Evaluation of Dredging Effort by the Delaware Bay Oyster Fishery in New Jersey Waters

Abstract

As part of a study to assess the effect of commercial dredging for eastern oysters Crassostrea virginica on their beds in Delaware Bay, we evaluated the total dredging effort for the Delaware Bay oyster industry in New Jersey waters for 1999-2000 and examined some of the behavioral and gear-related factors that determined this total effort. In a standard 8-h fishing day, a one-dredge boat traverses about 3.8 ha (38,000 m²) of oyster bed. A two-dredge boat traverses nearly twice that area, about 6.4 ha. Oyster boats typically fish in a single area for most of the day, yet catch per unit effort (CPUE) does not decline during the course of the day. Catch per unit effort is stable because of low dredge efficiency. Although an oyster dredge is capable of routinely achieving efficiencies of 10-60%, dredge efficiency during fishing is usually only 4-7%. Low dredge efficiency means that swept-area coverage (area of the bottom traversed by a dredge) is high for the number of oysters taken. The most heavily fished Delaware Bay oyster beds were completely dredged one to eight times in a single year. Oyster catch is typically measured in bushels (1 bushel = 35 L). The CPUE, calculated as the number of bushels of oysters caught per hectare swept by the dredge, averaged 10-12 in 1999-2000. The CPUE did not vary significantly between one- and two-dredge boats but varied by more than a factor of five among oyster beds. Nevertheless, CPUE was not correlated with the number of bushels landed from each bed. Hence, factors other than the rate of capture, such as market quality, determined the spatial distribution of fishing effort.

Notes

² Each permitted vessel receives a yearly allocation for harvest, as determined by the annual stock assessment program (HSRL 2000).

³ Market quality tends to increase with increasing salinity in Delaware Bay due to the influence of increased phytoplankton standing stock, and hence increased food supply, at higher salinity on the oyster condition index (Powell et al. 1997; HSRL 2002).