Abstract
For the Atlantic sea scallop, Placopecten magellanicus, rotational access area management has gained support as an effective management tool to conserve and enhance the scallop resource. Multiple optical and towed gear surveys are conducted annually to provide information on the abundance and distribution of adult and juvenile scallops, enabling managers to respond to changes in biomass in different subunits. Recently, a significant divergence in area-specific biomass estimates was observed between different survey methods, with optical survey estimates exceeding those from towed dredge surveys. The leading theory for these differences is that the survey dredge would be saturated during the standard 15-minute tows in high-density areas, leading to underestimates of stock biomass. This study developed a methodology to directly observe and count scallops in the dredge path during survey operations. Seventy-three tows were conducted off Southern New England in the Nantucket Lightship Area and the Great South Channel, including in known high-density scallop beds. Estimates of catch efficiency were calculated by comparing the number of scallops in the dredge path to those caught in the dredge. The catch efficiency of the survey dredge was significantly reduced when sampling in high-density scallop beds, declining from 0.42 at low-density to 0.09 in the high-density beds. High catch volumes of scallops in the dredge were observed to cause the sweep chains to lose contact with the seafloor, resulting in loss of scallops under the dredge. Ultimately, a better understanding of the survey dredge performance and catch efficiency in different density areas will help reduce uncertainties in biomass estimates.
•An in-situ camera system was developed to count scallops along the dredge path, directly measuring survey dredge efficiency.•Catch efficiency declined with density, from 0.42 at low densities to 0.09 in high-density beds, confirming dredge saturation.•Saturation occurred at catches between 6000–8000 scallops, beyond which the dredge no longer reflected true seabed abundance.•Video showed the dredge sweep lost bottom contact at high densities, allowing scallops to escape underneath.•Dredge angle did not predict saturation, meaning other metrics are needed to monitor dredge performance during surveys.