Low-frequency target strength and abundance of shoaling Atlantic herring (Clupea harengus) in the Gulf of Maine during the Ocean Acoustic Waveguide Remote Sensing 2006 Experiment

Abstract

The low-frequency target strength of shoaling Atlantic herring ͑Clupea harengus͒ in the Gulf of Maine during Autumn 2006 spawning season is estimated from experimental data acquired simultaneously at multiple frequencies in the 300-1200 Hz range using ͑1͒ a low-frequency ocean acoustic waveguide remote sensing ͑OAWRS͒ system, ͑2͒ areal population density calibration with several conventional fish finding sonar ͑CFFS͒ systems, and ͑3͒ low-frequency transmission loss measurements. The OAWRS system's instantaneous imaging diameter of 100 km and regular updating enabled unaliased monitoring of fish populations over ecosystem scales including shoals of Atlantic herring containing hundreds of millions of individuals, as confirmed by concurrent trawl and CFFS sampling. High spatial-temporal coregistration was found between herring shoals imaged by OAWRS and concurrent CFFS line-transects, which also provided fish depth distributions. The mean scattering cross-section of an individual shoaling herring is found to consistently exhibit a strong, roughly 20 dB/octave roll-off with decreasing frequency in the range of the OAWRS survey over all days of the roughly 2-week experiment, consistent with the steep roll-offs expected for sub-resonance scattering from fish with air-filled swimbladders.

Feasibility of Acoustic Remote Sensing of Large Herring Shoals and Seafloor by Baleen Whales

“…The expected target strength of a single herring in a vertical layer is determined by parameters such as mean layer depth, shoal thickness, neutral buoyancy depth, and herring length. With mean and standard deviation of the parameters empirically determined in Reference [24] (Table 2), TS is determined using a swimbladder resonance model [31] for deep shoals as described in Appendix A. Similar analysis is performed for shallow shoals given measured constraints.…”

“…An OAWRS experiment was conducted on the northern flank of Georges Bank during the peak annual Atlantic herring spawning period in the Fall of 2006 [1,22,24]. The spatial distribution of herring was instantaneously imaged and continuously monitored [22].…”

“…The conditional expectation over the sound speed and the density in Equation (5) is determined by averaging 10 Monte-Carlo realizations, where the Green functions are calculated along the entire propagation path in range and depth for each realization. Each Monte-Carlo realization employs a different sound speed profile measured during the OAWRS 2006 experiment [24] every 500 m [33,34] along the propagation path. SL is the whale call source level [1], T is the time duration,f is the center frequency of baleen whale vocalizations relevant for target detection, BW 1/3 is the one-third octave bandwidth centered at frequencyf , DT is the detection threshold, AG is the array gain given whale acoustic parameters, L is the spatial distance between the two omni-directional receiver with spatial coherence, α is the waveguide propagation factor [35], β is the constant baseline ambient noise intensity, and n is the power law coefficient of wind-speed-dependent ambient noise.…”

“…Typical well documented scenarios during the herring spawning season on Georges Bank [1,2,22,24,25] are investigated to determine ranges up to which detection of large herring shoals and the seafloor is feasible for given whale acoustic parameters. Detection of large herring shoals and the seafloor is found to be highly dependent on local ambient noise conditions at whale locations, herring shoal distribution, baleen whale vocalization parameters such as time duration and source spectra, as well as oceanographic conditions including sound speed structure, bathymetry, and seafloor scattering amplitudes along the acoustic propagation path.…”

“…Detection of large herring shoals and the seafloor is then investigated using theoretical and numerical methods developed for analyzing Ocean Acoustic Waveguide Remote Sensing (OAWRS) of fish shoals [21][22][23][24][25] given these active sensing parameters. Sensing resolution in cross-range is determined by spatial array theory and in range by incoherent energy analysis [1,2,[21][22][23][24][25][26], since evidence suggests temporally coherent auditory processing is unlikely [27,28].…”