The scientists who study cod populations have tried a lot of different ways to figure out where cod aggregate and to observe their behavior, like trawl surveys, sonar, and even underwater video cameras. But recently, a team of federal and state fisheries scientists have developed a new way to observe groups of cod. Rather than watching them, they’re listening to them—and they’re hearing some pretty interesting stories that could help us protect this depleted species.

This story starts with a state fisheries employee out fishing on his day off. Three miles off the coast of Gloucester, he stumbled on a large group of spawning cod on an otherwise nondescript gravel sandbar. Recognizing the opportunity to study this unusual group of fish, researchers later returned to install passive acoustic monitoring equipment.

This underwater laboratory works in two ways—first, it records the sounds cod in the area make (cod vocalize by inflating and contracting their swim bladders, making faint grunting noises that can be difficult to hear on a recording). Second, it picks up information on the location of individual cod that the researchers catch and tag with acoustic signals. This monitoring has allowed the scientists to track where male and female cod are over time, at what depth they’re swimming, and when they’re making noise.

The researchers have already discovered some pretty interesting things about spawning cod. First, they noticed that female cod and male cod make sounds during the day, but only male cod make noise at night. This pattern reflects what the researchers saw on video—at night, male cod move from the school into smaller groups where they compete for the attention of females. That means that cod actually spawn at night, not during the day as was previously thought.

The scientists also discovered that cod tend to spawn near the surface—potentially to avoid fishing gear dragged along the bottom.

While this information about spawning behavior is interesting on its own, it could also have even bigger implications for the way we protect our cod populations. Now that scientists know what a group of spawning cod sounds like, researchers can scan the ocean—potentially with self-propelled robots equipped with microphones—to locate previously undiscovered spawning sites. As scientist Sofie Van Parijs told the Cape Cod Times, “Killing them where they spawn is a great way to drive a species to extinction.” Finding groups of spawning cod could help fisheries managers create temporary or permanent areas off-limits to fishing to protect these fish when they’re at their most vulnerable.

Scientists have already used this type of monitoring to improve the management of another iconic New England species—the endangered North Atlantic right whale. An array of underwater microphones currently listens for right whales’ distinctive, upward-swooping calls. When the whales are detected in a shipping lane, nearby vessels are alerted and diverted to help avoid a collision.

Scientists believe this technology could be similarly helpful for cod, but there are some challenges standing in the way of putting it to use. First, there is limited funding for more research. Second, there is currently no set way to include this information in fisheries management process, so scientists will have to work closely with managers to see if it can be considered when setting up new areas closed to fishing. Lastly, the oceans are noisy. Between all the sounds made by other marine animals and the deep rumbles of commercial boats, it can be difficult for microphones and scientists to hear the noises cod make.

If these problems can be resolved, the quiet grunts of cod could mean a big step forward for the conservation of this depleted species.