Written by Brijonnay Madrigal
We are pleased to share a new publication in the Journal of the Acoustical Society of America, which describes the first killer whale call catalog for the Chukchi Sea and identifies the likely ecotype present. This paper was written by Brijonnay Madrigal and colleagues at the NOAA Alaska Fisheries Science Center as part of her Master’s thesis work conducted at Moss Landing Marine Laboratories prior to entering the PhD program at the Hawai’i Institute of Marine Biology.
The manuscript is entitled:
Call repertoire and inferred ecotype presence of killer whales (Orcinus orca) recorded in the southeastern Chukchi Sea
Authors: Brijonnay Madrigal, Jessica Crance, Catherine Berchok, Alison Stimpert
Summary:
Killer whales occur in the Arctic but less is known about the killer whale populations in the Chukchi Sea. The lack of detailed acoustic analysis of killer whale ecotypes in this region is in part a result of a lack of dedicated effort until recent years, due to the difficulties of accessing the Chukchi Sea. Calling behavior differs among ecotypes, which can be distinguished based on pulsed call type, call rate, and bandwidth. In this study, using passive acoustic monitoring we sought to identify killer whale ecotype presence at a site in the southern Chukchi Sea by characterizing pulsed calls recorded during three consecutive summers. We predicted that transients (Mammal-eating killer whales) would be the primary ecotype detected at this site, based on prey availability, previous observations, and home range. Identifying ecotype presence at this site will increase our knowledge of the spatio-temporal distribution of killer whales in the Arctic and have implications for ecosystem management in this area. This study provides the first comprehensive catalogue of transient killer whale calls recorded in the Chukchi Sea, as well as reports on previously undescribed calls. These data provide new insight into transient acoustic behavior and call diversity in the Chukchi Sea and can serve as a baseline for future acoustic work on killer whales in the Arctic.
Methods:
Data used in this study were collected as part of the Arctic Whale Ecology Study (ARCWEST). Passive acoustic data were collected using Autonomous Underwater Recorders for Acoustic Listening (AURAL) devices, deployed on subsurface moorings in the southeastern Chukchi Sea. Data were recorded from a mooring location approximately 75 km southwest of Point Hope (Figure 1). A semi-automated energy detector in MATLAB was used to extract killer pulsed calls and a combination of manual and hierarchical cluster analysis was used to categorize call contours into sub-categories.
Results:
A total of 1323 killer whale pulsed calls were detected on 38 of 276 days during the summers (June-August) of 2013 to 2015. The majority of calls (n = 804, 61%) were recorded in 2013 with the most calls recorded in July (76% of total calls). Calls were grouped into six categories: multi-part, downsweep, upsweep, modulated, single modulation, and flat (Figure 2). Most detections were flat (n = 485, 37%) or multi-part calls (n = 479, 36%), which contained both high- and low-frequency components. Call comparisons with those reported in the published literature showed similarities with other transient populations in fundamental frequency contour point distribution and median frequency (Figure 3).
Key Findings:
One of the most distinguishing features in the Chukchi Sea dataset was the presence of multiple call components in the pulsed calls. CHp1-CHp5 call types (characterized by 2-5 distinct call parts) were not found in any other published data set and yet comprised 54% of the CHp call type.
The frequencies of calls recorded at PH1 suggest they are produced by transients. The high prey availability for transients at our recording location, combined with historic sightings including observations of killer whale predation on gray whales near Point Hope, support this ecotype determination.
Based on the location and the acoustic results presented here, it is likely that these Chukchi Sea transients are from the Alaska/Aleutian Islands/Bering Sea transient population in Alaskan waters, although we cannot discount the possibility of other whale populations (e.g. Russian killer whales).
Figure 1: Map of the southern Chukchi Sea and study site, PH1 (red star), 75 km southwest of Point Hope, AK.
Figure 2: Representative examples of the subcategories within each call: (a) multi-part (CHp), (b) flat (CHf); (c) downsweep (CHd); (d) upsweep (CHu); (e) modulation (CHm); and (f) single modulation (CHs). All CHp call exemplars contain brackets indicating the different parts of the call including low frequency components (L) and high frequency components (H) as well an asterisk (*) indicating primary LFC. All spectrograms FFT size 512, 16 kHz, Hamming, 50% overlap.
Figure 3: Median frequency of LFC fundamental frequency of killer whale calls from the Chukchi Sea compared to the eastern North Atlantic, North Pacific resident, and transient populations from Filatova et al. (2015) Figure 2. Sample size is shown in parentheses.
Full citation details: Madrigal, B.C., Crance, J.L., Berchok, C.L., and Stimpert, A.K. (2021). Call repertoire and inferred ecotype presence of killer whales (Orcinus orca) recorded in the southeastern Chukchi Sea. The Journal of the Acoustical Society of America, 150(1), 145-158 https://doi.org/10.1121/10.0005405
The full publication can be downloaded here: https://asa.scitation.org/doi/abs/10.1121/10.0005405
If you are unable to download the article, please email bcm2@hawaii.edu for a pdf copy.
https://mlml.sjsu.edu/
Twitter: @brijonnay, @MMRP_UH
Instagram: @mosslandingmarinelabs, @mmrp_uh