At first glance, fish don’t appear to have ears, but that doesn’t mean they can’t hear. While there are usually no openings on a fish’s head for sound to enter, they do have inner ears that pick-up sound through their body. In fact, many fish rely on their ears to find habitat and mates, as well as spawn, swim, and avoid predators. This makes sense when you consider that sound transmission in water is approximately 4 times faster than air, allowing fish to communicate through sound quickly and over relatively large distances.
Despite the fast transmission of sound in water, not all fish have notable hearing. Indeed a fish’s ability to hear sound varies drastically depending on the design of the inner ear. Fish that have a connection between the inner ear and a gas-filled cavity, for instance, generally have better hearing than other fishes. Fish usually hear best within the 30-1000Hz range with some species that can detect up to 5000Hz and other very exceptional species that are sensitive to infrasound or ultrasound (for comparison, people can generally hear between 20 to 20,000Hz, though are most sensitive to waterborne sounds between about 400 to 2,000Hz).
One example of how fish use sound, is to attract and find mates. Male midshipman fish, for example, sing to attract females; serenading females to come from afar to drop their eggs in the male’s nest. Mysteriously, only fertile females respond to these songs. Scientists have suggested that the reason only fertile females respond is increased estrogen (which fertile females have in abundance). These higher levels of estrogen have been seen to enhance the female’s ability to hear the high-frequency mating songs of the males. Indeed, this study was one of the first to suggest a reason why many vertebrates, even humans, have estrogen receptors in their ears.
Another example of how fish use sound comes from a study that used sounds recorded from different habitat types to see how juvenile fish responded. They found juvenile fish used sounds from particular habitats to orient and guide nighttime movements to desired reef habitats. This is an important result as disruptions to these auditory cues could inhibit nighttime migrations of juvenile fish to those more protective reef habitats.
Knowing the vital role sound can play in the survival and reproduction of some fish, one can imagine how altering a fish’s ability to hear could have a significant impact on those fish. There are many factors that affect fish hearing. One obvious example is simply noise. The noise in our aquatic environments has changed over the last century as more and more people are using motorized boats in coastal areas, and with increased coastal development, oil and gas exploration, and shipping. One only has to sit in a boat with a running motor to imagine how a fish might feel in an area with heavy boat traffic. But how does this affect these fish and their likelihood to reproduce and survive. The current answer is that we don’t really know.
Another factor that may affect fish hearing is ocean acidification. The rate at which carbon dioxide (CO2) is taken up by the ocean increases as the concentration increases in our atmosphere, resulting in a more acidic ocean. The resulting decrease in pH can decrease calcification in marine organisms. A potential problem for fish hearing, which relies on a calcium carbonate structure in the inner ear (called an otolith). While a previous Fisheries Blog article highlighted a study where the otolith of juvenile sea basses were larger rather than smaller from acidification (see why here), this study did not evaluate how those changes would affect fish hearing. Another study from the University of Miami found similar results for Cobia, a large tropical fish, and suggested that acidification may improve their hearing. However, another recent study evaluated how CO2-enriched conditions influenced the hearing of juvenile clownfish to daytime reef noise, demonstrating that more CO2-enriched conditions lowered the ability of fish to hear and respond to predatory reef noises. A result that could have detrimental impacts on the survival of these juveniles.
Unexpectedly, another impact on fish hearing may be fish farming. Otoliths are normally composed of aragonite (a stable calcium carbonate mineral) with the rare occurrence of vaterite instead (a less stable form of calcium carbonate) in wild fish. However, fish raised in hatcheries were found to be up to 10 times more likely to have vateritic otoliths than their wild counterparts, and were also suggested to have experienced hearing loss as a result. The reason for this occurrence is still unknown, but is an important consideration for restocking programs based on captive-bred fish.
There are many factors, besides those listed here, that can affect fish hearing and cause potentially detrimental impacts on those fishes that rely on their ears to survive and reproduce. For many of these factors, we are only just beginning to understand how or realize that human activities may be interfering with fish hearing. It is important that we understand how human activities, both in obvious and unexpected ways, impact the ability of fish to reproduce and survive to maintain healthy fisheries as well as protect our aquatic ecosystems.
By: Dana Sackett
References and other reading material:
Bass AH. 2016. Hearing and hormones: paying homage to the comparative approach. In: Hearing and hormones Eds: Bass AH, Sisneros JA, Popper AN, Fay RR. Springer Handbook of Auditory Research. Springer International Publishing Switzerland. 57. DOI: 10.1007/978-3-319-26597-1_1
Bignami S, Enochs I, Manzello D, Sponaugle S, Cowen RK. 2013. Ocean acidification alters the otoliths of a pan-tropical fish species with implications for sensory function. Proceedings of the National Academy of Sciences USA. doi:10.1073/pnas.1301365110
Popper AN, Fay RR. Rethinking sound detection by fishes. Hearing research 273:25-36.
Radford CA, Stanley JA, Simpson SD, Jeffs AG. 2011. Juvenile coral reef fish use sound to locate habitats. Coral Reefs. 30:295-305.
Reimer T, Dempster T, Warren-Myers F, Jensen AJ, Swearer SE. 2016. High prevalence of vaterite in sagittal otoliths causes hearing impairment in farmed fish. Nature.com: Scientific Reports DOI: 10.1038/srep25249
Simpson SD, Munday PL, Wittenrich ML, Manassa R, Dixson DL, Gagliano M, Yan HY. 2011. Ocean acidification erodes crucial auditory behavior in marine fish. Biology Letters. 7:917-920.
Slabbekoorn H, Bouton N, van Opzeeland I, Coers A, ten Cate C, Popper AN. 2010. A noisy spring: the impact of globally rising underwater sound levels on fish. Trends in Ecology and Evolution 25:419-427.