Impact of underwater noise pollution on the early life history of cartilaginous fishes.

Understanding how human-made noise affects sharks, rays and chimaeras 

The oceans are getting louder. Boats, construction, and other human activities create underwater noise that can impact marine life. While we know that sound can stress fish and change their behaviour, most research has focused on bony fishes. Our project turned attention to a group of animals that has been largely overlooked: cartilaginous fishes, which include sharks, rays, and chimaeras (also called ghost sharks). These animals are especially vulnerable to human impacts. They grow slowly, reproduce late in life, and have only a few offspring, making it harder for populations to recover from threats. In some species, embryos develop inside tough egg cases on the seafloor for many months, exposed to everything happening around them, including underwater noise. 

Our research explored how noise pollution affects these early life stages, focusing on three species: 

• The Port Jackson shark (Heterodontus portusjacksoni) (Figure 1) 
• The Australian ghost shark (Callorhinchus milii) (Figure 2)
• The southern fiddler ray (Trygonorrhina dumerilii) (Figure 3A) 

What we did 

We combined field recordings, lab experiments, and high-resolution imaging to explore three key questions: 

1. What does the underwater soundscape sound like in areas where these animals lay their eggs or give birth? 
2. How do baby sharks and rays respond when exposed to human-made noise? 
3. Does noise affect how their ears develop? 

In the field, we deployed underwater microphones in Port Phillip Bay and Western Port Bay (Figure 3B) in Melbourne, Victoria, recording soundscapes over many months.

We also began building a tool that can automatically detect boat noise in these recordings. In the lab, we raised shark and ghost shark embryos in tanks where we played either natural underwater sounds or boat noise. We tracked their growth and behaviour. After hatching, we studied their ears using advanced techniques like microCT (Figure 4) and confocal microscopy to examine the tiny hair cells that enable fish hearing and balance. We also ran short-term boat noise exposure experiments with adult Port Jackson sharks, adult ghost sharks and juvenile fiddler rays, observing their behaviour – did they swim more, seek out places to hide, change how they breathed? Their brains and ears were preserved for further study to assess how sound may affect development.

What we found 

• Boat noise affects behaviour. Both adult ghost sharks and adult Port Jackson sharks changed their behaviour during noise exposure. They altered their swimming, avoided the sound source, or showed signs of stress such as increased gill movements. 
• Inner ear development may be impacted. Embryos exposed to noise had differences in the size and shape of their inner ears, and changes in their sensory hair cells, which are critical for hearing and balance. 
• Port Jackson sharks are a valuable model. We created the most detailed images to date of their inner ears, showing how these structures grow and function. This work will be useful for future hearing studies in sharks.
• Real-world recordings are essential. The soundscape data we collected will help identify when and where noise is most disruptive. Our developing noise detector will support ongoing monitoring and management of this threat.

Why this matters 

Sound plays a vital role for many fish. They use it to navigate, avoid danger, and find mates. If their hearing is damaged early in life, they may struggle to survive. Our research provides some of the first clear evidence that human-made noise can affect shark and ghost shark embryos, even before they hatch. We also found that noise can disrupt the behaviour of adult sharks and ghost sharks, potentially causing them to avoid important areas like breeding grounds or nursery habitats. Repeated stress response, such as increased gill movements or altered swimming, may have longer-term impacts on health and reproduction. These findings highlight the need to include cartilaginous fishes in conservation planning and noise regulations. We hope our work will help shape future guidelines to protect these important animals and the habitats they depend on.