A single dredge will vary in efficiency depending on scope, speed of towing, vessel size and even vessel operator, in addition to state of and direction of the tide. Bias in favor of live animals over shell has been noted by both Powell at al.
If the Young of the Year (YOY, spat) are near the surface then the sampling is not representative. A dredge is size selective because it samples the surface more so than the underlying reef. 2002 and current MD DNR swept area surveys with rejection/repeat of tows that are overflowing on retrieval). Thus the only conditions under which a dredge can be used quantitatively are for very short tows (e.g., Powell et al. The dredge fills over the course of the tow to become a plow, after which it does not sample. As such the dredge scoops but does not plane, so it does not sample representatively over the course of a tow, especially when the towpath moves over varying bottom type as would be found on an oyster reef. The cumulative design in tow mode functions by alternately digging and releasing, hence the action of a towline over the stern of the vessel during dredging. An additional hinge point is the attachment of the line to the dredge. The dredge is towed by a flexible line of variable length with variable scope and “hinge points” at either the winch or the passage of the towline over the stern of the vessel. Potential problems include the following: The oyster dredge is such a piece of gear – the challenge in its operation is to sample consistently over non-uniform bottom. These studies highlight the limitations of gear that was initially employed as a qualitative rather than a quantitative sampling tool. Over the past decade, the science supporting swept area approaches has been intensively examined and refined by NEFSC NMFS- industry-academic collaborative efforts in support of the surf clam, ocean quahog, and sea scallop fisheries. Multiply density by the spatial footprint of the habitat, correct for dredge efficiency (this is less than 100%) and a total stock is estimated.
If these requirements can be met then a simple division of the number of oysters captured by the area covered gives density (oysters per unit area). The base requirement for a dredge to be used in quantitative surveys is that it can be assumed to cover a defined swept area with known capture efficiency on a consistent basis. 2.2 Link to Patent Tongs as Sampling Tools
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