Density-dependent capture efficiency of a survey dredge and its influence on the stock assessment of eastern oysters (Crassostrea virginica) in Delaware Bay

A reliable measure of gear capture efficiency is required to calculate unbiased estimates of population size and fishing mortality from survey data in a stock assessment. However, capture efficiency can vary spatially and temporally due to changes in abundance, stock area, the environment, and the sampling gear itself. Therefore, periodic reassessment of this parameter is necessary to ensure that the catchability coefficients being applied accurately reflect the capture efficiency of the survey sampling gear, especially when catchability is being estimated outside of the stock assessment model. Using data from field experiments conducted in 1999, 2000, 2003, and 2013, we evaluated spatial and temporal variability in capture efficiency for a commercial dredge used to conduct a fishery-independent survey of the eastern oyster (Crassostrea virginica) population in Delaware Bay, USA. A spatial gradient in capture efficiency was detected, but no temporal trend. Capture efficiency was a function of the density of oysters in the sampled area. To our knowledge this is the first time density-dependent capture efficiency has been identified for a sessile invertebrate stock survey. Since density dependence in capture efficiency leads to hyperstable catch-per-unit-effort, caution is advised when deriving oyster abundance from dredge survey catch-per-effort data, especially at low oyster density and when high spatial resolution estimates of survey dredge capture efficiency are not available.