New Remote Sensing Technologies for Mapping Subtidal Oyster Reefs
Problem and Justification
Populations of the eastern oyster, Crassostrea virginica, have been in long-term decline in many areas, including the proposed study area. Hence, oysters are a major concern of coastal managers. In New Hampshire, state and federal agencies have ongoing programs concerned with the overall health of oyster populations. A major hindrance to effective oyster management generally has been the lack of a methodology for accurately and economically obtaining data on distribution and abundance. The proposed development project will test the effectiveness of newly developing acoustic, visualization and mapping technologies for characterizing subtidal oyster reef habitat.
We are proposing a preliminary study on only three reefs. Although it has been demonstrated that various acoustic techniques (e.g., sidescan sonar, multi- and single-beam sounders) can identify oyster reef bottom, the extent to which reef characteristics (e.g., live vs. dead coverage) and even reef spatial coverage in some cases, can be inferred has not been adequately tested. The present proposed research will provide a test of the capabilities of acoustic techniques in this respect. The overall project for which we at Jackson Estuarine Laboratory (JEL) are seeking development funds will be a collaborative effort involving NH Fish & Game (NHF&G) and UNH's Center for Coastal and Ocean Mapping (C-COM), each of which will be funded with non-Sea Grant money. Hence, the requested Sea Grant funds will in effect be matched by other agencies also interested in the research. The present proposal describes the activities of all three groups.
Project Objectives and Expected Outcomes
1) Conduct an acoustic survey of three major oyster reefs in Great Bay (C-COM).
2) Characterize and determine the spatial extent of oyster coverage (and associated bottom types in order to delimit reef boundaries) based on acoustics data (C-COM).
3) Obtain quantitative data via video and quadrat sampling on oyster sizes and densities, and substrate type (NHF&G, JEL).
4) Compare reef data obtained acoustically (objectives 1 and 2) with data from traditional techniques (objective 3) (JEL).
5) Produce graphical displays of all data, including a map of the spatial extent and characteristics of each reef (JEL).
6) Assess the results of the project with respect to future research, including the potential for preparation of a full research proposal (C-COM, NHF&G, JEL).
Objective 1 (acoustics survey) will be accomplished using two different single beam (vertical incidence) echosounding systems owned by C-COM. The first sounder provides digitized waveform envelope data of the signals returned from scattering surfaces such as the bottom, allowing their characterization. The second is a specially adapted echosounding system owned by C-COM. This system is able to digitize the full waveform data. The survey platform, positioning system and technical support for collecting these data will also be provided by C- COM. These methods are closely linked to the C-COM's "New applications for seafloor data and seafloor characterization" research theme, particularly the effort to characterize the seafloor from single beam echosounders. As part of this, algorithms are being developed for the characterization of coastal habitats such as oyster reefs. Three reefs (Nannie, Adams Point, Oyster River) will be surveyed.
Objective 2 (characterize reefs using acoustics data) will involve using existing software to analyze the acoustic data from objective 1. Also, new software will be developed for the specific purpose of linking the acoustic parameters from objective 1 to habitat characteristics.
Objective 3 (video and quadrat sampling of reefs) will be accomplished with an underwater video system owned by JEL. Quadrat sampling by NHF&G divers will be used to refine the use of the video system. In other words, the video system will function as a kind of remote sensing that will be ground-truthed using quadrat sampling. The rationale is that video data can be collected much easier and much cheaper than diving and quadrat sampling. Hence, it will provide information from more areas on the reefs than would be possible with quadrat sampling alone. Multiple sites (number dependent on reef size) will be video-imaged on each reef. Quadrat counts will be made at half of the video sites. The bottom videography will be converted to digital still images that will be enhanced to maximize image quality. The stills will then be visually inspected and all oysters counted and measured. Counts of live vs. dead will be made when possible. Sediment samples will be taken from areas immediately adjacent to the study reefs and analyzed by standard methods for grain size and organic content. Sediment data mainly will be used to characterize the boundaries and composition of the study reefs, but will also provide information on the effectiveness of the acoustics data in identifying other bottom types.
Objective 4 (compare acoustics, video and quadrat data) represents the major unknown of the project: How much information on reef characteristics can be inferred from acoustics and video data? As discussed above, quadrat sampling primarily will provide groundtruthing for the video data. The video data will in similar fashion provide groundtruthing for the acoustics data. This objective will consist of an overall synthesis of the three techniques.
Objective 5 (graphical displays) will be accomplished with existing computers and software (including Arc View GIS) at JEL. GIS-based techniques are ideally suited to create and display spatial information. Map displays of oyster reef characteristics inferred from acoustics work, video and quadrat sampling as well as other bottom type information will be produced.
Objective 6 (future research) will consist of an overall assessment of the results (particularly objective 4) from the perspective of evaluating the potential of acoustic techniques for characterizing oyster reefs. Assuming the results are promising, appropriate directions for future research will be described.