Ocean Literacy · Marine Mammals · Ocean Noise

Hearing Tests at Taronga Zoo Sydney Measure What Human Noise Australian Sea Lions and Fur Seals Can Hear

Four trained animals have produced the first in-air hearing curves for an endangered sea lion and its fur seal neighbour, giving noise assessments for ports, shipping and offshore industry something species-specific to work from.

By SEVENSEAS Media | Ocean Conservation

Until this year nobody had measured what an Australian sea lion or a long-nosed fur seal can hear in air. Researchers from Curtin University’s Centre for Marine Science and Technology and Taronga Conservation Society Australia have now tested two animals of each species at Taronga Zoo Sydney, and found that both hear best at around 3.2 kilohertz, according to Curtin’s announcement of the work. The results appear in two papers, one on the sea lions in the journal Animals and one on the fur seals in Conservation.

The point of the exercise is noise. Ships, small boats, aircraft and coastal construction all reach the beaches and rocks where these animals haul out, and anyone assessing a new port or offshore project needs to know which of those sounds the animals can detect before arguing about whether they are harmed by them.

How you give a sea lion a hearing test

Taronga’s marine keepers trained the animals to press a paddle whenever they heard a tone, with a fish as the reward for a correct response, Curtin says. The researchers played tones between 100 hertz and 20 kilohertz to find the quietest sound each animal could detect at each frequency. Plotted together, those thresholds make an audiogram. The fur seal tests were run inside an anechoic chamber at the zoo, a room lined to absorb echoes, according to the Conservation paper.

The sea lion paper describes behavioural audiograms of this kind as the “gold class” of hearing curves, because they measure what the animal perceives rather than only how its ear and nerves respond to sound. That depends on the animals cooperating, and Dr Benjamin Pitcher, a behavioural biologist at Taronga, said they were given the choice.

“The seals and sea lions were given voluntary choice to participate in the hearing tests using positive reinforcement, allowing us to measure what they could actually perceive while keeping the animals engaged in the process,” he said in the announcement.

What the curves show

The two sea lions, both adult females, were most sensitive at 3.2 kilohertz, with thresholds of 13 and 14 decibels relative to 20 micropascals, the reference pressure also used for human hearing in air. Their range of best hearing ran from 600 hertz to 9 kilohertz in one animal and to at least 20 kilohertz in the other. The authors write that the curves broadly resemble those published for other sea lion species, but that the Australian animals hear best at lower frequencies.

The fur seals, an adult male called Bondi and an adult female called Eve, also peaked at 3.2 kilohertz, at 7 and 12 decibels. The younger animal, the female, picked up high frequencies better, and the overall shape of both curves was similar to those recorded for northern fur seals, the paper reports.

The team then laid the fur seal audiograms over recorded spectra of boats, ships, aircraft, trains and construction. Many of those sounds would be audible to the animals while they are hauled out on land, Curtin says.

Why port and offshore assessments need this

“Before we can understand whether a noise is affecting an animal, we need to know whether it can hear that noise in the first place.”

Professor Christine Erbe, Centre for Marine Science and Technology, Curtin University

Erbe, who led both studies, said the work matters most for the Australian sea lion, which she put at about 12,000 animals in total. The sea lion paper describes it as one of the most endangered of the eared seals, with bycatch, prey depletion, pollution and habitat loss among its threats, and says coastal and offshore industrial development disturbs it largely through noise. The fur seal paper names Australia’s expanding blue economy, including shipping, fishing and offshore renewable energy, as the source of the noise it is concerned with, and cites a seismic survey off New Zealand that changed long-nosed fur seal behaviour.

Without species data, assessors have had to borrow. In a December 2024 letter to Mid West Ports on the Geraldton Port Maximisation Project, published by Western Australia’s Environmental Protection Authority, Erbe’s own centre estimated in-air noise exposure for Australian sea lions using criteria from the US National Oceanic and Atmospheric Administration, which place the species in a generic category of “other marine carnivores in air”. The fur seal paper says its results are meant to feed exactly this kind of work, as baseline data for building auditory weighting functions and assessing noise from coastal and offshore developments.

Pitcher said the findings could inform environmental impact assessments and noise management “as shipping, coastal development and offshore industries expand”.

What the tests cannot say yet

Four animals is a small sample, and the two sea lion curves already differ noticeably at the high end. An audiogram also measures detection only. Knowing that a fur seal can hear a passing train says nothing on its own about whether the train wakes it, drives it into the water or keeps a mother away from her pup.

These animals also spend much of their lives at sea, and every result so far was measured in air. Pitcher said the team’s next step is to find out how noise affects them under water and when it begins to interfere with their behaviour or their hearing.

Read the studies: In-Air Behavioral Audiograms of Two Australian Sea Lions (Neophoca cinerea), Animals, and Aerial Audiograms of Two Long-Nosed Fur Seals (Arctocephalus forsteri), Conservation. Curtin University’s announcement is also online.

SEVENSEAS Media · Conservation Journalism for the Ocean