Effectiveness of exclusion zones and soft-starts as mitigation strategies for minimizing acoustic impact from underwater noise sources

Several recent studies have suggested that cumulative Sound Exposure Level (SEL) is an important metric for assessment of the impact of exposure to anthropogenic sound sources by various marine receptors including marine mammals and fish. This metric allows the cumulative exposure of an animal to a sound field for an extended period to be assessed against a predefined impact criteria. Two widely used mitigation strategies used by both industrial and military users to reduce potential impact on marine receptors are exclusions zones (zones from source where received levels exceed a certain threshold), a source is either not started or stopped if receptors are detected within this zone and ‘soft-starts’ or ‘ramp-ups’ (lower energy levels at the commencement of a noise source allowing a receptor to move out of the area). This paper discusses the relative effectiveness of these methodologies in terms of a cumulative exposure impact criteria. Cumulative exposure examples are given including typical marine piling operations for wind-farm construction and sub-surface piling operations for various receptor models, including, static, fleeing and transiting animals.