Scientists have this week been taking a listen in the waters around St. Croix, seeking out the “hot spots” where reef fish meet up to make babies. And they have been using technology to help them in that search. What they are looking for are spawning aggregations: gatherings of large numbers of fish – from tens to thousands – at specific times and places for the purposes of reproduction.
Such gatherings are vital to the life cycle of many reef fish but also leave the fish vulnerable to overfishing while they are aggregating, according to the National Oceanic and Atmospheric Administration.
Knowing where and when the reproduction aggregations happen can be a key piece of information in managing a fishery and keeping it sustainable, NOAA ecologist Chris Taylor said in a phone interview.
“We don’t know where these spawning events occur in most cases,” Taylor said. “So we’re using a glider to remotely record for spawning sounds.”
Scientific mission
Taylor and other scientists are participating in NOAA’s National Centers for Coastal Ocean Science 11th annual scientific mission in the U.S. Caribbean, which started Thursday and goes through April 3.
In addition to deploying the underwater glider to gather oceanographic data and listen for and record spawning noises, the mission will also map and explore sea floor habitats and fish distribution, according to NOAA.
For the mission in the Caribbean this year, the 185-foot NOAA Ship Nancy Foster is plying the waters off St. Croix’s south shore.
Listening for fish
The underwater glider that was deployed on the mission this week is a G2 Slocum Electronic Glider. The autonomous glider has no propulsion, and instead uses its wings and changes in its weight and buoyancy to move, “undulating up and down in the water column,” Taylor said. It has special sensors that can detect and record sounds.
As it turns out, some fish species make sounds when they’re spawning or preparing to spawn. Those include some grouper species, such as yellowfin and Nassau grouper, Taylor said.
The sounds may be interpreted as thumping sounds or trilling, according to Taylor.
“It depends on the species, and probably depends on what they’re trying to say to each other,” he said. “They may be trying to attract members of their own kind to an area or signal that it’s time to spawn. Or in some cases, they might be communicating aggression.”
The glider will cover more areas than traditional means. The plan was to have it patrol the southern shelf edge of St. Croix, collecting physical and acoustic data.
“The glider offers us a technical capability we didn’t have before,” said Tim Battista, NOAA’s chief scientist for this mission.
Part of the challenge in trying to learn more about spawning aggregations is being at the right place at the right time, he said.
“That can be difficult to do with divers or ships,” he said. “We think gliders offer a unique capability for us to detect them better.”
Remote control
The glider spends about three hours underwater, then comes to the surface, where it relays information by satellite to a pilot at the Naval Oceanographic Office in Mississippi, Taylor said. That pilot can then make adjustments as needed to correct the glider’s course.
The recordings from the glider may provide a first step in locating the spots where spawning aggregations occur, Taylor said.
Being armed with information about when and where spawning aggregations happens could allow resource managers to impose measures “that might reduce fishing pressure on these species when aggregating,” he said.
Such measures could include forming protected areas or having seasonal closures.
On Friday, the glider experienced technical problems.
Battista and Taylor said that there had been a minor failure in its internal system, causing the glider to abort its mission. On Friday afternoon, they were waiting for word on whether it could be repaired to complete the mission.
Other tasks
Other aspects of the Caribbean mission on the Nancy Foster this year include mapping the sea floor using acoustic technology – a multi-beam sonar system – that is mounted on the ship.
The resulting maps will provide a much more complete picture of the sea floor, including the habitats in different areas, Battista said.
For local agencies and groups that use the data collected, “these maps help them better understand the types, extent and conditions of those habitats,” he said.
The data collected on the NOAA missions – which themselves are guided by local needs – goes to local resource managers and decision-makers, he said.
“Our role in NOAA really is to support local management actions and help inform resource managers to allow them to make better decisions,” he said.
Those agencies that NOAA is partnering with in the project include the V.I. Department of Planning and Natural Resources, Caribbean Fishery Management Council, University of the Virgin Islands, V.I. Coastal Zone Management and the National Park Service.
“Where we go is largely driven by where they have identified the highest priority needs,” Battista said. “We bring the ship to focus on those locations and bring technology that isn’t available down here.”
During the mission, scientists also plan to deploy remotely operated underwater vehicles to take video and digital still images of the sea floor and fish.
“That helps us create a better map and creates a nice inventory of what’s there now,” Battista said.
The ship also has “some scientific-grade fish-finders on board that allow us to use sounds to locate fish,” Taylor said.
That technology transmits sound through water, which then bounces off fish and the air pockets in the side of their abdomens, and comes back to the receivers, which determine the size of the fish and through GPS, where it is located.
The information is being used to develop a map of the distribution of fish, Taylor said.
Data and information
The whole idea is to provide local agencies with information that they can use in making decisions.
“We like to think managers make informed decisions based on the information available,” Battista said, but noted that they need good information to guide those decisions.
Funding for the work is provided by the NOAA Coral Reef Conservation Program.
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