Assessing the exposure and behavioural response of grey seals (Halichoerus grypus) to shipping noise

Abstract

Marine mammals use sound to drive vital life functions such as communication, foraging and navigation, but the underwater soundscape upon which they rely is changing. Growth in global trade and manufacturing has driven a dramatic increase in the number and size of ships in the commercial fleet. These ships generate chronic underwater noise which has been associated with a number of negative ecological effects such as auditory damage, changes in behaviour and stress. As a result, regulatory bodies have recognised underwater noise as a pollutant. However, there is still a lack of data relating to the noise levels experienced by marine life, and how exposure to shipping noise will impact certain species, frustrating efforts to set targets for acceptable levels of noise from shipping. The grey seal (Halichoerus grypus) is a protected species with hearing sensitive to the dominant low frequencies of shipping noise. UK waters are home to approximately 38% of the global grey seal population but are also traversed by some of the world's busiest shipping lanes. As a result, there is high spatial overlap between grey seals and shipping traffic. However, knowledge of the impact of shipping noise on grey seals while at-sea is sparse. Consequently, this thesis aims to investigate the exposure and behavioural response of grey seals to underwater noise from shipping, and improve the efficiency with which predictions of shipping noise for this task can be calculated. Using an acoustic modelling approach, ship noise levels were reconstructed along the GPS location and dive tracks of grey seals in the English Channel and Celtic Sea. The m-weighted 24-hr cumulative sound exposure levels of seals ranged from 121 to 170 dB re 1 μPa²s. The exposure of seals was influenced by the maximum source level, number and closest point of approach of ships in relation to the location of the seal, and noise predictions varied with depth as seals moved throughout the water column. Main findings indicate that while the exposure of seals to shipping noise was not high enough to cause damage to auditory systems, it was great enough to result in changes in the diving behaviour of grey seals in both regions. Adult seals increased the ascent rate of benthic and shallow dives, and seal pups decreased the descent rate of pelagic dives as a result of exposure to shipping noise at median sound pressure levels of 122 and 111 dB re 1 μPa respectively. The efficiency with which predictions of exposure along the tracks of seals was calculated improved 5 fold by developing a grid with an adaptive cell size to aggregate ships. These findings contribute to our understanding of the potential risk that shipping noise poses to grey seals, facilitates more efficient and accurate assessment of underwater noise and informs future policy that seeks to protect marine ecosystems from shipping noise.