Aquatic animals live in an acoustic world, prone to pollution by globally increasing noise levels. Noisy human activities at sea have become widespread and continue day and night. The potential... Show moreAquatic animals live in an acoustic world, prone to pollution by globally increasing noise levels. Noisy human activities at sea have become widespread and continue day and night. The potential effects of this anthropogenic noise may be context-dependent and vary with the time of the day, depending on diel cycles in animal physiology and behaviour. Most studies to date have investigated behavioural changes within a single sound exposure session while the effects of, and habituation to, repeated exposures remain largely unknown. Here, we exposed groups of European seabass (Dicentrarchus labrax) in an outdoor pen to a series of eight repeated impulsive sound exposures over the course of two days at variable times of day/night. The baseline behaviour before sound exposure was different between day and night; with slower swimming and looser group cohesion observed at night. In response to sound exposures, groups increased their swimming speed, depth, and cohesion; with a greater effect during the night. Furthermore, groups also showed inter-trial habituation with respect to swimming depth. Our findings suggest that the impact of impulsive anthropogenic noise may be stronger at night than during the day for some fishes. Moreover, our results also suggest that habituation should be taken into account for sound impact assessments and potential mitigating measures. Show less
Anthropogenic sources increasingly contribute to the underwater soundscape and this may negatively impact aquatic life, including fish. Anthropogenic sound may mask relevant sound, alter behaviour,... Show moreAnthropogenic sources increasingly contribute to the underwater soundscape and this may negatively impact aquatic life, including fish. Anthropogenic sound may mask relevant sound, alter behaviour, physiology, and may lead to physical injury. Behavioural effect studies are often seen as critical to evaluate individual and population-level impact. However, behavioural responsiveness likely depends on context and characteristics of sound stimuli. We pose that ambient sound levels, signal-to-noise ratio (SNR), and pulse rate interval (PRI), could affect the behavioural response of fish. To study this, we experimentally exposed groups of tagged European seabass (Dicentrarchus labrax) to different impulsive sound treatments that varied in pulse level, elevated background level, SNR, and PRI. Upon sound exposure, the seabass increased their swimming depth. The variation in the increase in swimming depth could not be attributed to pulse level, background level, SNR or PRI. It may be that the current range of sound levels or PRIs was too narrow to find such effects. Show less
Anthropogenic noise has been shown to affect marine animals in various ways, this may have fitness consequences at individual and population level. This thesis aims to increase insight into the... Show moreAnthropogenic noise has been shown to affect marine animals in various ways, this may have fitness consequences at individual and population level. This thesis aims to increase insight into the quantification of sound-induced behavioural responses that are relevant to fitness, and into factors that modulate the responses. I addressed both knowledge gaps using captive and field studies on marine animals from multiple trophic levels. For the quantification of behavioural responses relevant to fitness, I examined the changes in time budgets of Atlantic cod in a net pen and basin in response to sound (chapter 2 and 3). To increase insight into factors that modulate sound impact, I examined the effect of various acoustic characteristics of sound stimuli and the environment on European seabass (chapter 4), the interaction between foraging shore crabs and common shrimps during noise (chapter 6), the cross-sensory interference by noise in foraging crabs (chapter 7), and habituation to repeated sound exposures by blue mussels (chapter 8). Future studies are needed to be able to link changes in time budgets to changes in energy budgets, and consequently to fitness. Additionally, studies into the factors that modulate the effects of sound are needed to fully understand the impact of sound. Show less
Fish live in a world that is not silent. In fact, fish hear low frequency sounds and may need them for navigation, predator-prey interactions and mate attraction. Due to their dependence on sounds... Show moreFish live in a world that is not silent. In fact, fish hear low frequency sounds and may need them for navigation, predator-prey interactions and mate attraction. Due to their dependence on sounds, fish may be particularly sensitive to changes in their acoustic world. Since the 1900s, the acoustic world of fish has been altered by a new prominent sound source: human activities. These activities generate a cacophony of high-intensity sounds. At close range, they may cause tissue damage and temporary hearing loss. At further distances, more moderate sounds may cause auditory masking, behavioural changes and reduced efficacy in foraging and avoiding predators. Assessing the severity of behavioural impacts of sounds is not straightforward and are often problematic.This thesis used European seabass to study behavioural effects of man-made sounds of different temporal structures. It also compared the effects of tank-based experiments with open-water experiments. Finally, this thesis studied the habituation of fish to sound exposure and how sound effects are influenced by various environmental factors. Show less
