Signals from the deep: Spatial and temporal acoustic occurrence of beaked whales off western Ireland

Abstract

Little is known of the spatio-temporal occurrence of beaked whales off western Ireland, limiting the ability of Regulators to implement appropriate management and conservation measures. To address this knowledge gap, static acoustic monitoring was carried out using eight fixed bottom-mounted autonomous acoustic recorders: four from May to December 2015 on Ireland's northern slope and four from March to November 2016 on the western and southern slopes. Recorders ran for 205 to 230 days, resulting in 4.09 TB of data sampled at 250 kHz which could capture beaked whale acoustic signals. Zero-crossing-based automated detectors identified beaked whale clicks. A sample of detections was manually validated to evaluate and optimize detector performance. Analysis confirmed the occurrence of Sowerby's and Cuvier's beaked whales and Northern bottlenose whales. Northern bottlenose whale clicks occurred in late summer and autumn, but were too few to allow further analysis. Cuvier's and Sowerby's clicks occurred at all stations throughout the monitoring period. There was a significant effect of month and station (latitude) on the mean daily number of click detections for both species. Cuvier's clicks were more abundant at lower latitudes while Sowerby's were greater at higher latitudes, particularly in the spring, suggesting a spatial segregation between species, possibly driven by prey preference. Cuvier's occurrence increased in late autumn 2015 off northwest Porcupine Bank, a region of higher relative occurrence for each species. Seismic airgun shots, with daily sound exposure levels as high as 175 dB re 1 μPa2·s, did not appear to impact the mean daily number of Cuvier's or Sowerby's beaked whale click detections. This work provides insight into the significance of Irish waters for beaked whales and highlights the importance of using acoustics for beaked whale monitoring.

Fig 1. Acoustic monitoring stations using AMARs in the Atlantic Margin study area off western Ireland during 2015 (stations 1–4) and 2016 (stations 3 and 5–8).

Fig 2

Waveform (top) and spectrogram (bottom) of beaked whale clicks. A Sowerby’s beaked whale click recorded at station 1 on 5 Sept 2015 at 14:26, a Cuvier’s beaked whale click recorded at station 3 on 29 Nov 2015 at 17:56, and a northern bottlenose whale click recorded at station 1 on 12 Sept 2015 at 20:10 (right; 512 Hz frequency resolution, 2.6 ms time window, 0.2 ms time step, Hamming window).

Fig 3. Daily and hourly occurrence of Cuvier’s beaked whale clicks per acoustic file at stations 1–8 represented as individual black dots (P = 0.87, R = 0.79).

Shaded areas indicate periods of darkness. The red dashed lines indicate the deployment and retrieval dates. Black diagonal lines indicate periods with no data.

Fig 4

Mean Cuvier’s beaked whale detections per recording day bubble plot (right) and with standard error bars (left) for stations 1 (SE = ±0.70), 2 (SE = ±0.59), 3 (SE = ±2.61), 4 (SE = ±4.48), 5 (SE = ±1.08), 6 (SE = ±1.38), 7 (SE = ±1.35), and 8 (SE = ±1.26) off western Ireland from May to Dec 2015 (stations 1–4) and Mar to Nov 2016 (stations 3 and 5–8). Results of Kruskall-Wallis tests are given and results from Tukey-Kramer multiple comparisons tests are represented by coloured means (left) with different colours denoting a statistically significant difference. *Station 8 differs from 7, but not from 3, 5, and 6.

Fig 5

Mean daily number of Cuvier’s beaked whale detections per month for each station that recorded data in 2015 (top) and 2016 (bottom), as well as the average across all stations with standard error bars. Results of Kruskall-Wallis tests are given and results from Tukey-Kramer multiple comparisons tests (applied to averages from 2015 and 2016 separately) are represented by coloured means with different colours denoting statistically significant differences.

Fig 6

Mean adjusted hours with Cuvier’s (top) and Sowerby’s (bottom) beaked whale clicks with standard error bars in four light periods: dawn, light, dusk, and dark. Light period times were obtained from Reda and Andreas (62). Results of Welch’s ANOVA are given for each species and results from Tukey-Kramer multiple comparisons tests are represented by coloured means with different colours denoting statistically significant differences. *The Sowerby’s light period does not differ significantly from any of the other light periods.

Fig 7. Daily and hourly occurrence of Sowerby’s beaked whale clicks per acoustic file at stations 1–8 represented as individual black dots (P = 0.96, R = 0.93).

Shaded areas indicate periods of darkness. The red dashed lines indicate the deployment and retrieval dates. Black diagonal lines indicate periods with no data.

Fig 8

Mean Sowerby’s beaked whale detections per recording day bubble plot (right) and with standard error bars (left) for stations 1 (SE = ±1.28), 2 (SE = ±0.56), 3 (SE = ±0.70), 4 (SE = ±0.81), 5 (SE = ±0.51), 6 (SE = ±0.37), 7 (SE = ±0.23), and 8 (SE = ±0.15) off western Ireland from May to Dec 2015 (stations 1–4) and Mar to Nov 2016 (stations 3 and 5–8). Results of Kruskall-Wallis tests are given and results from Tukey-Kramer multiple comparisons tests are represented by coloured means (left) with different colours denoting statistically significant differences. ^Station 4 does not differ from 3 or 5. *Station 7 does not differ from 8 or 6, but 8 differs from 6.

Fig 9

Mean daily number of Sowerby’s beaked whale detections per month for each station that recorded data in 2015 (top) and 2016 (bottom) as well as the average across all stations with standard error bars. Results of Kruskall-Wallis tests are given and results from Tukey-Kramer multiple comparisons tests (applied to average of 2015 and 2016 separately) are represented by coloured means with different colours denoting statistically significant differences. *November 2015 differs from July and October 2015, but not from June, August, September, or December 2015. ^October 2015 differs from August 2015.

Fig 10. Daily and hourly occurrence of northern bottlenose whale clicks per acoustic file at stations 1–8 represented as individual black dots (P = 1, R = 0.42).

Shaded areas indicate periods of darkness. The red dashed lines indicate the deployment and retrieval dates. Black diagonal lines indicate periods with no data.

Fig 11. Daily and hourly occurrence of seismic impulses (black dots) at stations 1–8.

Shaded areas indicate periods of darkness. The red dashed lines indicate the deployment and retrieval dates. Black diagonal lines indicate periods with no data. Blue solid lines indicate days when sonar was opportunistically observed.

Fig 12. Daily unweighted and marine-mammal auditory filter weighted [66] sound exposure levels for station 8 which had the highest seismic sound levels across all eight deployments.