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Calibration of electronic measurement boards and length values recorded in the Fisheries Scientific Computing System (FSCS)
  • Published Date:
    2015
Filetype[PDF - 1.61 MB]


Details:
  • Corporate Authors:
    Northeast Fisheries Science Center (U.S.)
  • Series:
    NOAA technical memorandum NMFS-NE ; 232
  • Document Type:
  • Description:
    Measured length distributions of Atlantic herring (Clupea harengus) were not identical between midwater trawl catches from the fall systematic herring survey and bottom trawl catches from the fall stratified-random survey conducted by the Northeast Fisheries Science Center (NEFSC) from 1998 to 2012. The Scantrol FishMeter fish measurement board was used during the stratified-random bottom trawl survey, while the Ichthystick II Electronic Fish Measuring Board was used during the systematic herring survey. Both sets of measurements were recorded by the Fisheries Scientific Computing System (FSCS). We investigated whether differences in length-measurement systems caused discrepancies in length distributions between the 2 surveys. Our initial concern was that the Scantrol board may have rounded up to the nearest 0.5 centimeter on measurement and that on recording into FSCS the measurement was again rounded up to the nearest whole centimeter. In the first set of measurements, dowels were cut to known lengths, randomized, measured with each board, and recorded into FSCS. Three different measurement boards of each type were compared to assess variability within each board type. The actual board measurements and the recorded FSCS values were compared against the known values. Systematic biases were observed in 2 of the Scantrol boards, prompting a series of systematic measurements on all Scantrol boards. All fish lengths stored in FSCS from Scantrol boards were positively biased by 0.25 cm, and some boards appear to have a mechanical offset that can increase the bias up to 0.50 cm when compared to true lengths. We cannot completely rule out instrument error as the cause of discrepancies in mean Atlantic herring lengths between surveys, but the effects of rounding by the Scantrol boards and FSCS do not appear to explain the magnitude of the differences. Biological and sampling factors should be investigated as the cause of length distribution discrepancies between surveys. [doi:10.7289/V5QZ27X1 (http://dx.doi.org/10.7289/V5QZ27X1)]