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Federal Register / Vol. 86, No. 106 / Friday, June 4, 2021 / Proposed Rules Description of NEFSCs Active Acoustic Devices NEFSCs fisheries surveys may use a wide range of active acoustic devices for remotely sensing bathymetric, oceanographic, and biological features of the environment. Most of these sources involve relatively high frequency, directional, and brief repeated signals tuned to provide sufficient focus and resolution on specific objects. The NEFSC may also use passive listening sensors i.e., remotely and passively detecting sound rather than producing it, which do not have the potential to impact marine mammals. NEFSC active acoustic sources include various echosounders e.g., multibeam systems, scientific sonar systems, positional sonars e.g., net sounders for determining trawl position, and environmental sensors e.g., acoustic Doppler current profilers.
The sources are characterized as nonimpulsive, intermittent sources.
Midand high-frequency underwater acoustic sources typically used for scientific purposes operate by creating an oscillatory overpressure through rapid vibration of a surface, using either electromagnetic forces or the piezoelectric effect of some materials. A
vibratory source based on the piezoelectric effect is commonly referred to as a transducer. Transducers are usually designed to excite an acoustic wave of a specific frequency, often in a highly directive beam, with the directional capability increasing with operating frequency. The main parameter characterizing directivity is the beam width, defined as the angle subtended by diametrically opposite half power 3 dB points of the main lobe. For different transducers at a single operating frequency the beam width can vary from 180 almost omnidirectional to only a few degrees.
Transducers are usually produced with either circular or rectangular active surfaces. For circular transducers, the beam width in the horizontal plane assuming a downward pointing main beam is equal in all directions, whereas rectangular transducers produce more complex beam patterns with variable beam width in the horizontal plane.
The types of active sources employed in fisheries acoustic research and monitoring may be considered in two broad categories here, based largely on their respective operating frequency e.g., within or outside the known audible range of marine species and other output characteristics e.g., signal duration, directivity. As described below, these operating characteristics
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result in differing potential for acoustic impacts on marine mammals.
The types of active sources employed in fisheries acoustic research and monitoring, based largely on their relatively high operating frequencies and other output characteristics e.g., signal duration, directivity, should be considered to have very low potential to cause effects to marine mammals that would rise to the level of a take, as defined by the MMPA. Acoustic sources operating at high output frequencies 180 kHz that are outside the known functional hearing capability of any marine mammal are unlikely to be detected by marine mammals. Although it is possible that these systems may produce subharmonics at lower frequencies, this component of acoustic output would also be at significantly lower SPLs. While the production of subharmonics can occur during actual operations, the phenomenon may be the result of issues with the system or its installation on a vessel rather than an issue that is inherent to the output of the system. Many of these sources also generally have short duration signals and highly directional beam patterns, meaning that any individual marine mammal would be unlikely to even receive a signal that would likely be inaudible.
Acoustic sources present on most NEFSC fishery research vessels include a variety of single, dual, and multi-beam echosounders many with a variety of modes, sources used to determine the orientation of trawl nets, and several current profilers with lower output frequencies that certain marine mammals may detect e.g., 10180 kHz.
However, while likely potentially audible to certain species, these sources also have generally short ping durations and are typically focused highly directional to serve their intended purpose of mapping specific objects, depths, or environmental features.
These characteristics reduce the likelihood of an animal receiving or perceiving the signal. A number of these sources, particularly those with relatively lower output frequencies coupled with higher output levels can be operated in different output modes e.g., energy can be distributed among multiple output beams that may lessen the likelihood of perception by and potential impact on marine mammals.
The acoustic system used during a particular NEFSC survey is optimized for surveying under specific environmental conditions e.g., depth and bottom type. Lower frequencies of sound travel further in the water i.e., good range but provide lower resolution i.e., are less precise. Pulse
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width and power may also be adjusted in the field to accommodate a variety of environmental conditions. Signals with a relatively long pulse width travel further and are received more clearly by the transducer i.e., good signal-to-noise ratio but have a lower range resolution.
Shorter pulses provide higher range resolution and can detect smaller and more closely spaced objects in the water. Similarly, higher power settings may decrease the utility of collected data. Power level is also adjusted according to bottom type, as some bottom types have a stronger return and require less power to produce data of sufficient quality. Power is typically set to the lowest level possible in order to receive a clear return with the best data.
Survey vessels may be equipped with multiple acoustic systems; each system has different advantages that may be utilized depending on the specific survey area or purpose. In addition, many systems may be operated at one of two frequencies or at a range of frequencies. We summarize characteristics of these sources below and in Table 2.
1. Multi-Frequency Narrow Beam Scientific EchosoundersEchosounders and sonars work by transmitting acoustic pulses into the water that travel through the water column, reflect off the seafloor, and return to the receiver.
Water depth is measured by multiplying the time elapsed by the speed of sound in water assuming accurate sound speed measurement for the entire signal path, while the returning signal itself carries information allowing visualization of the seafloor. Multifrequency split-beam sensors are deployed from NEFSC survey vessels to acoustically map the distributions and estimate the abundances and biomasses of many types of fish; characterize their biotic and abiotic environments;
investigate ecological linkages; and gather information about their schooling behavior, migration patterns, and avoidance reactions to the survey vessel.
The use of multiple frequencies allows coverage of a broad range of marine acoustic survey activity, ranging from studies of small plankton to large fish schools in a variety of environments from shallow coastal waters to deep ocean basins. Simultaneous use of several discrete echosounder frequencies facilitates accurate estimates of the size of individual fish, and can also be used for species identification based on differences in frequencydependent acoustic backscattering between species. The NEFSC operates Simrad EK500 and EK60 systems, which
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