April 18, 2013 — 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 themm, they are 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.
Read the full opinion piece at the New England Ocean Odyssey