Estimating sea scallop incidental mortality from photogrammetric before-after-control-impact surveys
Date
2016
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Publisher
University of Delaware
Abstract
After several decades of stock decline, the Atlantic sea scallop (Placopecten magellanicus) fishery is one of the most valuable in the United States due in part to the implementation of new management measures in 1994. The continued sustainability of the fishery is dependent on catch limits determined by yearly stock projection models. Incidental mortality is an important term in sea scallop stock projection models, but is historically difficult to measure. Current estimates are derived from experiments that relied heavily on qualitative observations and as a result lack precision. To better estimate incidental mortality, I used a Multiple-Before-After-Control-Impact (MBACI) experimental design to measure the effect of scallop dredging on the disposition of sea scallops that remain on the seafloor following dredging. An autonomous underwater vehicle (AUV) was employed to collect color photos and side-scan sonar images of the seafloor before and after controlled dredge treatments in the Mid-Atlantic and Georges Banks regions. Approximately 170,000 photos were annotated for instances of dredge-induced mortality. I found 2.5% and 8% incidental mortality for the Mid-Atlantic Bight and Georges Banks sites, respectively, a difference that is likely attributable to the relatively harder substrate of the scallop habitat on Georges Banks resulting in greater physical trauma. This study provides a quantitative estimate of incidental mortality using a precise and noninvasive platform. The spatial scale and distribution of the study sites are broader relative to past studies and represent the two principal stocks of the sea scallop resource. These results are lower than the incidental mortality values currently used in fishery stock models and suggest the existing values are conservative, but appropriate estimates.