A comparison between a suction dredge and a traditional oyster dredge in the transplantation of oysters in Delaware Bay

Journal of Shellfisheries Research, Dec, 2004 by Eric N. Powell, Kathryn A. Ashton-Alcox

ABSTRACT One mechanism to enhance oyster production is the timely transplant of oysters from nursery beds to beds used for commercial harvesting. Transplanting in Delaware Bay is normally done with a traditional oyster dredge. Such dredges can concentrate market-size oysters, a desirable characteristic for some transplant goals. Unfortunately, catch rates are slow. The suction dredge is much faster, but suction dredges likely do not concentrate large oysters and, by removing most surficial material, may reduce bottom shell coverage and decrease bottom complexity. We investigated the relative benefits of using a traditional oyster dredge and a suction dredge in a transplant program. In this study, traditional oyster dredges used for transplant operations had dredge efficiencies of approximately 5%, about 100 bushels of material being loaded per hectare swept. The tendency for the dredge to catch larger particles preferentially was negated by the tendency to operate the dredge at below-optimal efficiencies. Nevertheless, deck loads contained a factor of 2 to 3 more oysters per bushel than present on the bottom. The suction dredge operated very differently, although deck loads contained 1 to 3 times as many oysters as were present on the bottom. Catch efficiencies were high, between 19% and 58%. Swept area per bushel loaded was much lower, about 600 bushels being loaded per hectare swept. Catch efficiencies were highest for small particles. Dredge efficiency rose markedly after transplanting, from 6% to 28% on the plots worked by the traditional oyster dredge and from 11% to 56% on the plots worked by the suction dredge. Nevertheless, neither method proved deleterious to bottom complexity, cultch availability, oyster growth and mortality, or population health. In a sustainable transplant, the number of small oysters and amount of cultch moved should be minimized. This goal was not achieved. The suction dredge, by selective removal of smaller particles enriched in juveniles and cultch, risks a long-term decline in live oyster abundance and shell coverage. The traditional oyster dredge has the inherent capability of concentrating larger animals, but, as used in the transplant process, much of the selective advantage disappears. A behavioral shift to exploit the desirable selective advantage of the traditional oyster dredge may improve the efficiency of the transplant program.

KEY WORDS: oyster, Crassostrea, dredge efficiency, transplantation, fisheries management, Delaware Bay

INTRODUCTION

One mechanism to enhance oyster production is the timely transplantation of oysters from nursery beds to beds used for commercial harvesting (Owen 1953, Jory & Iversen 1985, Powell et al. 1997, Kraeuter et al. 2003). In Delaware Bay, nursery beds are in lower salinity water where lower predation rates and lesser disease prevalences increase oyster survivorship. Unfortunately, oysters frequently do not reach market size in harvestable numbers on these beds or, if they do, fail to achieve market standards of meat quality (Powell et al. 1997, Ford 1997, Kraeuter et al. 2003). Transplantation of these oysters downbay, however, normally results in a rapid increase in meat weight (condition) as well as increased growth (Kraeuter et al. 2003). Timed appropriately, such oysters can become marketable before the increased disease and predation pressure downbay markedly reduce their numbers. However, an efficient transplanting scheme would select from the nursery beds only oysters that can become marketable in one growing season, reserving on the seed beds smaller oysters requiring multiple growing seasons to achieve marketability; thereby minimizing loss to predation and disease.

In Delaware Bay, transplanting is normally done with a traditional oyster dredge. The dredge used by nearly all boats is a 24-tooth dry dredge, with tooth length approximately 44 mm, a mouth opening of 1.27 m x 51 cm, and a bag consisting of 17 rows of 50.8-mm rings (Fig. 1 in Powell et al. 2002). Dredge efficiency can exceed 50% (Powell et al. 2002), but is normally much lower (Chai et al. 1992, Powell et al. 2002), particularly when used during commercial harvesting (Banta et al. 2003). The dredge is less efficient for smaller oysters and much less efficient for cultch (Powell et al. 2002). A culling machine, an on-deck rotating drum with bar spacing set to retain only the larger oysters and shell, can increase the concentration factor for market-size oysters (HSRL 2003). Hence, use of the traditional dredge and a culling machine should minimize downbay transport of small oysters that require more than one growing season to achieve market size.

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Unfortunately, catch rates for dry dredges are slow. The average oyster boat moved 900 bushels ([dagger]) of oysters per day during the 2003 transplant program in Delaware Bay (New Jersey Department of Environmental Protection [NJDEP], pers. comm.). An alternative is the suction dredge. The suction dredge is essentially a large vacuum cleaner. Water is pumped through a hose from the bottom to the deck. As a consequence, the surface material under the dredge is vacuumed up through the hose to the deck, washed over a fine-mesh sorter to remove mud and sand, and then deposited on the deck. A suction dredge of 2.44-m swath can easily move 3,500 bushels in the time the dry dredge moves 900 bushels.