Vast assembly of vocal marine mammals from diverse species on fish spawning ground

Abstract

Observing marine mammal (MM) populations continuously in time and space over the immense ocean areas they inhabit is challenging but essential for gathering an unambiguous record of their distribution, as well as understanding their behaviour and interaction with prey species. Here we use passive ocean acoustic waveguide remote sensing (POAWRS) in an important North Atlantic feeding ground to instantaneously detect, localize and classify MM vocalizations from diverse species over an approximately 100,000 km(2) region. More than eight species of vocal MMs are found to spatially converge on fish spawning areas containing massive densely populated herring shoals at night-time and diffuse herring distributions during daytime. We find the vocal MMs divide the enormous fish prey field into species-specific foraging areas with varying degrees of spatial overlap, maintained for at least two weeks of the herring spawning period. The recorded vocalization rates are diel (24 h)-dependent for all MM species, with some significantly more vocal at night and others more vocal during the day. The four key baleen whale species of the region: fin, humpback, blue and minke have vocalization rate trends that are highly correlated to trends in fish shoaling density and to each other over the diel cycle. These results reveal the temporospatial dynamics of combined multi-species MM foraging activities in the vicinity of an extensive fish prey field that forms a massive ecological hotspot, and would be unattainable with conventional methodologies. Understanding MM behaviour and distributions is essential for management of marine ecosystems and for accessing anthropogenic impacts on these protected marine species.

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

“…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…”

“…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].…”

“…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].…”

“…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].…”