NOAA researchers meet a glider, an autonomous torpedo that has been listening for cod spawning noises. Scientists want to find where cod are spawning so they can be protected.
Dec 29, 2014 at 11:36 AM PLYMOUTH — Half-submerged in the ocean east of Scituate, the canary-yellow glider with swept-back wings looked like little more than someone’s errant model plane. But appearances can be deceiving, especially in this electronic age where more and more sophisticated technology is being loaded into ever smaller and sleeker packages.
From his perch over the deck, Bob Wallace, captain of the Stellwagen Bank National Marine Sanctuary research vessel Auk, swung the stern around so crew member Dave Slocum and Eric Matzen of the National Marine Fisheries Service could grapple the sophisticated torpedo that researchers hoped would provide valuable clues to the future of cod, the most iconic fish in New England.
In the Gulf of Maine, as many as 80 percent of legal-size cod die each year from fishing and natural causes and the size of the spawning stock is considered to be just 3 percent of what constitutes a healthy population. NMFS is considering reducing the annual catch to a level so low that it could shut down all fishing well before the limits on other, more plentiful species are caught.
Everyone agrees that finding cod spawning grounds is critical to rebuilding the species. Although significant anecdotal and research data exist on where and when cod spawning occurs, the areas and timing are so broad that shutting them down could result in significant closures and potentially cause fishermen unnecessary pain and economic losses. “It’s all been very carefully tracked by the (state) Division of Marine Fisheries and others,” said Leila Hatch, a marine ecologist at the Stellwagen Bank National Marine Sanctuary, and acoustic study collaborator with Sofie Van Parijs, a passive acoustics scientist at the service's Northeast Fisheries Science Center in Woods Hole.
But scientists believe that cod spawning areas may be so small that they are going largely undetected and unprotected. Because cod group tightly together when they are spawning, fishermen often target these aggregations, if regulations haven’t closed the area. Five years ago, fishermen discovered a true honey hole, three miles off Gloucester where as many as 30,000 large cod gathered each year to spawn. The state ultimately closed it down after fishermen and others voiced concern about the high catch rates. Regulators and scientists worry about other important spawning groups that may have gone unnoticed or have shifted their location due to fishing pressure or environmental factors.
When the state closed the area, the DMF set up what was essentially an underwater lab with a gridwork of acoustic buoys to listen to the signals from tagged cod that identified individual fish, and used video and ultrasound to observe movements within the school night and day. Cod mate mainly in inshore state waters, in densely packed cone-shaped schools researchers call haystacks. Males emit grunting sounds to attract females and warn off competitors.
In 2011, Van Parijs and Hatch partnered with the DMF, UMass Dartmouth’s School for Marine Science and Technology and local fishermen to deploy and retrieve acoustic receivers and analyze data. They added a second set of buoys capable of recording vocalizations from marine mammals and fish like cod.
The group was able to save money by piggybacking their research onto existing whale vocalization studies being conducted by the Stellwagen sanctuary and NMFS. But unlike right whales, which can throw their voice eight miles across the breadth of Massachusetts Bay, cod grunts travel for only about a football field. “It was a needle in a haystack operation, and these are tight haystacks,” Hatch said.
This year, the study added The Nature Conservancy and the Woods Hole Oceanographic Institution, and broadened its reach by including the two Slocum gliders from WHOI. The glider uses batteries to power an interchangeable array of instruments and sensors and to empty and refill ballast tanks. Changes in buoyancy cause the torpedo-shaped vessel to descend, gliding forward thanks to its swept wings, then rise as ballast tanks empty. in a saw-tooth pattern that can be repeated for hundreds of miles for up to six weeks. The crew of the Auk deployed the two gliders on Dec. 1, delaying the release of the second torpedo 12 hours so that both night and daytime spawning was monitored. The two gliders followed a pre-programmed route that zig-zagged through the state’s field of acoustic buoys, but also explored far beyond those limits to see if spawning was also occurring elsewhere. “The biggest struggle is that there are so few cod left to find,” Van Parijs said. “We are hoping the gliders will find the areas we don’t know about.”
Along its route, the glider surfaced every two hours, the antenna in its tail held up above the waves by the ballast bag in the stern, to transmit data to a satellite, which relayed it to researchers. A black tube strapped to the top of the gliders listened for signals from tagged fish. while a bulky amber bulb projecting off its nose recorded noise and vocalizations. While it is capable of distinguishing the calls of various whale species, scientists will have to download the recordings when the gliders are returned to WHOI and they run them through an instrument built to detect cod grunts.
The location and environmental information like temperature and depth accompany each vocalization, and hopefully will give clues as to why spawning occurs in specific spots and not in others. “None of these technologies is a silver bullet,” said Hatch. “But we are throwing a lot of technology at the problem because it is a critical problem.”
The cod project is one of a suite of acoustic studies undertaken by the Fisheries Science Center and its partners that is funded for the next three years. Van Parijs and Hatch hope the acoustic studies will result in meaningful regulations that get the highest benefit with the least pain from closing spawning areas.