Underwater Noise Pollution from Fishing Vessels in the Northern Waters of Central Java: Acoustic Characteristics and Ecological Implications

Imagine the ocean not just as a place of water and fish, but as a giant, bustling library where every creature relies on sound to read the room. In this underwater library, whales chat across miles, fish coordinate their schools like a synchronized dance team, and crabs navigate the muddy floor by listening to the world around them. For millions of years, the "background music" of the ocean was mostly the gentle hum of waves, the crackle of shrimp, and the songs of other sea life. But lately, a new, loud guest has been crashing the party: human ships. This field of study, known as underwater acoustics, investigates how the noise from our boats, engines, and propellers changes the ocean's soundscape. The big worry is that if the music gets too loud, the library's patrons can't hear each other. They might get lost, miss a dinner date, or even get so stressed they stop growing. Scientists are now asking a critical question: just how loud is the ocean getting, and who is making the most noise?

A team of researchers from Jenderal Soedirman University decided to investigate this noisy problem in the northern waters of Central Java, a busy stretch of the Java Sea known as Fisheries Management Area 712. Think of this area as a super-crowded highway where thousands of fishing boats zoom back and forth every day. The researchers wanted to see two things: first, what kind of boats are driving on this underwater highway, and second, what kind of "engine roar" they are making. They didn't just guess; they went out with special underwater microphones (called hydrophones) and high-definition cameras to record the sounds and match them to the specific boats passing by. They looked at everything from tiny, single-person wooden boats to massive industrial vessels, analyzing the volume (how loud), the pitch (how high or low), and the rhythm of the noise they produced.

Here is what they found: The underwater highway is overwhelmingly dominated by the smallest drivers. Between 2019 and 2024, the vast majority of boats were tiny vessels, weighing 5 tons or less. These little guys are usually powered by outboard engines (the kind you see on the back of a boat) that sputter and rev up and down as the fishermen maneuver. Because these engines are small and the boats are light, they don't make a steady hum. Instead, they create a chaotic, "impulsive" noise—like a thousand tiny, erratic drumbeats or a room full of people shouting in short, sharp bursts. The researchers measured these sounds reaching up to 140 decibels (a very loud level), but the noise was often jagged and unpredictable.

In contrast, the bigger boats (those over 5 tons) were fewer in number but made a very different kind of noise. These vessels use heavy diesel engines mounted inside the hull. Their sound is more like a deep, continuous, and steady drone, similar to a large air conditioner running non-stop. While the little boats make "spiky" noise, the big boats make a "wall" of low-frequency sound that travels far and wide through the shallow water. The study suggests that while the big boats are individually louder and more persistent, the sheer number of tiny boats operating all at once creates a massive, cumulative wall of noise that fills the entire area.

The ecological implications of this noisy mix are serious. The researchers suggest that this constant din is likely drowning out the natural sounds fish and other sea creatures need to survive. The tiny, erratic noises from the small boats might startle fish or make it hard for them to hear predators, while the deep, steady drone of the big boats could mask the calls fish use to find mates or stay in a group. Since the Java Sea is a shallow, semi-enclosed space, these sounds don't just fade away; they bounce around and pile up. The paper concludes that this "acoustic smog" is likely causing stress, changing how fish behave, and potentially hurting the health of the entire ecosystem. It's a reminder that even though we can't always hear the ocean's noise from the shore, the creatures living there are hearing it loud and clear, and it's changing their world.