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Puerto Rico & Vieques 2008 Cruise, day six: March 31, 2008
Hydroacoustic Fish Surveys
Survey Technician,
Melody Ovard,
monitors the
progress of
the multibeam
sonar unit
(Credit:
Amy V. Uhrin)
Click to enlarge
Previous research has shown that key members of reef fish communities, most notably grunts and snappers, migrate from reefs 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, researchers often use hydroacoustic (sonar) technology to assess nocturnal fish communities. We began using this approach in night surveys last year at Isla Vieques and will continue this year.
Close up of screen capture showing fish movement above the reef Click to enlarge |
Each evening after dive operations are complete, the fisheries sonar unit is activated. This unit is a scientific fish finder used to estimate reef fish biomass in specific areas around Isla Vieques. The unit 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. Reflected "echoes" are monitored on a laptop computer and recorded onto its hard drive for later analysis and biomass estimations. The images above on the right are actual echo recordings from Isla Vieques. The use of fisheries sonar permits observations during all hours and under most ocean conditions.
Survey
Technician
aboard the
NANCY FOSTER,
about to deploy the
CTD (Credit: Amy V.
Uhrin)
Click to enlarge
A CTD (Conductivity, Temperature, and Depth) is a standard oceanography tool used for measuring variations in conductivity and temperature along depth profiles in the water column. The CTD also calculates the speed of sound underwater. The speed of sound is then used to calibrate the ship’s multibeam sonar.
We continue to have new fish sightings every day. Today’s top picks include the butter hamlet (Hypoplectrus unicolor) and the sand tilefish (Malacanthus plumieri). Typically, hamlets with like color patterns and markings will mate together. However, when a similar appearing mate is not available, different appearing individuals will mate, producing hybrids. Because of this hybridization, there has been much scientific debate regarding the classification of hamlets.
Sand tilefish create burrows out of sand and rubble and can be difficult to photograph as they are quite wary and will retreat head first into their lair when approached.
Butter hamlet (Credit: Brian Degan) Click to enlarge |
Sand tilefish (Credit: Christine Addison) Click to enlarge |