Stanton, T. K., Chu, D., Jech, J. M., and Irish, J. D. 2010. New broadband methods for resonance classification and high-resolution imagery of fish with swimbladders using a modified commercial ...broadband echosounder. – ICES Journal of Marine Science, 67: 365–378. A commercial acoustic system, originally designed for seafloor applications, has been adapted for studying fish with swimbladders. The towed system contains broadband acoustic channels collectively spanning the frequency range 1.7–100 kHz, with some gaps. Using a pulse-compression technique, the range resolution of the echoes is ∼20 and 3 cm in the lower and upper ranges of the frequencies, respectively, allowing high-resolution imaging of patches and resolving fish near the seafloor. Measuring the swimbladder resonance at the lower frequencies eliminates major ambiguities normally associated with the interpretation of fish echo data: (i) the resonance frequency can be used to estimate the volume of the swimbladder (inferring the size of fish), and (ii) signals at the lower frequencies do not depend strongly on the orientation of the fish. At-sea studies of Atlantic herring demonstrate the potential for routine measurements of fish size and density, with significant improvements in accuracy over traditional high-frequency narrowband echosounders. The system also detected patches of scatterers, presumably zooplankton, at the higher frequencies. New techniques for quantitative use of broadband systems are presented, including broadband calibration and relating target strength and volume-scattering strength to quantities associated with broadband signal processing.
A field program to measure acoustic propagation characteristics and physical oceanography was undertaken in April and May 2001 in the northern South China Sea. Fluctuating ocean properties were ...measured with 21 moorings in water of 350- to 71-m depth near the continental slope. The sea floor at the site is gradually sloped at depths less than 90 m, but the deeper area is steppy, having gradual slopes over large areas that are near critical for diurnal internal waves and steep steps between those areas that account for much of the depth change. Large-amplitude nonlinear internal gravity waves incident on the site from the east were observed to change amplitude, horizontal length scale, and energy when shoaling. Beginning as relatively narrow solitary waves of depression, these waves continued onto the shelf much broadened in horizontal scale, where they were trailed by numerous waves of elevation (alternatively described as oscillations) that first appeared in the continental slope region. Internal gravity waves of both diurnal and semidiurnal tidal frequencies (internal tides) were also observed to propagate into shallow water from deeper water, with the diurnal waves dominating. The internal tides were at times sufficiently nonlinear to break down into bores and groups of high-frequency nonlinear internal waves.
Observations of the temporal evolution of the geometric properties and migration of wave‐formed ripples are analyzed in terms of measured suspended sand profiles and water velocity measurements. Six ...weeks of bedform observations were taken at the sandy (medium to coarse sized sand) LEO‐15 site located on Beach Haven ridge during the late summer of 1995 with an autonomous rotary sidescan sonar. During this period, six tropical storms, several of hurricane strength, passed to the east of the study site. Ripples with wavelengths of up to 100 cm and with 15 cm amplitudes were observed. The predominant ripples were found to be wave orbital scale ripples with ripple wavelengths equal to 3/4 of the wave orbital diameter. Although orbital diameters become larger than 130 cm during the maximum wave event, it is unclear if a transition to non‐orbital scaling is occurring. Ripple migration is found to be directed primarily onshore at rates of up to 80 cm/day. Suspended transport due to wave motions, calculated by multiplying acoustic backscatter measurements of suspended sand concentrations by flow velocity measurements, are unable to account for a sufficient amount of sand transport to force ripple migration and are in the opposite direction to ripple migration. Thus it is hypothesized that the onshore ripple migration is due to unobserved bedload transport or near‐bottom suspended transport. Bedload model calculations forced with measured wave velocities are able to predict the magnitude and direction of transport consistent with observed ripple migration rates. Sequences of ripple pattern temporal evolution are examined showing mechanisms for ripple directional change in response to changing wave direction, as well as ripple wavelength adjustment and erosion due to changing wave orbital diameter and relative wave‐to‐current velocities.
Field observations of current profiles and temperature, salinity, and density structure were used to examine vertical mixing within two pockmarks in Belfast Bay, Maine. The first is located in 21 m ...water depth (sea level to rim), nearly circular in shape with a 45 m rim diameter and 12 m rim-to-bottom relief. The second is located in 25 m water depth, more elongated in shape with an approximately 80 m (36 m) major (minor) axis length at the rim, and 17 m relief. Hourly averaged current profiles were acquired from bottom-mounted acoustic Doppler current profilers deployed on the rim and center of each pockmark over successive 42 h periods in July 2011. Conductivity–temperature–depth casts at the rim and center of each pockmark show warmer, fresher water in the upper water column, evidence of both active and fossil thermocline structure 5–8 m above the rim, and well-mixed water below the rim to the bottom. Vertical velocities show up- and down-welling events that extend into the depths of each pockmark. An observed temperature change at both the rim and center occurs coincident with an overturning event below the rim, and suggests active mixing of the water column into the depths of each pockmark. Vertical profiles of horizontal velocities show depth variation at both the center and rim consistent with turbulent logarithmic current boundary layers, and suggest that form drag may possibly be influencing the local flow regime. While resource limitations prevented observation of the current structure and water properties at a control site, the acquired data suggest that active mixing and overturning within the sampled pockmarks occur under typical benign conditions, and that current flows are influenced by upstream bathymetric irregularities induced by distant pockmarks.
As wild fish stocks decline, marine aquaculture is expected to play an increasing role in satisfying the global need for seafood. Since the expansion of near-shore aquaculture is becoming more ...difficult because of multi-use issues and environmental impact concerns, the feasibility of moving aquaculture into the open ocean is being studied. To enable the optimum design and evaluation of fish cage and mooring performance in the energetic open ocean, physical and numerical modeling techniques are being utilized. To validate these methods, the dynamics of a fish cage and mooring system deployed in the Gulf of Maine are simulated with physical and numerical models and compared with field observations. Assuming that the system can be modeled as a linear system, a stochastic approach was used to analyze the motion response (heave, surge and pitch) characteristics of the fish cage and the load (tension) response in an anchor line to wave forcing. Transfer functions were calculated from field observations for storm events and used to understand the system dynamics and to validate the models for the deployed system. Fish cage heave and surge motions were found to be overdamped, while pitch exhibited a resonance at low frequencies (less than 0.1 Hz). Transfer functions for anchor line tension were consistent with observations over most of the wave frequencies. The physical model clearly revealed the pitch resonance, while the numerical model was better at predicting mooring line tension. Results also provided insight concerning dynamical processes that require further study, including fluid–net panel interaction, transfer function amplitude dependence and the nonlinear relationship between steady current and wave fluid velocity on drag and its effect on system geometry and therefore response.
Three-dimensional numerical simulations of a tidally dominated estuary within the Gulf of Maine are performed using the Regional Ocean Modeling System (ROMS) and validated with observations of sea ...surface elevation and velocity time series obtained between 1975 and 2016. The model is forced at the ocean boundary with tidal constituents (M2, S2, N2, O1, K1), a time series of observed subtidal elevations and discharge from seven rivers that drain into the estuary. Harmonic analysis is used to determine the tidal dissipation characteristics and generation of overtides within the system. Amplitude decay and phase shift of the dominant semidiurnal (M2) tidal component shows good agreement with observations throughout the main channel of the Piscataqua River and over the channels and mudflats of the Great Bay. The model simulates harmonic growth of the overtides across the spectrum, and indicates a spatial evolution of the tide consistent with a shoaling wave that evolves from a skewed elevation profile with ebb dominance in the lower parts of the estuary, to a more asymmetric, pitched-forward shape consistent with flood dominance. The M4 constituent has spatial variation qualitatively similar to the observations but has magnitudes that are under-predicted in the complex bathymetric region of the Piscataqua River where much of the M2 tidal dissipation occurs. The M6 tidal constituent agrees well with the observations throughout the estuary suggesting that frictional effects on harmonic growth are well modeled. Root-mean-square model-data differences in velocities (~0.05 m/s) and sea surface elevation (~0.1 m) agree to within about 10% of the tidal amplitudes. Differences between model simulations with and without subtidal oscillations in the estuary are small, suggesting that interactions between the tide and other low frequency (subtidal) mean flows are weak and can be ignored when considering tidal dynamics. Including average fresh water discharge in the model does not affect the behavior of the tidal flows, but can generate high frequency baroclinic velocities potentially important to mixing within the estuary.
•Implementation and validation of a high-resolution 3D hydrodynamic model•Model was compared with observations of water levels and currents.•Tidal amplitude decay and tidal phase change in agreement with observations.
A study is conducted to validate a numerical model for calculating mooring system tensions of a large fish farm containing 20 net pens in the absence of waves. The model is forced using measured ...current velocity values obtained outside of the farm. Mooring line tensions calculated with the numerical model are compared with load cell field data sets. The approach considers current velocity reduction and load characteristics that occur through the net pen system for both clean and fouled net conditions. Without accounting for the reduction, the numerical model produces excessively conservative results. With reduction, a substantial improvement occurs. Understanding these differences will help to establish appropriate safety factors when designing large marine fish farms using the model. Additional validation studies should be conducted with wave and current forcing to investigate the modeling of large fish farms for exposed or open ocean sites.
Surface wind stress, heat, and freshwater fluxes were estimated over the southern flank of Georges Bank during February–August 1995 using moored measurements made at ST1 located on the 76‐m isobath, ...roughly halfway between the tidal mixing and shelf/slope fronts. Wind stress variability was dominated by a succession of atmospheric lows that passed Georges Bank during the deployment. A transition between frequent lows and strong wind stress events (“winter”) to less frequent lows and weaker wind stresses (“summer”) occurred in mid‐May. In winter, wind stress fluctuations tended to be omnidirectional, with maximum stresses above 0.5 N/m2 during four storms, one a classic “nor'easter”, while summer fluctuations were weaker but strongly polarized in the along‐bank direction. The ST1 surface heat flux was dominated by shortwave heating, which increased from a winter mean of 130 W/m2 to 230 W/m2 in summer. Long‐wave cooling decreased from 50 W/m2 (winter) to 30 W/m2 (summer), while mean sensible and latent fluxes increased from −20 and −40 W/m2 (winter) to +10 and 0 (summer) respectively. Overall, winter was characterized by weak net heating (30 W/m2) with shortwave gain offset by long‐wave, latent, and sensible heat loss. In summer, increased shortwave gain and reduced long‐wave loss and weak sensible and latent fluxes combined to produce strong net heating (210 W/m2). ST1 precipitation was highly episodic with little seasonality while evaporation occurred mostly during winter, resulting in a net evaporation of −15 cm and net freshwater flux of +48 cm over the deployment.
The LISST (Laser In Situ Scattering and Transmissometery) instrument was designed by Sequoia Scientific to measure the particle size distributions and concentrations of sediment suspensions in the ...field environment. To understand the LISST's performance with natural sediments from a variety of marine sources, several experiments were performed to compare the LISST's results to traditional sieving, filtering and weighing techniques. The LISST was able to correctly locate the peak of a unimodal particle size distribution and resolve the two peaks of a bimodal distribution if they are separated by at least 1
φ for sediment sizes of 5 to 250 μm. Unlike a single frequency scattering sensor for sediment concentration, which require sediment size specific calibration constants, the LISST is able to accurately measure sediment concentration with a single calibration constant for varying size distributions within the size range of 5 to 250 μm. The LISST was also found to adequately represent the particle volumetric size distribution for two different samples from marine environments.
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