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Underwater Videography: An Innovative Way to Assess Oyster Reefs
We use custom-made underwater videography systems to map the seafloor for a variety of purposes. In addition to mapping oyster reefs, recent projects include assessments of seafloor conditions relevant to macroalgal production in estuarine and shallow shelf waters in southwest Florida, coral habitats in the United Arab Emirates, and shallow shelf communities in the Gulf of Maine (see reports and presentations). For oyster reefs, we use both towed and “drop” systems to determine the boundaries and general characteristics of the reefs. The cameras are made by SeaViewer underwater video systems, and include several different models. One system has dual LED lights, and is capable of working in water depths up to about 50 m. The sleds include two models differing in the size and weight, and the cameras can be installed on any of the deployment systems. The onboard components include a digital video recorder, GPS overlay unit that imprints position data onto the imagery, color monitor, and 12-volt battery that powers everything; all enclosed in a water-tight case.
The series of figures below illustrate the overall process typically used for mapping oyster reefs with towed video. For oyster reefs (or any other targets covering large areas), parallel transects positioned as close to one another as practical are navigated across the mapping area, with video imagery and position recorded continuously. The imagery is viewed real-time onboard so that image quality and position data are continuously monitored. The recorded imagery is later classified in the laboratory (typically at 2-second intervals) using a classification scheme appropriate for each project; “oyster bottom” and “non-oyster bottom” in the figures shown. The classified shiptracks are then plotted on a basemap and polygons representing the spatial extent of each bottom type are manually drawn as polygons. ArcGIS software is used for the entire process, and the final polygons are constructed as shape files.
Using Video to Ground-truth Sonar Data
In some cases, underwater video can provide a low-cost alternative to single beam sonars (and other methods) for seafloor mapping. In fact, towed video has become the method of choice for periodic mapping of boundaries and general condition of the oyster reefs in New Hampshire for management purposes (see Grizzle et al. 2005 and 2008 for studies comparing various sonars to video and other methods for mapping oyster reefs). However, there are limitations to its use. The most obvious is that water clarity must be sufficient to allow unambiguous identification of the features being mapped. We have found that even in estuaries where turbidity levels are typically quite high, there are times when underwater visibility is sufficient for obtaining useful video imagery. Another limitation is tow speed. Useful imagery is typically limited to tow speeds of about 2 knots, a bit faster when the water is clear but less under turbid conditions. In any case, this limits the linear distances that can be surveyed on an hourly basis, and thus affects the cost of each project.
Another potential issue with video mapping is positional accuracy. We typically use GPS units that have advertised positional accuracies of <3 m. We could use instruments with greater accuracy but the GPS systems are not the primary control on positional accuracy; it is the position of the camera relative to the GPS receiver. In general, the shallower the water the easier it is to maintain the position of the camera directly under the GPS receiver which is onboard the vessel. As water depth increases the ability to control the position of the camera relative to the GPS receiver decreases. Thus, we have found that positional accuracies of 5 to 10 m are more realistic for seafloor maps mainly based on towed underwater video.
Squamscott River Oyster Reef
At least some portions of an oyster reef at the mouth of the Squamscott River have persisted at high oyster densities since the late 1990s. The entire area is closed to harvesting of oysters, and salinity can vary from well below 5 psu during high-runoff events to >30 psu when rainfall is low. Available recruitment and disease data do not suggest obvious differences compared to other reefs in the Great Bay system, yet this reef persists as an example of a population of dense adult oysters typically consisting of several year classes and apparently challenged by both major diseases, MSX and Dermo. The photos below are stills extracted from some our recent video imagery of the reef showing the level of vertical structure in some areas and other features of the reef.
More Examples of Still Images From Underwater Videography