Passive acoustic monitoring of beaked whale densities in the Gulf of Mexico

Abstract

Beaked whales are deep diving elusive animals, difficult to census with conventional visual surveys. Methods are presented for the density estimation of beaked whales, using passive acoustic monitoring data collected at sites in the Gulf of Mexico (GOM) from the period during and following the Deepwater Horizon oil spill (2010-2013). Beaked whale species detected include: Gervais' (Mesoplodon europaeus), Cuvier's (Ziphius cavirostris), Blainville's (Mesoplodon densirostris) and an unknown species of Mesoplodon sp. (designated as Beaked Whale Gulf - BWG). For Gervais' and Cuvier's beaked whales, we estimated weekly animal density using two methods, one based on the number of echolocation clicks, and another based on the detection of animal groups during 5 min time-bins. Density estimates derived from these two methods were in good general agreement. At two sites in the western GOM, Gervais' beaked whales were present throughout the monitoring period, but Cuvier's beaked whales were present only seasonally, with periods of low density during the summer and higher density in the winter. At an eastern GOM site, both Gervais' and Cuvier's beaked whales had a high density throughout the monitoring period.

Figure 1. Three sites in the Gulf of Mexico with detections of beaked whales (dots): Green Canyon (GC), Mississippi Canyon (MC), and Dry Tortugas (DT); and two sites with no beaked whale detections (open circles): Main Pass (MP) and DeSoto Canyon (DC).

Deepwater Horizon site (red star) and cumulative surface oil during April-August 2010 (dark gray area). The black line denotes the 1000 m contour. Surface oil is cumulative NESDIS SAR composite from: http://gomex.erma.noaa.gov. Map generated using GMT (http://gmt.soest.hawaii.edu/projects/gmt).

Figure 2. Acoustic signatures for GOM beaked whales. Time series (upper), spectrogram (middle) and mean spectra (lower; n indicates the number of FM pulses used for calculation of mean) are presented for Cuvier’s, Gervais’, Blainville’s, as well as an unknown species designated as the “BWG” beaked whale.

Dotted lines show mean noise floor.

Figure 3

Group size distribution for (A) Gervais’ and (B) Cuvier’s beaked whales derived from acoustic encounters at MC (green), GC (blue), and DT (red) sites. In each plot the bars of each color add to 100%.

Figure 4

Estimated detection probability for (A) Gervais’ and (B) Cuvier’s beaked whale clicks (line) and group (dash) based on a simulation using sound propagation modeling for site MC. Only MC is shown due to the similarity of other sites. Vertical bars show +/− one standard deviation.

Figure 5

Weekly density estimate for Gervais’ beaked whale at (A) MC, (B) GC, and (C) DT sites based on click counting (above) and group counting (below). Circles denote estimates and vertical lines show +/− one standard error. Shaded areas lack recording effort.

Figure 6

Weekly density estimate for Cuvier’s beaked whale at (A) MC, (B) GC, and (C) DT sites based on click counting (above) and group counting (below). Circles denote estimates and vertical lines show +/− one standard deviation. Shaded areas lack recording effort.