Anthropogenic noise has increased unprecedentedly in the last century both on land and underwater, being considered a global pollutant of major concern (e.g. US National Environment Policy Act and EU Marine Strategy Framework Directive). In addition to increasing noise level, human activities originate noise that differs from natural noise in frequency distribution, intensity fluctuation, duty cycle and impulsiveness (R1), presenting a novel challenge to animals. The detrimental effect of man-made noise is well established in terrestrial habitats, impacting both behaviour and physiology in humans and other animals (R2,R3). For example, traffic noise impacts health in humans (increases stress levels, causes sleep disturbances and cognitive impairment, and increases the risk of heart diseases; R4) and reproductive success in song birds (R5).
In the marine environment, where sound propagates faster and over longer distances than in the air, noise from offshore constructions, underwater explosions, seismic surveys, sonar emissions, shipping, and recreational boating has become a dominant feature (R3). From these, shipping and boating are likely the most common and chronic form of noise pollution increasing noise levels in the low frequencies (<1.500 Hz) (R6,R7). Low frequency sounds emanating from the environment are commonly used by aquatic organisms (including fish) in ecologically relevant processes, such as navigation, finding suitable habitats, food and mates, or avoiding predators (R3). Boat traffic has thus the potential to mask acoustic information essential for reproduction and survival and hence to impact fitness in marine organisms. However, while a number of studies have demonstrated a detrimental effect of noise in a wide range of marine animals (R3,R8-11) direct assessment on individual fitness and resulting changes at the population/community level are needed (R12,R13). Most importantly, there is a clear need for experimental studies to be carried out with free-living animals under natural acoustic conditions. Though such experiments have the highest ecological validity they are logistically challenging. Most studies have therefore been carried out in the laboratory, where animals are not able to escape the noise source and are exposed to an altered acoustic field, especially in terms of particle motion, the sound component that fish and invertebrates are mostly sensitive to (R1,R14-16). In addition, most experiments to date have failed to characterize the noise treatment in terms of particle motion (R15).
To provide evidence on impacts of noise pollution for mitigation and management measures an integrative approach is warranted. Noise can impact individuals and populations by both bottom-up (responses at the genetic, cellular and physiological level) and top-down (responses modulated by species-specific behavioural ecology and habitat requirements) mechanisms (R17). Experiments should take into account both approaches as bottom-up mechanisms are shared by many taxa and allow for broad predictions across taxa, while top-down mechanisms are dependent on behavioural traits and are thus more specific, allowing a better understanding of response variability (R17). Yet such integrative approach is lacking in the literature.
Fishes are the largest extant vertebrate group, and are of major societal importance: they are a main food source, contribute to 12% of the world's population livelihoods, and have significant cultural and social relevance to communities (R18). Fishes are well-established model systems for bioacoustics and behavioural ecology studies: they exhibit widespread evolution of hearing mechanisms and sound production during social behaviour, and some species are amenable to laboratory and field studies (R19,R20*). In addition they play a relevant ecological role in marine ecosystems including in coastal areas where the impact of human activity and derived noise is prominent. Nevertheless, information on the impact of man-made noise on fish is still scarce (R21). Recent studies have shown that noise can mask fish communication signals (R22*,R23*) key for reproductive success (R24*,R20*), affect development (R25), foraging success (R26), and predation (R27). But there is a paucity of data regarding negative impacts of noise on direct fitness in fish, such as mating success (R28) or number of surviving offspring (R25), and there are almost no reports of experimental field work (R27). In addition, no study has addressed long-term physiological effects of noise pollution in fish such as changes in genetic and cellular level including biomarkers, which could inform us of the organism physiological ability to cope with deleterious effects of increasing noise levels in the ocean (R29*).
In summary, the impact of man-made noise on fish is an overlooked issue. Fish are of major biological, ecological and societal value and this gap of knowledge needs to be addressed adequately to design appropriate mitigation, conservation and management measures. In particular, relevant input could be provided to noise regulation in Portugal, which is still under consideration.