Humans and their activities in the marine environment introduce noise into the ocean in various ways. Each activity that generates noise may have different effects, depending on the noise generated, and its frequency, intensity and composition (intermittent, pulsed or continuous sounds).

The main anthropogenic activities producing high levels of noise are seismic surveys of sub-bottom strata (usually using air-gun arrays), active sonars (military, scientific surveying, echo sounders), explosions (associated with military exercises and port construction), pile-driving (wharf construction, offshore platforms), vessels (particularly dynamically positioned vessels), dredging and drill rig activities. Lower levels of noise are produced through general ongoing vessel activity and offshore renewable energy operations. To date:

• seismic surveys have been concentrated in the main oil and gas regions of the North West Shelf and Bass Strait (Figure MAR18)
• military sonar has been concentrated in maritime exercise areas such as Sydney and Perth
• dynamically positioned vessels are associated with offshore facilities
• pile-driving and dredging associated with port development have been concentrated in the north-west and north-east, although dredging activities are routinely conducted in port environs throughout Australia (see the Coasts report for further detail).

Because of the extent of marine activities generating noise in the marine environment, anthropogenic noise is considered to have high impact.

For most marine animals, sound is important for communication; for locating particular features, prey and peers; and for short-range and long-range navigation (Erbe et al. 2015). Sounds from anthropogenic sources can mask vocal communication, disrupt normal behaviours, and cause temporary or permanent threshold shifts in hearing. Explosions can cause physical damage to tissues and organs. These pressures can, ultimately, have adverse impacts on foraging and reproduction, and individual health and fitness, which can manifest as population effects (OSPAR 2009, Cato 2010).

Enough is known about hearing in marine mammals and, to some extent, fish to model the effects of noise on hearing and, to some extent, on masking. Far less is known about behavioural responses and their significance. Within Australian waters, most of the emphasis has been on experimental assessment of the impacts of air-gun noise on marine mammal behaviour, with fewer studies of the impacts on fish. More recently, there have been some studies on invertebrates, and so the baseline understanding of impacts—in particular, acute impacts—is growing (McCauley et al. 2003; Cato et al. 2012, 2016; Fewtrell & McCauley 2012; Miller & Cripps 2013; Dunlop et al. 2016). However, chronic effects associated with ongoing low-level noise are not well understood across all species groups (Hawkins et al. 2015).

Substantial data on noise have been collected globally, allowing the characterisation of noise from most anthropogenic sources. Records of activities generating noise can be used to estimate trends in the occurrence of sources of anthropogenic noise (e.g. Figure MAR19). However, this does not directly equate to the amount of noise entering the marine environment, because activities can generate varying levels of sound depending on the source, and sound propagates through the water column in varying ways depending on the location, time of year and regional oceanography.

Before 2008, regular monitoring of noise in the Australian marine environment was spatially and temporally limited, and the establishment of soundscapes and any trends in soundscapes was therefore also limited. With the establishment of the IMOS network of National Reference Stations (Lynch et al. 2014), coordinated ongoing monitoring of noise in the marine environment now occurs at several stations, predominantly in the southern half of Australia. Monitoring efforts are facilitating characterisation and quantification of the marine soundscape (Erbe et al. 2015; Figure MAR20), and the establishment of trends at the station sites. Analysis of acoustic data from the National Reference Station moored offshore from Perth has identified that soundscapes are dominated by physical and meteorological (wind, waves and rain), biological (marine animals such as whales and fish) and anthropogenic (predominantly marine vessel activity, which was present 25 per cent of the time) sound sources (Erbe et al. 2015). However, the limited nature of monitoring of noise in the marine environment precludes an assessment of recent trends. Therefore, trends of anthropogenic noise remain unclear.

Considerable variability exists in the outlook for the major activities generating noise, with flow-on effects on the types of noise expected to contribute to the marine environment. High point-source activities are expected to remain stable (sonar, pile-driving, dredging) or decrease (seismic surveys). Activities generating lower levels of noise, such as marine mining, marine renewable energy and vessel activity, are expected to increase. How increases in these activities will affect the Australian marine environment is currently not clear, but they may result in a shift in anthropogenic contributions to marine soundscapes towards ongoing low-level noise.