Towed Diver Surveys
Within the depth limits of safe, no-decompression SCUBA diving (generally to 90’ depth), NOAA-certified CRED divers conduct towed diver surveys as a method of assessing relatively large areas of reef habitat. This method involves towing two divers behind a small surface craft that is moving at a velocity of 1-2 mph. Although the driver of the surface craft attempts to follow a depth contour, the divers also actively maneuver the “towboards” they are holding onto so as to maintain a relatively constant elevation above the surface of the reef.
- One of the two towboards is equipped with a digital videocamera that is pointing in the forward-looking direction so as to document both the “rugosity “, or 3-dimensional topography, of the reef, as well as the fish associated with the reef. The other towboard is equipped with a digital videocamera that is pointing in the downward-looking direction, so as to record the benthos over which the diver and camera are moving.
- Divers maneuvering the towboards on which the cameras are mounted record selected data pertaining to fish and to habitat, using datasheets mounted on the towboards. These in-situ observations are used to assist with preliminary summaries of the fish communities and the reef habitat, and as a general reference during more detailed, computer-assisted analyses.
- Both towboards are equipped with a SEABIRD sensor that records depth and water temperature every 5 seconds, so that biotic observations can be linked to salient physical parameters.
- A GPS unit on the surface tow vessel records the trackline over which the divers, and recording cameras, are being towed, so that the results that are later generated by analysis of the digital videos can be linked to geographic position.
- A complete towboard survey is 50 minutes in length, and covers close to 2 miles of reef.
Analysis of Towboard Benthic Videotapes
Using computer-assisted technology, the images captured by the downward-pointed camera of the benthic habitat are quantified as percent cover of the principle components that can be recognized from the videotapes. These components include: live coral, unencrusted (recently dead) coral, macroalgae, turf algae, coralline algae, other invertebrates, sand, rock, pavement, and rubble.
The suite of corals that can be identified from the videotapes depends upon the assemblage of corals present in the area being surveyed (e.g., the Northwestern Hawaiian Islands are less speciose than the U.S. Line & Phoenix Islands), and in general is limited to the genus level of identification. Similarly, algae can typically be identified from the videotapes only according to their functional class (macro, turf, coralline).
Thus, while towed diver surveys are capable of assessing large areas of reef, it requires more stationary divers working at smaller spatial scales during REA surveys to achieve a higher degree of taxonomic resolution.