Christopher Bassett

Handheld hydraulic tools, including hammer drills and chipping hammers, are used in offshore construction. A hydrophone was used to measure underwater sound radiated by drilling and chipping hammers during anchor maintenance activities in Kaneohe Bay, Hawai'i. The recordings were used to calculate mean-square and zero-to-peak sound pressure levels, sound exposure levels, and to estimate acoustic source levels. Both tools had estimated source levels between 158 and 177 dB re 1 μPa-m with peak sound pressures of approximately 1.6 kPa at 10 m during the most intense periods of operation. The sound produced by both tools is highly percussive and intermittent with peak intensities in the 1–10 kHz band. Cumulative, unweighted sound exposure levels associated with both activities range from approximately 150 dB re 1 μPa2-s after 5 min to 160 dB re 1 μPa2-s or more after 40 min at a range of approximately 90 m. Although the most intense sound corresponding to the drilling and chipping activities results in modest (<8 dB) decreases in sound exposure levels for high-frequency hearing specialists, this same group is most likely to experience temporary or permanent threshold shifts because of these activities given their lower onset thresholds.

The turbulent energy cascade of ocean mixing remains poorly understood at geophysically large Reynolds numbers (). Here we propose a new conceptual model for the structure of small‐scale mixing grounded in high‐resolution multibeam echosounding observations from the mouth of the Connecticut River, a shallow salt‐wedge estuary. Tidal forcing and bottom topography slope the pycnocline, sustaining interfacial shear and Kelvin–Helmholtz instabilities well below the marginal instability threshold on a vertical scale of order 1 m. Acoustic backscatter, used here as a proxy for salinity microstructure dissipation, provides time‐resolved two‐dimensional imagery of the structure and evolution of turbulent mixing. At , we find that mixing is dominated not by the collapse of slowly evolving billow cores as at , but instead by fast turbulence within the m thin, shallow‐sloping braids that connect them, energized by baroclinic shear. We interpret these observations using two‐dimensional direct numerical simulation at field‐matched parameters, which predicts the emergence of secondary Kelvin–Helmholtz instabilities and turbulence within the braids, pre‐empting further steepening and primary overturn. We then estimate turbulent dissipation and scalar variance dissipation at the pycnocline and braid scales using physics‐based scalings involving only the local slope, shear and layer thickness. Laboratory experiments in an inclined duct, an analog of the turbulent braid regime, visualize mixing down to dissipative scales, where it remains organized in myriad thin, sheared, intense filaments rather than overturns. We conclude that high‐ mixing hotspots with sustained shear forcing follow fundamentally different dynamics than previously thought.

The risk of collisions between animals and operating tidal turbines remains a concern in the scientific and regulatory communities. A sensor package including optical cameras was deployed to monitor animal interactions with a small-scale (1 m ² ) cross-flow tidal turbine. The turbine was deployed in Washington State, USA for 141 days at a site with peak flow speeds of 2.5 m/s. We analyze optical camera imagery spanning 109 days of turbine operation. The analyzed images contain 1044 observations of fish, fish schools, seabirds, or seals in the vicinity of the turbine. No instances of collision with seabirds or seals were observed. Seabirds were only observed during daylight hours and while the turbine was stationary. Both seals and fish were observed during both day and night and while the turbine was stationary and rotating. Four fish were observed colliding with the moving turbine and in all but one case the animals swam away following the collision. Over the same period of time, over fifty times more fish (224 individual fish and 5 fish schools) were observed passing the moving turbine without collision. Fish encounters were likely under counted due to the difficulty in discerning small fish from plant matter in the water column. These observations represent the first optical camera imagery showing fish, bird, and marine mammal interactions with a tidal turbine in North America. In addition to quantitative and qualitative discussion of the implications of our observations for collision risk, we discuss lessons learned on sampling schemes and deployment of machine learning for detection of animals to inform future data collection strategies in future monitoring campaigns.

The potential for injury or mortality because of collisions between animals and operating current energy turbines remains an important research question with consequences for management of proposed marine energy projects. During a 141-day deployment of a small-scale (1 m2) tidal turbine, a sensing package including optical cameras and imaging sonars was used to record near-field interactions between animals and the turbine. Over 1000 events were captured by the optical cameras, including several species of fish, at least two species of diving birds, and harbor seals. Both seals and fish were observed in the vicinity of the turbine while it was operational, and a range of behaviors were captured. Seals and fish were also observed while the turbine was stationary, although the observed behaviors were different. Diving birds were observed regularly, but only during daytime hours and when the turbine was stationary. In this work, we summarize trends in animal behavior with respect to tidal elevation, time of day, and turbine operational state and discuss specific observations of collisions (only observed for fish), evasion (fish and seals), avoidance, and attraction. Although the results focus primarily on the optical camera data because optical images better resolve near-field interactions with the rotor, co-temporal sonar data are used to provide additional context to the interactions. Based on our experiences and findings we also recommend approaches for future projects seeking to acquire, analyze, and report on similar data sets.

Radiated noise from marine energy harvesting is of environmental and engineering interest. Here, drifting hydrophones measure underwater noise in the vicinity of a relatively small wave energy converter. A statistical approach is demonstrated for attributing range-dependent, commonly occurring sounds in the frequency band from 90 to 600 Hz. Time-delay-of-arrival localization is then demonstrated for attribution of individual acoustic events likely associated with the power takeoff and wave-hull interactions. Because the radiated noise from the wave energy converter falls below ambient levels at a range of approximately 150 m, it is unlikely to substantially affect marine life at greater distance.

The degree to which walleye pollock (Gadus chalcogrammus, hereafter pollock) move between the US and Russian zones of the Bering Sea is a key source of uncertainty for fisheries management. To study transboundary migrations across the US–Russia maritime boundary and explore how climate variability might influence these migrations, four seafloor-mounted echosounder moorings were deployed from July 2019 to August 2020 in the northwestern Bering Sea. The observations indicated that a substantial amount of pollock moves between the US and Russia seasonally, with a period of southeast movement into the US as winter as sea ice forms and northwest movement into Russia in early summer as waters warm. Over the deployment period, 2.3-times more pollock backscatter moved into the US zone in fall and winter than exited the subsequent spring and summer. We hypothesize that the difference in the net movement between regions was driven by pollock moving farther into Russia during the historically warm conditions at the start of deployment period and reduced northwest return migration the following summer when temperatures were relatively cooler. This supports the hypothesis that temperature affects pollock distribution, and that continued warming will lead to a larger proportion of the stock in Russian waters.

Frontal boundary features with strong gradients in water properties and velocity are common features in coastal environments where fresh discharge meets ambient ocean waters. Convergence at these fronts results in two features that support the entrainment of bubbles. First, horizontal velocity gradients causes surface waves to steepen and break at these boundaries. Second, localized downwelling can further entrain and transport these bubbles. In experiments using broadband echosounders (45–420 kHz) at the Connecticut River ebb plume front and James River tidal intrusion front, bubble entrainment was observed to cause frequency-dependent excess attenuation. Seabed backscattering near fronts is used as a baseline, whereas a decrease in observed backscattering from the seabed under bubble plumes was attributed to excess attenuation due to bubble concentrations. Despite observing significant (>10 dB in some cases) excess attenuation in lower-frequency channels, we hypothesize that these fronts are unlikely to significantly affect propagation in the horizontal at these frequencies and that factors, such as refraction and three-dimension structure, are more operationally significant. [This work was funded by ONR.]