PREP Publications

Abstract

In New Hampshire, the locations, sizes, and shapes of the major oyster, Crassostrea virginica, reefs have been determined using a variety of techniques. The most recent survey occurred in 2001 when four (Nannie Island, Woodman Point, Adams Point, and Oyster River) of the six largest reefs were mapped using a combination of acoustic sounders, videography, and quadrat sampling. The present project required mapping the boundaries in order to determine the size of the remaining two major reefs in the Great Bay Estuary: Squamscott River and Piscataqua River. Underwater videography was used in the present study to determine the boundaries of these two reefs.

Continuous video imagery was acquired along three or four parallel transects spanning the longest axis of each reef and seven to ten transects perpendicular to them. At approximately 90 points for the Squamscott reef and 115 points for the Piscataqua reef, stationary (for 3 to 5 seconds) video imagery was taken within the overall matrix of transects. Concurrently and synchronized with respect to time with the imaging, DGPS output was logged at 0.5 second intervals to provide geo-referencing of all the imagery. Stills taken from each stationary imagery site were assigned a classification of "non-reef" (<10% bottom coverage by oyster shells), "low density reef" (10% to 50% coverage by oyster shells), or "high density reef" (>50% coverage by oyster shells). The classification types were then plotted on the base map and polygons were constructed manually, drawing each boundary line approximately midway between bottom type classes. Areas of polygons for "high density reef" and "low density reef" were determined by ArcView for each of the reefs. One representative still image from each stationary video site was assembled in a systematic grid overlaid on the overall imaging area to provide a photomontage of bottom images for each reef.

The video imagery was of sufficient quality to allow classification of "shell bottom" into two density classes: "low" (10% to 50% bottom coverage by oyster shell) and "high" (>50% bottom coverage by oyster shell). If it is assumed that "low" and "high" density oyster shell coverage reflect oyster reef bottom, the Piscataqua reef had an areal extent of 19.9 acres (Fig. 6) and the Squamscott reef covered 3.9 acres (Fig. 7). If only "high density" bottom represents oyster reef bottom, the Piscataqua reef covered 12.5 acres and the Squamscott 1.9 acres. If it is assumed that at least the high density areas would have been considered oyster reef bottom in previous studies, then areal coverages from the present study compare well with recent previous surveys suggesting that total bottom areal coverage may not have changed appreciably for either reef since the 1990s.

The use of underwater videography for routine monitoring of oyster reefs is in the early development stages. At this time, we think it can be recommended that video be considered as a tool for routine inspection of reefs, and to better design the traditional sampling programs based on quadrat sampling. Our laboratory recently was awarded a 2- year NH Sea Grant project to develop a general protocol for routine monitoring of oyster reefs. This research will consider underwater video along with several acoustics techniques and quadrat sampling.

Publication Date

8-24-2004

Publisher

New Hampshire Estuaries Project

Document Type

Report

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