When listening to beluga whales, the sound of a crunch or a clapped jaw may be a reliable indication that a beluga whale just successfully captured or missed a fish. In a 2021 paper published on beluga whales in Alaska, scientists analyzed sound data, collected over several years, to monitor beluga whale calls and other data. With these data they are gaining new insights into belugas’ diet, feeding behavior, and feeding habitats.
This is a visual representation of sound. Time is on the horizontal axis, frequency is on the vertical axis, and sound amplitude or volume is a color gradient from dark (quiet) to vivid (loud).
“Our goal is to collect data to help understand and recover Cook Inlet beluga whales, an endangered species and NOAA Fisheries Species in the Spotlight,” said Manuel Castellote, NOAA Affiliate and lead author for the study from the Cooperative Institute for Climate and Ocean and Ecosystem Studies, University of Washington. “There is a paucity of basic ecological knowledge, such as prey preference, about this population of whales. This impedes our efforts to help recover the population. The population is estimated to be only 279 animals, and was in decline during the 10-year period from 2008-2018, the most recent time period for which we have data.”
Given the endangered status of the Cook Inlet belugas, there are limited studies that are permitted on this population. So scientists conducted research using tagging technology and other methods on a comparable surrogate—an abundant population of beluga whales in Bristol Bay, Alaska. The Bristol Bay beluga population is estimated to be between 2,000 and 3,000 animals.
Scientists successfully used satellite tags, digital acoustic recording tags, and stomach temperature transmitters that are swallowed on eight beluga whales from Bristol Bay. The tags enabled researchers to understand movements and the sounds the belugas made when socializing and feeding. Stomach temperature readings also provided insights on whether the belugas had fish in their stomachs. Lower temperatures indicated that fish ingestion occurred.
Belugas are known to be among the most vocal whales. They emit three types of social communication signals:
- Narrowband frequency modulated tones termed whistles
- Broadband bursts of pulses termed calls
- Combination of these two previous types, emitted simultaneously, termed mixed or combined calls
These social signals range in frequency (pitch) from approximately 200 Hz to 20 kHz(.mp3).
Using Echolocation to See in the Dark
Beluga echolocation clicks, used to find food and navigate, extend upward of 120 kHz (ultrasonic). Echolocation is a technique used by mammals such as toothed whales, dolphins, and bats to determine the location of objects using reflected high-frequency sound. This allows the animals to move around in darkness or in very muddy water, so they can navigate, hunt, identify potential mates and predators, and avoid obstacles.
“The vocal repertoire of the Cook Inlet beluga whale population is not well known, only one descriptive study has been published so far, and no studies have been published for the Bristol Bay population,” said Castellote. “However, in other beluga and marine mammal populations, the highest vocal activity tends to occur during socialization. When feeding, typically there is no vocal activity or it is very low, just related to echolocation.”
For example, in dolphin and killer whales, a buzz sound is associated with both socializing and feeding. Scientists are often able to distinguish between these two activities if they know where the animal is in the water column. If it is near the surface, it is usually found to be socializing. However, when the animal is on a dive, it is believed to be searching for or chasing a fish.
Both Bristol Bay and Cook Inlet beluga habitats have very similar depth ranges, rarely exceeding 100 meters, and belugas in both populations are typically found nearshore. Although the dive data have yet to be summarized for Bristol Bay, dive depths in Cook Inlet average only 4 meters. Thus, using depth as a way to distinguish between social and feeding buzzes is not applicable for these populations. To learn more about Bristol Bay and Cook Inlet beluga whale calling behavior, scientists had to try a new approach.
Trying Tags to Collect Sounds
In Bristol Bay scientists collected sounds made by tagged beluga whales during feeding. Tagged data also helped them learn more about beluga feeding behavior, prey preferences, and areas where they were feeding.
“We identified feeding and social periods in Bristol Bay belugas based on stomach temperature sensing and acoustic behavior recorded in tagged animals. We characterized echolocation buzzes from both behavioral contexts, which allows us to identify when belugas are feeding,” said Castellote.
In Cook Inlet, they used data collected from a stationary mooring attached to the seafloor and equipped with passive acoustic equipment (an underwater microphone). These particular moorings are located in the Cook Inlet beluga critical habitat— near important foraging grounds. This non-invasive listening device captures sounds year round, including whale calls and echolocation signals.
Scientists were able to identify the type of echolocation signals used by Bristol Bay and Cook Inlet belugas when feeding. They analyzed sound recordings collected from Cook Inlet in May to September 2018. They reviewed fish counts in the river during their travel to spawn from the near-by Susitna River. This information was used to describe the relationships between the timing of spawning runs of salmon and eulachon, a small smelt, and beluga feeding occurrence.
Feeding Occurrence and Prey Preferences in Beluga Whales
From years of study, scientists know that passive acoustic monitoring via anchored moorings is an effective way to monitor the year-round occurrence of belugas in Cook Inlet. Now data from these moorings also can be used to learn more about these endangered whales. Scientists are able to use echolocation signals collected by the moorings to help describe feeding occurrence.
Beluga presence and the timing of feeding events reflect a clear preference for the mouth of the Susitna River during the spawning runs by eulachon, Chinook, pink, and coho salmon.
“The application of remote sensing technology is proving to be an effective way to identify when and where belugas feed. It is helping us to learn more about beluga whale prey preferences. With this, resource managers will be able to make more informed decisions to aid in the recovery of this endangered population,” said Paul Wade, project lead for Cook Inlet beluga research, Alaska Fisheries Science Center, and coauthor on the paper.
"We are pleased to see the success of the acoustic tagging study, to have been collaborators on the Bristol Bay work, and by sharing fisheries data and funding to contribute to the Cook Inlet data. We look forward to continuing this collaboration with current and on-going studies working towards Cook Inlet beluga recovery," said Lori Polasek, marine mammal program coordinator, Alaska Department of Fish and Game.