Kenneth C. Baldwin

Acoustic alarms reduce porpoise mortality

The most serious danger to dolphins and porpoises around the world is the threat from various forms of gill-net fishing. One potential way to reduce the number of deaths of marine mammals is the use of active acoustic alarms to warn animals about the presence of nets. Until now, acoustic alarms have not been tested in field experiments with sufficient statistical power. Here we describe a field experiment showing that acoustic alarms are effective at reducing the number of deaths of harbour porpoises (Phocoena phocoena) in sink gill-nets.

Protocols for calibrating multibeam sonar

Development of protocols for calibrating multibeam sonar by means of the standard-target method is documented. Particular systems used in the development work included three that provide the water-column signals, namely the SIMRAD SM2000/90- and 200-kHz sonars and RESON SeaBat 8101 sonar, with operating frequency of 240 kHz. Two facilities were instrumented specifically for the work: a sea well at the Woods Hole Oceanographic Institution and a large, indoor freshwater tank at the University of New Hampshire. Methods for measuring the transfer characteristics of each sonar, with transducers attached, are described and illustrated with measurement results. The principal results, however, are the protocols themselves. These are elaborated for positioning the target, choosing the receiver gain function, quantifying the system stability, mapping the directionality in the plane of the receiving array and in the plane normal to the central axis, measuring the directionality of individual beams, and measuring the nearfield response. General preparations for calibrating multibeam sonars and a method for measuring the receiver response electronically are outlined. Advantages of multibeam sonar calibration and outstanding problems, such as that of validation of the performance of multibeam sonars as configured for use, are mentioned.

Open Ocean Aquaculture Engineering: Numerical Modeling

Finite element analysis (FEA) is used to predict the dynamic performance of an offshore fish cage and submerged mooring grid system. The system has been deployed at an exposed demonstration site in 55 metes of water south of the Isles of Shoals, New Hampshire. Computer simulations were performed to investigate the dynamics of the cage motion and to calculate mooring line tensions. The results were used to establish the baseline design specifications and to evaluate the overall performance of the system. Both surface and,submerged positions of the net pen are considered. It is shown that the extreme environmental loading conditions at the demonstration site produce 60% less mooring tension in the case of a submerged cage. According to the analysis, the case when one of four mooring legs becomes disabled will not produce the failure of the mooring system. The problem of adequate modeling of net is addressed. A simple technique is proposed to approximate the effect of netting on the overall dynamic response of fish cage/ mooring systems.

Open ocean aquaculture engineering : System design and physical modeling

An open ocean aquaculture net pen system was developed and deployed for an exposed demonstration site south of the Isles of Shoals, New Hampshire in 55 meters of water. This component of the project is part of an interdisciplinary effort at the University of New Hampshire involving engineers, biologists, economists and commercial fishermen. Initially, two cages were specified for the growout of summer flounder (Paralichthys dentatus). The design process included physical model testing conducted in a wave/tow tank using 1/22.5 scale models. To select an optimum system, experiments were performed using gravity-type and central spartype cages. Vertical taut line and catenary moorings were evaluated. Data acquisition included drag and wave forces on the cages, mooring line forces and heave, pitch and surge motion of the cages. After comparison of the results and holding a design review including outside experts, a central spar configuration was selected for both cages. This system exhibited both reduced force loadings and less extreme motion, and its rigid frame would resist volume changes under storm conditions. In the final mooring design, each cage was separately deployed using a four anchor system to ensure redundancy. A mid-depth, square, horizontal grid was employed in order to reduce anchor footprint area, which is very expensive under New Hampshire permit rules. Mooring system components were sized to meet loadings scaled up from the physical model results. The system was deployed in June of 1999 and has performed well in all weather forcing conditions to date.

Open ocean aquaculture engineering : Mooring & net pen deployment

An open ocean aquaculture net pen system was developed for offshore deployment south of the Isles of Shoals, New Hampshire, USA in 55 meters of water. Two cages were specified for the growout of summer flounder as part of an interdisciplinary effort at the University of New Hampshire involving engineers, biologists, economists, and commercial fishermen. This effort included the design of mooring system suitable for the offshore environment. Assembly and deployment of these cages and associated moorings occurred in the summer of 1999. An overview of the procedures and techniques used during these efforts are presented here.

Multibeam sonar calibration: target localization in azimuth

Multibeam sonar requires calibration for use in absolute backscattering measurements of water-column targets. In an initial study, a least-squares method has been developed to locate a standard calibration target in the sonar transmit plane from the individual transducer element receive signals. By enabling the target position to be specified in azimuth, the overall transmit and receive sensitivity may be measured simultaneously for a number of beams, considerably rationalizing the amount of work required for calibration by standard target. The localization technique has been applied to data derived from test measurements with the Simrad SM2000 Multibeam Echo Sounder, operating at 200 kHz, with 128 receive beams of nominal beamwidth 1.5/spl times/20 deg. The approximate root-mean-square phase mismatch is about 0.1 rad, and the resultant error of the predicted azimuth angle is less than 0.1 deg.

Seismic expression of Quarternary climatic cycles in the peri-platform carbonate ooze of the northern Bahamas

Stratigraphic, physical, and acoustic properties of peri-platform carbonate sediments in an 18-m-long piston core (core 2218, 655 m) from Northwest Providence Channel, Bahamas, have been investigated. The sediments range in age from recent to about 1 m.y. old. There is a close correlation between changes of compressional wave velocity and Quaternary climate as indicated by δ 18 O. Velocity correlates negatively to aragonite content, water content, and porosity, whereas it correlates positively to grain size, saturated bulk density, and observed degree of cementation. Changes in the degree of cementation exert a particularly strong influence on velocity variations. All parameters correlate to Quaternary climate/sea-level fluctuations. During glacial/low-sea-level stages, the aragonite content, water content, water content, and porosity decrease, but grain size, saturated bulk density, and the degree of cementation increase; all of these increase velocity. In general, water content, porosity, and the aragonite content decrease slightly with core depth, but grain size, saturated bulk density, degree of cementation, and velocity increase slightly. Two processes may contribute to these trends: (1) shallow subsurface diagenesis upon burial and (2) variations of both the interglacial production of bank-top sediments and the intensity of glacial sea-floor diagenesis. Major changes in velocity are associated with transitions from glacial to interglacial stages. Acoustic reflectors correlate one-to-one with the velocity changes occurring at isotopic stage boundaries 7-8, 9-10, and 11-12. At the site of core 2218, deeper reflectors have an interference origin because velocity changes are more closely spaced. The reflectors that correlate with isotopic stage boundaries can be used to study the detailed history of Quaternary peri-platform carbonate deposition and place it into the context of global climate/sea-level cycles.

Calibrating a 90-kHz multibeam sonar

Quantitative use of multibeam sonar requires calibration. Protocols for calibrating multibeam sonar in an absolute sense by means of a standard target are being developed. These protocols are illustrated for the Simrad SM2000 Multibeam Echo Sounder, with 90-kHz operating frequency and 300 beams over a 90-deg sector (/spl plusmn/45 deg), each of nominal beamwidth 1.5/spl times/1.5 deg. In a sea well on Iselin Dock at the Woods Hole Oceanographic Institution, directional characteristics of the sonar were measured at the farfield range of 23 m in a sea well. At the large freshwater indoor tank at the University of New Hampshire, similar characteristics of directivity were measured at the nearfield range of 11.7 m. Beamforming was accomplished by digital signal processing. Beam patterns are presented and compared at both ranges. In addition, measurements of the on-axis response were made for each available time-varied-gain function, enabling linearity and dynamic range to be assessed.

Multibeam sonar calibration: removal of static surface reverberation by coherent echo subtraction

Calibration of multibeam sonar in confined environments may require treatment of surface reverberation. In the case of tanks, the method of coherent echo subtraction may be employed to remove static surface reverberation, allowing the tanks to be used for standard-target calibration. This is illustrated for the Simrad SM2000 Multibeam Echo Sounder operating at 200 kHz. The signal-to-noise ratio is typically improved by about 25 dB.

Modeling Flow Through and Around a Net Panel Using Computational Fluid Dynamics

Flow through and around nets is computed using computational fluid dynamics implemented using FLUENT software. The presented study is focused on net panels with different angle of attack, but once the approach is validated on net panels, it should be applicable to 3D aquaculture net-pens. The approach is to model the net as a thin sheet of porous material with a pressure drop that is a function of velocity through the porous material. The objective is to find out what porous media model has the best agreement with measured data and how to assess the material constants for the porous media. The results show that a thin sheet of porous media with pressure drop proportional to velocity squared has the best agreement with measured data. Optimal comparisons were achieved when the proportionality coefficient for tangential flow was less than that for the normal direction