## 3 Results
### Selectivity and Cod Length Structure
Probabi  
Figure 18: A box and whisker plot of percentage of undersized cod (<35 cm) in one haul in years 2019–2023. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline  & **UC + DD + DK** & **UC + JD + JK** & **UC** & **T90 + DD + DK** & **T90 + JD + JK** & **T90** & **Herring** \\ \hline Mean [\%] & **1.231** & 5.119 & 10.361 & 8.729 & 10.666 & 17.011 & 67.036 \\ \hline Variance [\%] & **0.878** & 8.661 & 8.619 & 28.290 & 18.366 & 38.443 & 158.913 \\ \hline SD [\%] & **0.937** & 2.943 & 2.936 & 5.319 & 4.286 & 6.200 & 12.606 \\ \hline Observations & **38** & 34 & 20 & 9 & 20 & 22 & 35 \\ no. & - & 39 & 21 & 8 & 20 & 22 & 34 \\ \hline t-stat & - & \(-7.377\) & \(-13.550\) & \(-4.214\) & \(-9.724\) & \(-11.860\) & \(-30.804\) \\ \hline P(T \(\leq\) t) & - & \(3.27\times 10^{-9}\) & \(3.76\times 10^{-12}\) & \(1.47\times 10^{-3}\) & \(2.53\times 10^{-9}\) & \(2.49\times 10^{-11}\) & \(1.00\times 10^{-26}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Statistical comparison of % catch of undersized cod using UC + DD + DK coded vs. other codons. the last row of Table 3 (P(T \(\leq\) t) one-tail), notably very low in observations, mean that for a practical significance level of alpha = 0.05, one has P(T \(\leq\) t) < alpha and a t-statistic lying within the one-sided critical region of the t-statistic distribution, thus enabling one to reject the null hypothesis. It can be concluded that UC + JD + JK also significantly outperforms other types of codend in terms of selectivity, though probability data provided in the last row of the Table 4 mean that rejection of null hypothesis is weakest for T90 + DD + DK. This justifies the research assumption that addition of innovative devices slowing the water flow to that of a standard T90 codend will also significantly reduce the number of undersized fish in a catch, although not as much as when using them with the more selective UC codend. Further research on this issue is necessary as the number of hauls with T90 + DD + DK was the lowest due to technical failure of the codend sustained in 2019. Table 5 and Figure 19 present differences between the percentage of undersized cod caught with the most selective UC + DD + DK codend and the non-selective herring codend using double bottom trawls in 2023. The mean difference between the percentage of undersized cod caught by the UC + DD + DK coded and non-selective herring codend using double bottom trawls in 2023 was 61.7% points with an SD of 8.4% points. The distribution of the frequency of measured differences allocated into 5% point classes (see Figure 19) has not been validated due to the low sample number (the number of comparative hauls by double bottom trawling was 10), but one can assume that there is no or negligible correlation between percentages of undersized cod with the UC + DD + DK coded and the non-selective herring codend as correlation coefficients of Pearson and Spearman are, respectively, 0.09 and 0.05 (a relationship such as a higher percentage for the non-selective codend and a higher percentage for the selective one does not exist--the variables do not tend to change together at either a constant or a non-constant rate). \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & UC + JD + JK & UC & T90 + DD + DK & T90 + JD + JK & T90 & **Herring** \\ \hline Deg. of freedom & - & 40 & 9 & 30 & 27 & 38 \\ \hline t-stat & - & \(-6.330\) & \(-1.958\) & \(-5.121\) & \(-8.405\) & \(-28.275\) \\ \hline P(T \(\leq\) t) one-tail & - & \(8.11\times 10^{-8}\) & 0.041 & \(8.27\times 10^{-6}\) & \(2.57\times 10^{-9}\) & \(1.98\times 10^{-27}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 4: A \(t\)-test of % catch of undersized cod using UC + JD + JK codend vs. other codends.