A method to identify time periods when industry and observer scale weights are incongruent aboard North Pacific (Alaska) at-sea processors
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A method to identify time periods when industry and observer scale weights are incongruent aboard North Pacific (Alaska) at-sea processors
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  • Description:
    In the Federal groundfish fisheries off Alaska, vessel- and species-specific catch and disposition information is accurately assessed in real time onboard catcher processors and motherships -- vessels that have their own factory on board and process at sea -- using electronic scales and observers who sample nearly every haul on every trip. Flow scales continuously record weight across load cells as catch is moved inside the factory with conveyor belts. To be useful these scales need to be accurate, so a flow scale test must be performed within a 24-hour period and pass within a 3% accuracy. However, flow scale accuracy outside of these tests has not been evaluated for over 20 years, and there may be considerable bias introduced into catch weights if flow scale readings are manipulated after tests are performed. We compared sample weights from flow scales and observer scales under normal fishing operations from over 150,000 hauls from 39 vessels over a 5-year period. Change Point Analyses were used to identify time periods in which the mean difference among scales exceeded permissible amounts for several days in a negative direction indicating weights were biased low. This semi-quantitative approach yielded 12 time periods of concern from five vessels during which the mean differences among scales exceeded permissible amounts, with the vessel scales reading lower. The results of this study were used to focus law enforcement efforts for more detailed investigation and potential action. We conclude with a potential method on how to produce this analysis in a timely fashion to aid fisheries enforcement activities without unduly jeopardizing observer safety.
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