## 3 Results
### Selectivity and Cod Length Structure  
During research in 2019, 7891 kg of cod was caught in trawls, of which the lengths of 12,757 cod were measured, the mass of which equalled 5965 kg. During research in 2020, 3850 kg of cod was caught in trawls, of which the lengths of 3509 cod were measured, the mass of which equalled 2222 kg. During research in 2021, 3319 kg of cod was caught in trawls, of which the lengths of 2593 cod were measured, the mass of which equalled 1595 kg. During research in 2023, 11,047 kg of cod was caught in trawls, of which the lengths of 9406 cod were measured, the mass of which equalled 2685 kg. The main reason for the difference in measured cod mass in relation to their number observed in 2023 (average  
Figure 15: The drag of three variations of trawl sets with codons T90, UC + JD + JK, and UC + DD + DK. mass 285 g) was largely the non-selective catch of undersized cod by the herring codend used comparatively in double bottom trawling. Figures 16-18 show statistics of the percentage of undersized cod number (<35 cm) and of mass of cod in one haul using various codends with and without innovative devices in the years 2019-2023. In Figure 16, the means are shown for both catch mass (blue columns refer to the left vertical axis) and selectivity % (red line refers to the right vertical axis) in one haul. The average percentages of undersized cod which were caught using various codend variants were as follows:  
1. UC + DD + DK: 1.2% \(\pm\) 0.9% (SD);
2. UC + JD + JK: 5.1% \(\pm\) 2.9% (SD);
3. T90 + DD + DK: 8.7% \(\pm\) 5.3% (SD);
4. UC: 10.4% \(\pm\) 2.9% (SD);
5. T90 + JD + JK: 10.7% \(\pm\) 4.3% (SD);
6. T90: 17.0% \(\pm\) 6.2% (SD);
7. Herring coded: 67.0% \(\pm\) 12.6% (SD). In the box and whisker plots in Figures 17 and 18, the left and right sides of each box are the lower and upper quartiles. The box covers the interquartile interval, where 50% of the data are found. The horizontal line that splits the box in two is the median. The mean  
Figure 16: The percentage of undersized cod (<35 cm) and the average mass of cod in one haul in years 2019–2023. Figure 17: A box and whisker plot of mass of cod in one haul in years 2019–2023. is indicated by a cross. The whiskers are the two lines outside the box that go from the minimum to the lower quartile (the start of the box) and then from the upper quartile (the end of the box) to the maximum. A variation of the box and whisker plot restricts the length of the whiskers to a maximum of 1.5 times the interquartile range. The data outside this range are outliers. All distributions except the mass of cod in the herring coded and the percentage of undersized cod in the UC coded are approximately symmetric, because both half-boxes of the bottom and top side of the median are almost the same length. The most concentrated distribution is for the percentage of undersized cod in the UC + DD + DK coded, which is also confirmed by the lowest SD value. In order to properly assess the differences in particular coded performances in view of the most important issue of the study--the selectivity--a \(t\)-test (for comparing means) with unequal variances (due to different SDs in coded populations) was conducted. This is a widely accepted technique in early statistical investigations. Details of the \(t\)-test results are presented in Table 3 (comparison against main benchmark UC + DD + DK) and 4 (comparison against UC + JD + JK as the second coded in the selectivity assessment). It can be concluded that UC + DD + DK significantly outperforms (from a statistical point of view) other types of coded in terms of selectivity, including the simpler innovative construction of UC + JD + JK. Namely, assuming a significance level of alpha = 0.05, one can reject in all cases the null hypothesis, thus confirming that the selectivity means are quite different, and additionally the mean for UC + DD + DK is much lower than for other coded variants, which is also presented in Figures 16 and 18.