Underwater noise generated by the detonation of historical ordnance in the Baltic Sea, Lithuania: potential ecological impacts on marine life

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Date: Dec. 2013
From: Baltica: The International Journal on Geosciences(Vol. 26, Issue 2)
Publisher: The Nature Research Centre
Document Type: Report
Length: 3,085 words
Lexile Measure: 1520L

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Abstract The removal of ordnance and ammunition followed by high levels of impulsive noise is implemented yearly in the Lithuanian area of the Baltic Sea. During the international naval exercise Open Spirit (Summer 2013), an acoustical survey (using submersible cable hydrophone) was conducted in order to measure the underwater impulsive noise levels produced by controlled detonation and to assess their potential ecological impacts. The findings indicate a high noise energy level of explosions having a particularly small weight of charges, reaching up to 190 dB (in low-frequency bands) and theoretical estimations of the initial shock wave of 276 dB.

Keywords .Acoustics * Underwater explosion * Aquatic life

INTRODUCTION

In the marine environment, explosives are used for several purposes including construction, mining, removal of unwanted structures, ship shock trials, military operations or exercises, as well as small charges to deter marine mammals i.e. seal bombs or catch fish i.e. blast fishing (Hildebrand 2009). Underwater explosions are one of the strongest point sources of anthropogenic sound in the marine environment (UNEP 2012). Moreover, underwater explosions are the contributor to the overall background underwater noise level (Frisk et al. 2003; UNEP 2012). Approximately 175,000 mines were laid in the Baltic Sea during the World Wars of the 20th century. Thus, in a former mined areas, -10-30% (in some areas, up to 80%) of the mines remain on the seabed. Subsequent studies of historical minefields have identified 1,985 minefields on the seabed (Fig. 1) (Moller 2011). Consequently, mine clearance operations are implemented yearly in the Lithuanian area of the Baltic Sea. Within the period of 1997-2013, 159 different mines were exploded (Lithuanian NAVY 2013, pers. comm.). Underwater explosions by themselves generate low-frequency shock waves and subsequent pulsations of the bubble sphere at high pressure (Tan 2008), which propagates for long distances, due to decreased attenuation of a low-frequency sound in Seawater (Withlow, Hastings 2008). A high level of impulsive noise has an ecological impact and leads to death risks of animals at the very close proximity to the explosion i.e. < 100 m (Ketten 1995). At the longer range, acoustic traumas (permanent threshold shift) or temporary hearing impairment (temporary threshold shift) can occur. At relatively remote distances, a perturbation of animal's life cycle has been reported (Ketten 1995; Southall et al. 2007).

In the Baltic Sea area, few studies have addressed the impact of explosions on marine animals. In a survey conducted by Schmidtke et al. (2009), the TNT charges of 1 kg and 300 kg were exploded in German waters at a 20 m depth to assess the noise mitigation measures. Sundermeyer et al. (2012) used a series of a small test charges in shallow coastal German waters to examine noise impacts on harbour porpoises (Phocaena phocaena). Here, the author presents the results of recent acoustic surveys of five noise events produced by detonating charges of a small weight in the shallow (<50 m) of the Lithuanian sector of Baltic Sea. The noise propagating in shallow water attenuates faster than in deeper regions...

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Gale Document Number: GALE|A356038665