Coral Reef NOAA
May 26, 2016  

What is a Towed Diver Survey?

underwater image of two divers being pulled on towboards to survey a reef
underwater image looking up at a towboard diver from below
Top: A team of towboarders surveys a Pacific reef. Photo Credit: NOAA PIFSC CRED; Bottom: CRED diver conducts a towed-diver survey at Wake Island.. Photo credit: NOAA PIFSC CRED photo by Paula Ayotte

Towed diver surveys, or towboard surveys, is a survey method coral researchers use to assess relatively large areas of reef habitat.

How it Works

This reef assessment method is primarily used by our researchers in the Pacific. The NOAA-certified SCUBA divers can safely utilize this method at depths up to 90 ft below the surface; if visibility is good, they can assess deeper reefs without having to go deeper themselves. How does it work? Envision waterskiing on your stomach—underwater. Typically, two divers are towed behind a small boat that is moving at a velocity of 1-2 mph. The divers can actively maneuver the “towboards” they are holding onto so that they maintain a relatively constant elevation above the surface of the reef and avoid any obstacles in their path. One of the two towboards is equipped with a digital videocamera that is positioned facing forward while the other towboard is equipped with a digital videocamera that faces down to the ocean floor. Both towboards are equipped with a SEABIRD sensor that records depth and water temperature every 5 seconds. A GPS unit on the boat records the path over which the divers, and recording cameras, are being towed. A complete towboard survey is 50 minutes in length, and covers close to two miles of reef! [a]

What Data is Collected

The forward-facing camera is used to record the three-dimensional topography of the reef, as well as the fish associated with the reef. The downard-facing camera records the benthos—the community of organisms which live on, in, or near the seabed—over which the diver and camera are being towed. Computer analysis of the video allows researchers to quantify the percent cover of different components of the reef ecosystem. The genus and sometime species of the corals and algae on the video can sometimes be identified. The SEABIRD sensor data allows researchers to link the visual data with physical parameters in the environment above the reef and the GPS allows the recordings to be linked to geographic position. [a]