Captive Marine Mammals: Assessing Acoustic Welfare

In the U.S. alone, hundreds of marine mammals like dolphins, orcas, and sea lions are kept in captivity. For intelligent and social animals like these, being kept in captivity can cause brain damage and other poor outcomes. While there are some welfare guidelines for captive marine mammals, the authors of this review argue that they could be improved by using “acoustic monitoring” — measuring the sounds in animal enclosures. They describe some of the approaches used by the U.S. Navy’s Marine Mammal Program.

Many marine mammals rely on sound to communicate and hunt, and their hearing is highly sensitive. For example, bottlenose dolphins can hear sounds with frequencies as high as 140kHz, which is seven times higher than what humans can hear. We can measure both the sounds that marine mammals hear and make to better understand and improve their welfare.

Causes Of Marine Mammal Hearing Damage

For marine mammals in captivity, there are many possible sources of loud noise. Animals in enclosures bordering the sea may hear the noises of nearby ship traffic or sonar from recreational and military vessels. Animals kept in artificial enclosures are exposed to the sounds of machines that pump and clean water. Humans often don’t notice these noises as sound travels differently in water and may be at frequencies too high for us to hear.

When animals are exposed to loud noises, they can suffer “threshold shifts,” where previously quiet sounds become inaudible. These shifts can be temporary or permanent. Because there is a lot of research on how these shifts happen for marine mammals, sound levels can be easily measured to alert keepers when they get too high. This is especially important when things change, for example when the animal is moved to a new enclosure. The authors recommend that regular sound measurements should be taken to identify and fix sources of loud noise before they cause hearing damage.

Hearing loss isn’t always caused by noise pollution, making it important to regularly test the hearing of captive animals. Hearing tests can be based on animal behavior (for example, by training a dolphin to touch a target when they hear a certain sound) or by directly measuring brain signals. The authors consider behavioral tests to be the most effective, but direct measurement might be necessary when animals can’t be trained. For seals and sea lions, direct measurement is much harder because the animals must be anesthetized. The authors recommend that hearing tests should be done twice a year to identify hearing loss that might cause welfare problems.

Understanding The Noises Animals Make

Many marine mammals rely on sound for communication so the sounds they make can give us information about their welfare. Acoustic monitoring of vocalizations is already done with farmed animals on land, such as spotting sickness by recording animal coughs. When groups of dolphins are kept captive together, they whistle to communicate. It’s possible to identify changes in the rate of whistles, which can alert keepers of stress or sickness in the group. In order to do this effectively there needs to be good data on how the dolphins are expected to behave normally. The authors recommend that sounds should be continually monitored to alert keepers of any behavioral changes.

Some marine mammals make individual, complicated calls that change in different circumstances, such as during sickness or transport. If facilities know a lot about the sounds made by each captive animal, they can then use machine learning models to better understand their welfare states. The authors recommend that “vocal catalogs” of individual captive animals in different conditions are compiled and used for precise monitoring.

Takeaways

Knowing how marine mammals respond to their acoustic environment deepens our understanding of the impacts of captivity on their lives. Because the needs of marine mammals are very complex, it’s hard to ensure they have high welfare in captivity. Animal advocates should be aware of acoustic welfare for marine mammals, and should pressure organizations that keep marine mammals captive to implement the higher welfare standards described in this work. When campaigning for abolition, the complexities of acoustic welfare can be used to show the difficulty of ensuring adequate welfare in captivity. Some of the methods described here may also be useful for measuring the welfare of marine mammals in the wild. This could help us understand how the welfare of captive mammals differs from those in their natural state.