By Jessica Perelman
Leonardo da Vinci first observed that ships could be heard at long distances underwater back in the late 1400s, and physicists first measured the speed of sound in water in 1826. But little did these scientists know just how important this energy is to the ocean’s many inhabitants, even if we ourselves cannot hear very clearly underwater. Flash-forward to the twenty-first century and the field of ocean acoustics– that is, the study of sounds and their behaviors in the sea- has emerged with a wide variety of applications.
Scientists are now able to use sound as an effective tool to study many ocean-related factors. Climate and weather observations, seismic exploration, sonar, and navigation are just a few advancements that have stemmed from determining the mechanics of underwater sound. Not only does this knowledge benefit human interests, but oceanographers are beginning to understand the importance of acoustics as an indicator of ocean health. Monitoring the sound patterns associated with certain ecosystems over time (think: coral reefs) helps determine the condition of marine environments facing climate change and disruptive human activity.
Sounds in the ocean have many sources. Depending on the location and time of year, marine life can be a major driver of the ambient noise that forms ocean soundscapes, from marine mammals and fish to invertebrates. For many of these animals, sound functions as an essential component of communication and foraging. In fact, auditory cues are often the primary form of communication in an underwater world where vision can be highly limited. Social marine mammals like cetaceans (whales and dolphins) use combinations of echolocation clicks, whistles, and songs not only to communicate, but to gather information about their surroundings and locate prey. Oyster toadfish vocalize by vibrating muscles around their swim bladder to generate a ritual mating call, which is particularly important in the turbid waters where they live. A family of small, cryptic crustaceans called snapping shrimp produce loud ‘snaps’ when displaying territorial aggression or perhaps when capturing prey, although the extent of this snapping behavior is not fully understood. However, the continuous crackling sound created by snapping shrimp colonies is one of the most prevalent sources of biological noise in coastal temperate and tropical habitats.Many natural and man-made sources contribute to ocean soundscapes. Credit: Mike Thompson, NOAA/SBNMS.
So, the ocean is certainly far from silent.
But all of these natural, ambient sounds that characterize marine habitats are no longer the only sources of acoustic variation in the ocean. Increasing noise from human activities like shipping, sonar, and seismic airgun testing is becoming a larger component of ocean soundscapes. This can be a major stressor for marine life. As scientists examine the impacts of anthropogenic noise on different organisms, studies have revealed behavioral changes, hearing loss, and masked communication in many animals. In some cases, mass strandings of marine mammals such as beaked whales have been attributed to the sounds produced by naval activity and military sonar exercises, although this could also be a result of constant shipping maneuvers.
These invasive disturbances have gained a great deal of attention in the world of marine bioacoustics, but this question remains: does noise affect all animals equally? Not exactly. Marine organisms, like terrestrial, have a wide range of hearing thresholds, and thus perceive noise in different ways. Animals that hear best at lower frequencies, such as baleen whales that can produce sounds less than 10 Hz, are more likely to hear background noise dominated by distant ships and other far-traveling anthropogenic sounds. Those with higher frequency hearing ranges, like toothed whales that echolocate with clicks as high as 100,000 Hz, might not even pick up many of these human-generated sounds, and instead hear a background of breaking waves, bubbles and water spray.
Growing knowledge about the hearing capabilities of marine mammals and mature fishes has sparked interest in understanding the acoustic senses of smaller organisms, especially those that are critical to marine food webs. Biologists like Dr. Aran Mooney of the Woods Hole Oceanographic Institution are interested in how marine animals sense the world around them, and how different species use this sense to avoid predators, search for food, and communicate. In a 2014 study that investigated behavioral responses to sound, Dr. Mooney and graduate student Julia Samson tested cuttlefish, a shell-less mollusk similar to squid, and identified the acoustic frequency of these creatures. They determined that cuttlefish seem to use the same range as fish, (80-1000 Hz), meaning that in addition to reef sounds and other biological activity, they are easily susceptible to anthropogenic noise. Larvae and other tiny organisms can also detect sounds in the ocean, and some of Mooney’s current work aims to determine if larval settlement patterns and local biodiversity in Caribbean coral reefs might be influenced by reef soundscapes. The study has shed light on just how drastic the acoustic variation between every reef (and every marine ecosystem) can be, even those in very close proximity. Such projects help environmental policy makers understand the effects of noise on marine organisms, and ultimately shape regulations on potentially detrimental human activities.
New technology allows scientists to explore, study, and monitor noise in changing marine environments, and the growing field of marine bioacoustics is providing insight into the ways animals perceive their surroundings. With this awareness, agencies like the National Oceanographic and Atmospheric Administration (NOAA) are able to develop initiatives that address noise impacts to marine species and habitats, such as the recently released Ocean Noise Strategy Roadmap. As simple as it may seem above the surface, sound in the ocean provides a unique system for studying the marine environment, and an invaluable resource to better understand the complexities of marine life.
Jessica Perelman is a recent graduate of the University of Southern California with a bachelor’s degree in biological sciences. She currently works at the Woods Hole Oceanographic Institution and plans to attend graduate school to pursue a career in marine conservation science.