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Hydroacoustic Fish Surveys
Previous research has shown that key members of reef-fish communities, most notably grunts and snappers, migrate from reef to surrounding soft-bottom habitats at night to feed. At dusk these fishes concentrate in schools at the edge of reefs, providing an opportunity to estimate the biomass of these communities. Observing these behaviors at night using divers is very difficult due to low light conditions. To overcome this issue, researchers often utilize hydroacoustic (sonar) technology to assess nocturnal fish communities. A new component of our Isla Vieques project involves using this technology for nocturnal surveys of fish distribution.
Dr. Chris Taylor, a Research Associate with North Carolina State University, is heading up these surveys with the help of Research Technician Warren Mitchell. During nighttime operations, fisheries sonar, which is a scientific fish finder, is used to estimate reef fish biomass in specific areas around Isla Vieques. The unit specifically consists of a Simrad EK60 split-beam echo sounder coupled with a Simrad EM1002 multibeam sonar. The transducer for the sonar, located on the underside of the NANCY FOSTER, sends out 15 pings per second. The pings reflect off the bottom as well as fish in the water column. These reflected “echoes” are monitored on a laptop computer and recorded onto the hard drive for later analysis and biomass estimations.
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The fish sonar surveys will be paired with habitat surveys conducted by the NANCY FOSTER using the ship’s multi-beam sonar tracking over the exact same locations. These fish biomass estimates, linked to specific locations and habitat types, will be useful for prioritizing conservation efforts for Isla Vieques and site selection for Marine Protected Areas.
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The sonar transducer is mounted on a long pole that is lowered into the “moon pool” of the ship. The moon pool is a room, with an airtight door, that has an open floor that extends right through the bottom of the hull of the ship. A moon pool allows researchers to lower their instruments down into the water column. The actual opening that the transducer is mounted through is 10 inches in diameter. Here we see Chris and Warren, with the help of Nancy Foster Chief Bosun Greg Walker, lowering the transducer pole into the moon pool. Occasionally, it is necessary to check on the transducer to make sure that the unit is not fouled with line or other debris. In order to do this, the ship must turn off all of its engines and power down so that divers can enter the water and swim under the ship. Here we see Nancy Foster Ensign Marc Weekley examining the transducer.
A CTD (Conductivity Temperature and Depth) is a standard oceanographic tool used for profiling changes in conductivity and temperature, associated with depth, in the water column. The CTD is used to calculate the speed of sound underwater. Information gathered by the CTD is necessary to calibrate the ship’s multibeam sonar.