GNGTS 2019 - Atti del 38° Convegno Nazionale

GNGTS 2019 S essione 3.2 601 EFFECT OF OFFSHORE SEISMIC PROSPECTING ON MARINE FAUNA: NEED TO RECONCILE SCIENTIFIC RESEARCH, BLUE ECONOMY, AND ENVIRONMENTAL SUSTAINABILITY A. Affatati, A. Camerlenghi, F. Zgur Istituto Nazionale di Oceanografia e di Geofisica Sperimentale - OGS, Trieste, Italy Marine seismic exploration and underwater noise. The marine environment is rarely silent. Noise is produced not only by natural sources, but also, and primarily, by the rapidly increasing human use of the sea (Hildebrand, 2009). Seawater is a poor medium for the propagation of light, so marine animals, especially mammals, have learned to use sound for communication, navigation, prey detection, and much more. Therefore, anthropogenic activities generating underwater noise can affect these functions and the spatial scale of impacts can be overwhelming due to the long-range nature of sound propagation (Gill et al., 2012). Airguns are used extensively worldwide for offshore seismic surveys, both for scientific and hydrocarbon exploration ( Gisiner, 2016) (Fig. 1). They are designed to produce impulses of high amplitude sound waves with source levels in the region of 220–248 dB re. 1 μPa @ 1 m. The acoustic output has its highest energy in the relatively low-frequency range of 10–200 Hz. However, because the impulses include also medium to high frequency components, a potential to interfere with marine mammals use of sound is implicit. Airguns are normally deployed in array configurations designed to direct sound pulses towards the seabed, nevertheless, sound energy is radiated also in other directions, since each airgun is an omnidirectional sound source (Hermannsen et al., 2015). Seismic surveys are currently experiencing an unprecedented level of public attention. The term “airgun” can be misleading. Far from being weaponry, they are impulsive sources of compressed air that produce elastic waves, not shooting any object in the water nor producing ultrasonic shock waves that can lead to barotrauma, as explosives do. Due to the growing concern about the effects on marine life, and to reduce source energy not used in geophysical imaging, a variety of novel sources- such as marine vibroseis- are being explored as potential replacements for airguns (Rassenfoss, 2016). Impact onmarineanimals. Inspiteof the importanceof assessing thepotential environmental impacts of marine seismic surveys on commercially valuable species, fundamental knowledge gaps still exist which hinder our understanding of the matter (Carroll et al., 2017). There is growing concern for the need to evaluate in which ways exposure to noise may eventually affect ecosystem dynamics and population growth rates. Furthermore, cumulative effects related to noise exposure need to be investigated (Wright et al., 2007). Potential effects on marine mammals exposed to seismic sources may include: acoustic masking and temporary - or permanent- threshold shift (TTS or PTS), behavioral alterations - such as avoidance responses-, displacement, changes in vocalizations. Moreover, additional indirect physical damage (“the bends”), due to masking or physiological effects, such as stress, may occur (Weir and Dolman, 2007). Physiological changes, if chronic, can inhibit the immune system and compromise the health. In fact, loud, impulsive noise may cause significantly increased mean norepinephrine, epinephrine, and dopamine levels (Romano et al., 2004). Marine mammals. Since most energy is produced at low frequencies in airgun pulses, studies of impacts on marine mammals have targeted primarily the effects on large baleen whales and sperm whales ( Physeter macrocephalus ), whose presumed better low frequency hearing most likely overlaps largely with the peak frequency of airguns (Madsen et al., 2002). However, studies concerned with considerable energy emitted at medium-high frequencies have resulted worrisome: seismic noise might also have the potential to unfavourably affect smaller marine mammals presenting more sensitive hearing at higher frequencies (Madsen et al., 2006; Au, 2000). In addition, determining an exposure level that is “safe” for marine mammals is