## 4 Discussion and Conclusions  
The conducted research trawls clearly indicate that the use of an innovative ultracross codend of 120 mm mesh with devices reducing the water flow speed (tarpaulin diffusers and 120 mm net confusors) significantly reduces the number of undersized cod (<35 cm) in a catch comparing to other codend solutions. 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 that the mean for UC + DD + DK is much lower than for other codend variants. The standard European T90 codend, without innovative devices reducing the water flow speed, retained, on average, 17% of undersized cod, while the UC + DD + DK innovative codend retained only 1.2% of undersized cod, which is 14 times less, and the UC with one tarpaulin diffuser and one 120 mm net confusor (UC + JD + DK) retained 5.1% of undersized cod, which is 3.3 times less. Comparison of the high selectivity achieved by UC + DD + DK for cod of length < 35 cm with the length structure of cod in the testbed area (Figure 20) leads to the conclusion that this codend enables the precise selection of fish quite close to the 35 cm limit (68% of cod caught non-selectively were of sizes from 24 cm to 38 cm, but only 1.2% of cod caught selectively were less than 35 cm). This selectivity is steady as no correlation between the percentages of undersized cod caught by UC + DD + DK and the non-selective herring coded while using a double bottom trawl set in 2023 was found and statistically confirmed. The obtained results are also very promising in terms of invisible discards and the subsequent mortality of escaped fish. At slower water speeds inside the trawl set, the fish which escaped during hauling had a greater chance of survival. Construction of a special divided small-mesh cover for the codend allowed for the determination of the amount as well as the length and mass of cod escaping from the codend while hauling the trawl gear. The average percentage of cod which managed to escape from the innovative codends was 36% \(\pm\) 5.4%(SD). At the same time, it should be borne in mind that one of the factors that determined the number of undersized cod during the trawl fishing research was the length structure of the Baltic cod population (as presented in Figure 20). Other factors such as (1) seasonality (especially spawning season), (2) fishery area, and (3) skipper competency/experience have not been studied during the present research. The most serious threat to cod fishery in the Baltic Sea is the decline in cod stock. In the current situation for fishermen, carrying out further research of innovative codends may provide reliable data that can be used to determine the economic profitability of fishing and precisely determine the size of Baltic cod resources. This will allow the fishing community to gain significant arguments in discussions regarding catch limits, fishing bans, fishing tools, and methods, which may in fact translate into the future of cod fishing in the Baltic Sea and the fate of fishing vessel owners, fishermen, and their families. Conceptualisation, P.N., K.B. and P.Z. ; methodology, P.N., P.Z., K.B. and J.A. ; software, P.Z., J.A., A.Z. and G.K.; validation, P.N., P.Z. and K.B. ; formal analysis, P.Z. ; investigation,K.B., G.K. and A.Z. ; data curation, K.B., G.K. and A.Z. ; writing--original draft preparation, P.Z. ; writing--review and editing, P.N., K.B., J.A. and A.Z. ; visualisation, P.Z., K.B. and J.A. ; supervision, P.Z. ; project administration, M.K. ; funding acquisition, P.Z., P.N. and M.K. All authors have read and agreed to the published version of the manuscript. This research was co-financed from EU 2014-2020 Operational Programme \"Fisheries and the Sea\" (OP 2 FISH 2014-2020), grant of Polish ARMA number 00009-6520.13-OR1600003/22/23. The data are not publicly available due to their proprietary and business-sensitive nature.