This introduction to a special issue on approaches to managing underwater noise in Canada provides a brief overview of recent efforts to better understand and reduce anthropogenic underwater noise. Recent programs have aimed to increase understanding of anthropogenic noise in the habitats of highly endangered whales and have supported management actions such as vessel slow downs. Technical workshops have advanced the development of quiet ship design and associated technologies. Collaborative research examined noise levels in the St. Lawrence Estuary and the Arctic Ocean. Efforts to better manage noise have gone beyond shipping: enhanced mitigation measures have been put in place for naval exercises near habitats used by southern resident killer whales, while other work has focused on the identification of appropriate metrics for measuring noise. To coordinate and advance these and other efforts, the Government of Canada is developing a national Ocean Noise Strategy.

Seagrass beds are recognized as pivotal and among the most vulnerable coastal marine ecosystems globally. The eelgrass Zostera marina L. is the most widely distributed seagrass species and dominates the temperate northern hemisphere. However, an alarming decline in seagrass has been occurring worldwide due to multiple stressors. Seagrass meadow degradation is particularly serious in the Bohai Sea, in temperate China; however, large areas (> 500 ha) of seagrass meadows and population recruitment have rarely been reported in this area. In the present study, we report on a large eelgrass bed in a eutrophic bay of the Bohai Sea. Sonar and field survey methods were used to investigate the distribution of seagrass and its population recruitment. We also analyzed the major threats to this large seagrass bed. Results showed that a large Z. marina bed with an area of 694.36 ha occurred in this area of the Bohai Sea, with a peripheral area of ~25 km². Seagrass canopy height and plant coverage had a significant correlation with water depth. Asexual reproduction principally occurred in autumn and played a dominant role in population recruitment in vegetated areas, where no seedlings successfully colonized. In contrast, a considerable number of seedlings survived in the seagrass meadow gaps, and thus played a critical role in the recruitment in these areas. The maximum reproductive shoot densities were about 100 and 70 shoots m⁻² at sampling site (S)-1 and S-2 in 2018, respectively, which was about two times more than in 2019 (50 and 20 reproductive shoots m⁻² at S-1 and S-2, respectively). The potential seed output per unit area in 2019 was about 1020 seeds m⁻² at S-1 and 830 seeds m⁻² at S-2, and the seed output in the study area was at a low level compared with global values. Overall, high spring and summer water temperature appeared to induce sexual reproduction of Z. marina in the study area, including reproductive effort, reproductive investment, and seedling development. Furthermore, eelgrass height, aboveground biomass, and density were significantly related to water temperature. Among the potential threatening factors to seagrass in this area, the activities of clam harvesting were intense with daily clam catches >2000 kg, leading to patchy seagrass meadows, especially in the fringe areas. The seagrass bed was also threatened by marine pollution (nutrient loading) and land reclamation. Therefore, the protection and restoration of this seagrass bed are strongly recommended. Our study will provide fundamental information for the conservation and management strategies of large eelgrass beds in the Bohai Sea.

Sunken oil incidents have occurred multiple times in the Bohai Sea over the past ten years. Currently, quick and effective sunken oil detection and classification remains a difficult problem. In this study, sonar detection experiments are conducted to obtain acoustic image samples using a multibeam echosounder (MBES) in a large seawater tank at the bottom of the area where the sunken oil is located. A series of MBES data corrections are constructed to generate backscatter strength images that can reflect the target characteristics directly. Meanwhile, eight-dimensional features are extracted, and a support vector machine (SVM) classification framework is built to classify the sunken oil and other interference targets. The results indicate that the MBES backscatter images provide an alternative approach for detecting and classifying sunken oil. The overall target classification accuracy reaches 88.5% by the SVM algorithm. Thus, this study provides a basis for further investigation of detecting sunken oil.

Fielding a well-trained, combat-ready military, and observing Canada's responsibility as environmental stewards are at times conflicting priorities for the Department of National Defence (DND). As new low frequency sources are introduced into service, DND must review and update policies and procedures regarding the use of active sonar to minimize its impacts on marine mammals with an evidence-based approach. Risk is mitigated primarily through avoidance, which requires an understanding of marine mammal distribution in order to avoid the most sensitive species and their habitats. In parallel, a research and development program evaluates and develops technological solutions to minimize the risk of harm. By first embracing an empirical framework to assess acute and chronic impacts, DND has been able to partner with other government departments and researchers to develop technology targeted towards the residual risks to marine mammals posed by sonar operations.

Underwater noise pollution from impulsive sources (e.g. explosions, seismic airguns, percussive pile driving) can affect marine fauna through mortality, physical injury, auditory damage, physiological stress, acoustic masking, and behavioural responses. Given the potential for large-scale impact on marine ecosystems, some countries are now monitoring impulsive noise activity, coordinated internationally through Regional Seas Conventions. Here, we assess impulsive noise activity in the Northeast Atlantic reported during 2015–2017 to the first international impulsive noise register (INR), established in 2016 under the OSPAR Convention. Seismic airgun surveys were the dominant noise source (67%-83% of annual activity) and declined by 38% during 2015–2017. Reported pile driving activity increased 46%. Explosions and sonar/acoustic deterrent devices both had overall increases in reported activity. Some increases were attributable to more comprehensive reporting in later years. We discuss utilising the INR for risk assessment, target setting, and forward planning, and the implementation of similar systems in other regions.

The potential effects of exposing marine mammals to military sonar is a current concern. Dose–response relationships are useful for predicting potential environmental impacts of specific operations. To reveal behavioral response thresholds of exposure to sonar, we conducted 18 exposure/control approaches to 6 long-finned pilot whales. Source level and proximity of sonar transmitting one of two frequency bands (1–2kHz and 6–7kHz) were increased during exposure sessions. The 2-dimensional movement tracks were analyzed using a changepoint method to identify the avoidance response thresholds which were used to estimate dose–response relationships. No support for an effect of sonar frequency or previous exposures on the probability of response was found. Estimated response thresholds at which 50% of population show avoidance (SPLmax=170dB re 1μPa, SELcum=173dB re 1μPa2s) were higher than previously found for other cetaceans. The US Navy currently uses a generic dose–response relationship to predict the responses of cetaceans to naval active sonar, which has been found to underestimate behavioural impacts on killer whales and beaked whales. The navy curve appears to match more closely our results with long-finned pilot whales, though it might underestimate the probability of avoidance for pilot-whales at long distances from sonar sources.