Ecosystem Scale Acoustic Sensing Reveals Humpback Whale Behavior Synchronous with Herring Spawning Processes and Re-Evaluation Finds No Effect of Sonar on Humpback Song Occurrence in the Gulf of Maine in Fall 2006

Abstract

We show that humpback-whale vocalization behavior is synchronous with peak annual Atlantic herring spawning processes in the Gulf of Maine. With a passive, wide-aperture, densely-sampled, coherent hydrophone array towed north of Georges Bank in a Fall 2006 Ocean Acoustic Waveguide Remote Sensing (OAWRS) experiment, vocalizing whales could be instantaneously detected and localized over most of the Gulf of Maine ecosystem in a roughly 400-km diameter area by introducing array gain, of 18 dB, orders of magnitude higher than previously available in acoustic whale sensing. With humpback-whale vocalizations consistently recorded at roughly 2000/day, we show that vocalizing humpbacks (i) were overwhelmingly distributed along the northern flank of Georges Bank, coinciding with the peak spawning time and location of Atlantic herring, and (ii) their overall vocalization behavior was strongly diurnal, synchronous with the formation of large nocturnal herring shoals, with a call rate roughly ten-times higher at night than during the day. Humpback-whale vocalizations were comprised of (1) highly diurnal non-song calls, suited to hunting and feeding behavior, and (2) songs, which had constant occurrence rate over a diurnal cycle, invariant to diurnal herring shoaling. Before and during OAWRS survey transmissions: (a) no vocalizing whales were found at Stellwagen Bank, which had negligible herring populations, and (b) a constant humpback-whale song occurrence rate indicates the transmissions had no effect on humpback song. These measurements contradict the conclusions of Risch et al. Our analysis indicates that (a) the song occurrence variation reported in Risch et al. is consistent with natural causes other than sonar, (b) the reducing change in song reported in Risch et al. occurred days before the sonar survey began, and (c) the Risch et al. method lacks the statistical significance to draw the conclusions of Risch et al. because it has a 98–100% false-positive rate and lacks any true-positive confirmation.

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

“…Baleen whale vocalization for potential active sensing is parameterized by call source level SL, time duration T, center frequencyf , and one-third octave frequency bandwidth BW 1/3 with center frequencyf (Table 1) following conventions for measuring these parameters in References [1][2][3][4].…”

“…In Equation (4), Φ re f = 1 µPa is the reference acoustic pressure in water, SL is the whale call source level with mean and standard deviation [1,2] given in Table 1 consistent with circular complex Gaussian random fields fluctuations [26], TLA is the depth-averaged two-way transmission loss to individual targets integrated over one resolution footprint area [23] given in Equation (5), TS = 10 log 10 S r re f k 2 is the expected target strength of a single herring, S is the plane wave scatter function of a single herring, k is the acoustic wavenumber, r re f = 1 m is the reference length, n A,re f = 1 fish/m 2 is the reference areal fish population density, and n A = N/A R (ρ C ) is the expected areal density of the targets within a spatially varying resolution cell centered at horizontal location ρ C . When the instantaneous bandwidth BW of baleen whale vocalizations [1,2] is greater than the one-third octave bandwidth BW 1/3 given in Table 1, an adjusted whale call source level SL adj = SL + 10 log 10…”

“…Recent research has found a high spatial and temporal correlation between certain baleen whale vocalizations and peak annual spawning processes of Atlantic herring (Clupea harengus) in the Gulf of Maine [1,2], indicating an apparent relationship between these vocalizations and feeding activities of baleen whales. Suggested functions of the vocalizations include communication [3][4][5][6], prey manipulation [7], and echolocation [8][9][10][11], given the fact that Atlantic herring is a keystone prey species common in the diets of many marine animals including large whales in the Gulf of Maine region [12][13][14].…”

“…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].…”

“…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.…”