Powell, EN; Ashton-Alcox, KA; Banta, SE; Bonner, AJ
The impact of commercial dredging on an oyster reef was evaluated at four sites chosen on New Beds, one of the most important commercial oyster beds in Delaware Bay. Dredging occur-red on two of these sites in late October 1999, early and late November 1999, April 2000, and July 2000. Dredging was conducted according to standard industry procedures. Each day, dredging was continuous during approximately an 8-h period. Both one-dredge and two-dredge boats were used. Market-size oysters were culled and sacked in the standard manner. Total dredge coverage for the study was about 240,000 m(2) on each experimental site. The most heavily dredged areas were completely covered by the dredge 4 to 6 times during the study. Two 8-h dredging events within a 10-day period produced barely detectable changes in the oyster population. Minor chipping and abrasion of the shell increased in frequency, but no other discernible impacts were found. Over the 10-mo study that included five dredging events, many of the taphonomic indicators of dredge damage showed time-dependent trends that differed between control and experimental sites. However, these effects were limited mostly to minor chipping and indications of abrasive wear, rather than the more serious aspects of shell damage defined as major chipping, breakage, cracking, and shell perforation. A variety of population health indicators were assayed during the study, including the ratio of live oysters to boxes, condition index, Perkinsus marinus infection intensity, and oyster size-frequency distribution, These indicators should have monitored growth, disease pressure, and mortality. Essentially no significant effects could be discerned for any of these measures. Over a very long time, dredging may significantly influence oyster bed physiography and community structure. However, once the bed has become a fished bed, this study suggests that moderate dredging that results in a yearly swept area of no more than four times the area of the bed is unlikely to result in significant further impact on the oyster populations living there.