Minke Whale Hearing -A project of the President’s Subcommittee on Ocean Science and Technology (SOST)
Over the last century, humans have increasingly put sound into the ocean. Human-made sound, or anthropogenic sound, is intentionally used to explore the ocean floor, to search for oil, in scientific research, in fisheries, and for military purposes (e.g. sonar). A dramatic increase in the use of cargo ships for transporting commercial goods and a proliferation of recreational vessels have also contributed to an increase in ocean sound in many parts of the world. Because sound travels efficiently through water, sounds produced in the ocean can travel great distances and can be detected thousands of miles away from the source of the sound if the sound is of a sufficiently high level.
Baleen whales, otherwise called mysticete whales, are the largest animals on earth.
These whales filter feed using baleen, which is a feathery, filter-like comb structure within the mouths of the whales. Baleen whales rely on sound to communicate, forage, and navigate the world’s oceans. Anthropogenic sound can potentially affect a baleen whale’s hearing, change its behavior, and interfere with its ability to communicate, forage, and navigate. It can potentially hinder a baleen whale’s ability to hear and avoid ships, and chronically high levels of sound might elevate levels of stress hormones that affect a whale’s health and ability to reproduce. In short, anthropogenic sound is noise to baleen whales and it has the potential to negatively affect them. As many baleen whales are endangered and their rates of producing calves are low, there is reasonable concern as to how anthropogenic noise might affect baleen whales and their populations.
Anthropogenic ocean noise covers a broad range of frequencies (i.e., from low to high pitches); whales might not be able to hear some of these frequencies of sound, but other sounds might occur at frequencies to which whales are very sensitive. Understanding what a whale can hear is the first step in determining what kinds of anthropogenic ocean noise might affect them. This knowledge ultimately helps in the development of measures to protect the whales from ocean noise. Importantly, no test of the frequencies that baleen whales can hear has ever been successfully performed, and the hearing abilities of baleen whales have only been estimated from ear anatomy and observed behavioral responses to sound exposure. As a result, the SOST Interagency Task Force on Ocean Noise and Marine Life has identified a lack of knowledge about baleen whale hearing as a critical impediment to protecting baleen whales from ocean noise.
The National Marine Mammal Foundation (NMMF) in collaboration with the Norwegian Defense Research Establishment (FFI) is undertaking an effort to acquire the first audiogram (hearing sensitivity curve) for a baleen whale. The research is funded by the SOST Interagency Task Force on Ocean Noise and Marine Life, which consists of multiple agencies including the Office of Naval Research, Bureau of Energy Management, National Oceanic and Atmospheric Administration, US Navy Living Marine Resources, and the Marine Mammal Commission. The effort will provide the first direct measures of hearing across frequency in any baleen whale and will be a step toward addressing a knowledge gap that is critical to protecting baleen whales form anthropogenic ocean noise.
The project will take place over several years in northern Norway during the late spring and early summer. During this time, the smallest of the baleen whales, the minke whale (Balaenoptera acutorostrata), migrates northward to Arctic waters. The research team will perform a catch-and-release study in which adolescent minke whales will be caught and temporarily held within a natural body of water barricaded by nets. The whale will then have its hearing tested by using auditory evoked potential (AEP) methods. After the test, the whale will be released to continue its migration.
The AEP method monitors electrical signals produced by the brain when it hears a sound. These tests are regularly performed on newborn human babies, and are also now a commonly used veterinary tool for testing the hearing of dolphins, porpoises, and other small whales. The hearing tests with minke whales will be conducted by NMMF scientists, who have extensive experience in performing these types of tests and are world leaders in the study of marine mammal hearing.
Adolescent minke whales will be targeted for this study. Adolescent minkes range from 3-5 m in length and are similar in size to adult beluga whales, for which the AEP method has been successfully used many times. Testing with the smaller individuals makes handling the whale easier and increases the chances of successfully performing the hearing tests.
Whales will be captured between two small islands that the whales are known to pass between on their migration. Once a whale passes between the islands, the entrances will be blocked with barrier nets. The waterway is large and a whale will have plenty of room to freely swim and dive while contained between the islands. After a marine mammal veterinarian has observed the whale and made a decision that it is healthy for testing, another net that spans the width of the waterway will be deployed between two boats. This net will be slowly maneuvered to direct the whale to an opening in a circular fish farm, which is a large netted enclosure for holding fish. When the whale enters the fish farm, the opening to the fish farm will be closed. The net of the fish farm can then be pulled up under the whale to create a hammock. This safely limits movement of the whale while also keeping it at the surface so its blowhole is kept above water but the lower body remains submerged. The hearing test is conducted while the whale is in the hammock. After the hearing test is completed, the whale will be released back into the fish farm and then back into the basin. The barrier nets to the basin are then opened so the whale can continue its migration.
Each whale that has its hearing tested will be fitted with a satellite tag prior to its release. This will allow researchers to monitor the whale to ensure that it continues on its normal migration. The tags should allow researchers to follow the whales for several months after testing.
The first field season of the SOST minke whale hearing project has come to a close. This season was focused on providing proof-of-concept that the system for catching and releasing whales would work and provide a safe environment for testing. Almost half of the season was used to set up and optimize the catch and release system, which consisted of a system of nets used to guide the whales into a basin that could be sealed once entered. Nearly 1.7 km of netting weighing more than 20 tons were deployed.
Twenty whales were observed in the area during the month of setup and operation, validating local knowledge about the migratory patterns of the whales used in the conceptual design of the system. Three whales entered the catch basin prior to completion of the catch and release (C&R) site and three others entered the basin after the team completed the C&R site and entered “catch” mode. This importantly demonstrated that whales migrating through the area could be diverted into the C&R site by the net system. Two of the three whales entering the C&R site were safely caught demonstrating the feasibility of containing whales once in the basin.
The information obtained from this first field season was critical to demonstrating proof-of-concept of the temporary catch and release of minke whales. No hearing tests were attempted this year, but the team will be shifting focus next year to this final phase of the experimental protocol. The hearing sensitivities will be the first ever measured for any baleen whale and will have important consequences to managing the impact of ocean noise on baleen whales.