By Mari Hansen 
One of nature’s silent wonders is the migration of whales. Animals the size of buses travel thousands of kilometers across entire oceans with amazing accuracy for months at a time. It’s difficult to avoid feeling awed when viewing tracking maps of these journeys, the slow arcs across the North Atlantic or Pacific. However, something subtle but concerning appears to be developing beneath the surface. The sound of the seas is growing louder. Additionally, whales—animals that evolved in an acoustic environment—are beginning to find it difficult to cope with the noise.
Underwater, sound is everything. Once you go below the surface, light quickly disappears, so the main way that marine life perceives their environment is through sound. In particular, baleen whales are excellent communicators. In perfect circumstances, their deep calls can travel hundreds of kilometers, enabling people dispersed over vast distances to maintain a loose connection. Researchers frequently refer to this as a “wisdom of the crowd” system, or distributed navigation network, in which a large number of whales traveling together increase the migration route’s accuracy. It’s a sophisticated system. Or it was, anyway.
| Category | Information |
|---|---|
| Primary Species Studied | Baleen whales (including minke, humpback, and blue whales) |
| Migration Distance | Up to 20,000 km annually |
| Main Communication Method | Low-frequency vocalizations that travel hundreds of kilometers |
| Main Human Noise Sources | Shipping traffic, sonar, offshore construction, seismic surveys |
| Estimated Rise in Ocean Noise | Over 20 dB increase since mid-20th century |
| Example Study Area | North Sea migration corridor |
| Key Scientific Method | Agent-based mathematical modelling of whale movement |
| Example Species Used in Model | Minke whale (Balaenoptera acutorostrata) |
| Reference Website | https://www.nationalgeographic.com/animals/mammals/facts/whales |
The oceans’ acoustic characteristics have undergone significant change in the last century. With their engines creating a continuous low-frequency hum that travels incredible distances through water, cargo ships now traverse important sea routes day and night. Pile drivers are used on offshore construction sites to pound the seabed. Military sonar covers a lot of ground. Seismic blasts are fired into the seabed by oil exploration vessels. What researchers sometimes refer to as the “modern ocean soundscape” is enhanced by each activity.
As a result, according to a number of recent modeling studies, ambient noise levels have risen by over 20 dB in certain areas since the middle of the 20th century. Although that figure may appear low, acoustically it indicates a rise in background noise intensity of almost a hundred times.
It’s hard not to wonder what that sound must sound like underwater when you’re standing close to a busy harbor and watching container ships glide by in the dingy morning light.
To investigate this question, whale migration researchers recently developed a sophisticated mathematical model. Modeling provides one of the few methods to mimic how massive populations might react to shifting ocean conditions because whales can’t exactly be brought into a lab. Hundreds of virtual whales navigated the North Sea using environmental cues and the calls of other whales within hearing distance as they followed migration routes in their simulations.
The results appeared almost graceful in quiet conditions, similar to the pre-industrial ocean. The majority of whales arrived within a few days of each other, and migration routes created narrow passageways leading to feeding grounds. Long-distance communication signals allowed whales dispersed hundreds of kilometers apart to maintain a loose sense of coordination. The researchers then presented contemporary shipping noise levels. The migration was instantly altered.
The model showed a collapse of about an order of magnitude in the distance that whales could hear one another. Suddenly, instead of long-distance acoustic networks, people were working almost alone. Migration continued, but it was much slower and occasionally took several more days. The graceful group navigation started to fall apart.
An unexpectedly human analogy is sometimes used by scientists to explain this phenomenon. Nowadays, trying to have a conversation underwater might be similar to trying to do so at a packed concert.
Loss of communication is only one aspect of the problem, though. Whales seem to be affected by noise in at least two additional ways.
First, avoidance behavior is frequently triggered by loud sound sources. Whales may exhibit negative phonotaxis, a reaction in which they simply avoid loud noises. In minor instances, this results in small detours around construction zones or shipping lanes. However, the models indicate that something more concerning might occur. Whales may effectively encounter what researchers refer to as a “wall of noise” if enough noisy areas line up across a migration route, stopping them from following their usual route.
The issue of confusion is another. It is thought that whales rely on natural acoustic landmarks, such as the sound of breaking icebergs, surf crashing along far-off coastlines, and possibly even the subtle sounds of ocean geography. The animals may lose important directional information when human noise obscures those signals. This occasionally led to whales in simulations veering off course and being carried into completely different regions by currents.
Which of these effects predominates in the wild is still unknown. Like a silent warning, that uncertainty looms over the research.
There is a growing sense that something subtle is changing beneath the waves as migration maps are created over decades. There have already been reports of some whale populations abandoning feeding grounds close to busy shipping lanes. Others seem to be rerouting around industrial areas that are noisy.
From the surface, the oceans still appear huge and deserted. Long white wakes that vanish in a matter of minutes are left behind by ships moving slowly across the horizon. However, the story might sound completely different underwater.