One calendar year of Automatic Identification System (AIS) ship-traffic data was paired with hydrophone recordings to assess ambient noise in northern Admiralty Inlet, Puget Sound, WA (USA) and to quantify the contribution of vessel traffic. The study region included inland waters of the Salish Sea within a 20 km radius of the hydrophone deployment site. Spectra and hourly, daily, and monthly ambient noise statistics for unweighted broadband (0.02-30 kHz) and marine mammal, or M-weighted, sound pressure levels showed variability driven largely by vessel traffic. Over the calendar year, 1363 unique AIS transmitting vessels were recorded, with at least one AIS transmitting vessel present in the study area 90% of the time. A vessel noise budget was calculated for all vessels equipped with AIS transponders. Cargo ships were the largest contributor to the vessel noise budget, followed by tugs and passenger vessels. A simple model to predict received levels at the site based on an incoherent summation of noise from different vessels resulted in a cumulative probability density function of broadband sound pressure levels that shows good agreement with 85% of the temporal data.

Flow-noise resulting from oceanic turbulence and interactions with pressure-sensitive transducers can interfere with ambient noise measurements. This noise source is particularly important in low-frequency measurements (f < 100 Hz) and in highly turbulent environments such as tidal channels. This work presents measurements made in the Chacao Channel, Chile, and in Admiralty Inlet, Puget Sound, WA. In both environments, peak currents exceed 3 m/s and pressure spectral densities attributed to flow-noise are observed at frequencies up to 500 Hz. At 20 Hz, flow-noise exceeds mean slack noise levels by more than 50 dB. Two semi-empirical flow-noise models are developed and applied to predict flow-noise at frequencies from 20 to 500 Hz using measurements of current velocity and turbulence. The first model directly applies mean velocity and turbulence spectra while the second model relies on scaling arguments that relate turbulent dissipation to the mean velocity. Both models, based on prior formulations for infrasonic (f < 20 Hz) flow-noise, agree well with observations in Chacao Channel. In Admiralty Inlet, good agreement is shown only with the model that applies mean velocity and turbulence spectra, as the measured turbulence violates the scaling assumption in the second model.

Broadband acoustic scattering techniques are not widely used in fisheries acoustics, but this may change due to the recent commercial availability of a broadband echosounder system operating at frequencies commonly used in fisheries surveys. A four-channel (15–150 kHz) broadband echosounder was used to investigate the potential of broadband methods to improve species discrimination during a walleye pollock (Gadus chalcogrammus) survey in the Gulf of Alaska. Narrowband echosounders combined with mid-water and bottom trawls were used to identify aggregations of interest for broadband measurements. Broadband frequency responses were measured for multiple pelagic and semi-demersal fishes as well as euphausiids. No clear patterns in the broadband frequency responses were identified that would aid in discrimination among the commonly encountered swimbladder-bearing species. The results are consistent with narrowband observations and suggest that both techniques face the same challenges when attempting to discriminate among acoustically similar species as frequency responses overlap within the measured bandwidth. However, examples are presented in which broadband frequency responses provide additional information about near-resonant scatterers. The benefits of broadband operations have not been fully realized and widespread adoption of broadband techniques and improved processing algorithms may yield improved acoustic-based species discrimination for use during fisheries surveys.