in looking at the distribution of distances to air-gun emission positions throughout the experiments. The exposures were in general closer in the Nordkappbank experiment. Analyzing the noise level data for the Vesterålen experiment showed on average cylindrical spreading, which is used in a simple acoustic spreading model estimating the integrated sound exposure levels at a central point in both areas. The analysis shows that the total noise energy exposed to an imaginary fish in these central locations are similar, but that the daily maximum total energy is higher for the Nordkappbank experiment. This gives some indication that the initial simple metrics gave a fair assessment of the exposures between the two cases