Until now, continental shelf environments have been monitored with highly localized line-transect methods from slow-moving research vessels. These methods significantly undersample fish populations in time and space, leaving an incomplete and ambiguous record of abundance and behavior. We show that fish populations in continental shelf environments can be instantaneously imaged over thousands of square kilometers and continuously monitored by a remote sensing technique in which the ocean acts as an acoustic waveguide. The technique has revealed the instantaneous horizontal structural characteristics and volatile short-term behavior of very large fish shoals, containing tens of millions of fish and stretching for many kilometers.
Until now, continental shelf environments have been monitored with highly localized line-transect methods from slow-moving research vessels. These methods significantly undersample fish populations in time and space, leaving an incomplete and ambiguous record of abundance and behavior. We show that fish populations in continental shelf environments can be instantaneously imaged over thousands of square kilometers and continuously monitored by a remote sensing technique in which the ocean acts as an acoustic waveguide. The technique has revealed the instantaneous horizontal structural characteristics and volatile short-term behavior of very large fish shoals, containing tens of millions of fish and stretching for many kilometers.
Ocean Acoustic Waveguide Remote Sensing (OAWRS) has recently been shown to be capable of instantaneously imaging and continuously monitoring fish populations over continental shelf-scale areas, covering thousands of km 2 . We show how OAWRS can be used in a variety of oceanic ecosystems to remotely assess populations and study the behavior of fish and other marine organisms, such as Antarctic krill, to help the study of marine ecology and the ecosystem-based approach to fisheries management.
An active sonar system is used to image wide areas of the continental shelf environment by long-range echo sounding at low frequency. The bistatic system, deployed in the STRATAFORM area south of Long Island in April-May of 2001, imaged a large number of prominent clutter events over ranges spanning tens of kilometers in near real time. Roughly 3000 waveforms were transmitted into the water column. Wide-area acoustic images of the ocean environment were generated in near real time for each transmission. Between roughly 10 to more than 100 discrete and localized scatterers were registered for each image. This amounts to a total of at least 30000 scattering events that could be confused with those from submerged vehicles over the period of the experiment. Bathymetric relief in the STRATAFORM area is extremely benign, with slopes typically less than 0.5 degrees according to high resolution (30 m sampled) bathymetric data. Most of the clutter occurs in regions where the bathymetry is locally level and does not coregister with seafloor features. No statistically significant difference is found in the frequency of occurrence per unit area of repeatable clutter inside versus outside of areas occupied by subsurface river channels.
Ocean remote sensing techniques often rely on autonomous buoys to measure and transmit real-time oceanographic and meteorological data. The operating lifetime, payload capacity, and sampling rate of such platforms are limited by onboard battery power. Here, we describe a rotarydrive, wave energy conversion device which utilizes the heaving motion of a surface buoy to generate power over a broad range of sea-states † . The device was demonstrated to generate over 50W of power in moderate seas at the Kilo Nalu Nearshore Reef Observatory.Index Terms-energy harvesting, heaving buoy, marine electronic equipment, marine harbors and ports, marine technology, ocean energy, remote sensing, wave spectra, wave energy conversion
The mean low-frequency target strength (TS) of spawning Atlantic herring populations in the Gulf of Maine is estimated from the experimental data acquired during September–October 2006 near the northern flank of Georges Bank. A low-frequency OAWRS system with an instantaneous imaging diameter of 100 km was deployed to provide spatially unaliased imaging of fish populations over wide areas. The OAWRS system’s scattering strength measurements are calibrated with areal fish population density estimates obtained from concurrent localized line-transect measurements with several conventional fish finding sonars (CFFSs). Trawl sampling at selected locations enables the identification of the imaged species. The mean TS estimates of herring individuals exhibits significant variation over OAWRS operating frequency range, in accordance with the results from a resonant scattering model for swimbladder-bearing fish. The neutral buoyancy depth of herring and the species composition in the imaged population is inferred by comparing the measured TS with those derived from the model. Our analysis indicates that the herring population has a neutral buoyancy depth of between 70 and 90 m and is therefore negatively buoyant between 120 and 180 m water depth at which it is commonly found. The herring populations instantaneously imaged with OAWRS often exceeds 200×106, of which over 150×106 individuals can be organized into a large shoal.
Several unified scattering and reverberation models were developed in support of the ONR Acoustic Clutter Program. They include a range-dependent model based on the parabolic equation that can be used to efficiently model scattering from a random spatial distribution of random targets that obey the sonar equation in the waveguide and a similar but range-independent waveguide model based on normal modes for scattering from extended objects. Both these models are bistatic and fully 3D, and the latter model also accounts for modal coupling between propagation and scattering caused by extended objects in the waveguide. These models are applied to examine both coherent and diffuse scattering measured after beamforming and match-filtering on an array from schools of fish, plankton, volume inhomogeneities in the sea bottom, roughness on the seafloor, extended seafloor and sub-bottom features such as river channels and reflective strata, and internal and surface waves. We provide a review of the dominant sources of clutter as well as background reverberation for long range active sonar based on comparison of model predictions with measured data from the Acoustic Clutter Experiments of 2001 and 2003.
Long range scattering from fish schools and bottom reverberation in the New Jersey Continental Shelf environment are modeled using a unified, range-dependent, bistatic scattering, and reverberation model based on the parabolic equation [Ratilal and Makris, J. Acoust. Soc. Am. 114, 2302 (2003)]. The fish swim bladder is approximated as an air-filled bubble, while the bottom reverberation from volume inhomogeneities is modeled using the Rayleigh–Born approximation. The broadband scattered field, in the frequency range from 390 to 440 Hz, is beamformed and spatially charted using two-way travel time. The model output is compared with scattered field levels from fish schools and background reverberation measured during the the Main Acoustic Clutter Experiment 2003 using a long range, bistatic sonar system. The fish school characteristics, such as size, distribution and density, are inputs to the model. These are obtained from measurements made by the fish finding sonar during the experiment. This calibrated model is then used to infer fish school distributions and densities in areas where fish finding sonar measurements are not available.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.