Comparison of length measurements of the blue whiting samples

Blue whiting (Micromesistius poutussou) form dense spawning aggregations along the Hebridean Shelf and Porcupine Bank, west of Scotland and Ireland, starting early spring until May. The International Council for the Exploration of the Seas (ICES) coordinated the international scientific acoustic survey targeting this spawning aggregation of the blue whiting which provide indices of abundance per age-class, which are an essential input data source for the assessment of this stock. The commercial fishing effort overlap with the survey in area and time. A first step towards the use of commercial length frequency data for survey index calculation is to investigate if there is any difference between survey and commercial length frequency data, and understand the origin of potential differences. In this study, the length frequency data collected by the self-sampling program on PFA vessels during the blue whiting fishing trips was compared with the survey measurements with a main focus on the 2017 and 2018 data. From different vessels contributing to the data set, only 4 vessels had adequate overlap in space and time in 2018, and 2 vessels in 2017. The results indicated that the mean length and the distributions are promising similar from two of the vessels contributing to this dataset in 2018. In addition, the third vessel showed very high similarity when the sampling dates are very close to the survey station while geographical distance is below 20 nautical miles. In 2017, the measurements from the two vessels had very close similarity to the survey measurements. As further supporting these close similarities, the mean length from a single survey samples (that were focus of the comparisons) remained within the 95% confidence interval of the measurements from the repeated hauls of PFA vessels in relatively close location. In 2018, one of the vessels contributing to the data did not have adequate similarity based on the statistical comparisons, however its samples also did not have very close overlap with survey samples in space and time compared to the other vessels.