## 2 Materials and Methods
### Fishing Boats, Codends, and Trawl Gear  
Fishing boats used in the research (see Figure 1) were WLA-187 and WLA-220 in 2023 and WLA-71 in 2019-2022, all operated by Necfish Co. from Wladyslawowo, Poland. The parameters of the fishing boats were as follows (see Table 1):  
Each trawler was equipped with two hydraulic net winches at the stern. They also had a sonar cable winch and two auxiliary electric winches. Two types of innovative codend of 120 mm mesh for a Baltic cod (single-species) fishery, minimising the bycatch of much smaller herring (_Clupea harengus_) and sprat (_Sprattus sprattus_), were designed, constructed, and analysed hydrodynamically, and their selectivity was compared with an ultracross codend of 120 mm square mesh (UC), a standard EU codend \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Parameter** & **WLA-187** & **WLA-220** & **WLA-71** \\ \hline Type & Cutter & Cutter & Cutter \\ \hline Gross register tonnage (GT) & 61 & 110 & 89 \\ \hline Length overall (LOA) [m] & 19.09 & 20.55 & 22.20 \\ \hline Breadth [m] & 5.80 & 6.00 & 6.01 \\ \hline Design draught [m] & 2.63 & 2.55 & 2.20 \\ \hline Engine type & four-stroke, MDO & four-stroke, MDO & four-stroke, MDO \\ \hline Maximum continuous rating & 294 & 368 & 289 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Fishing boats’ parameters. Figure 1: Fishing boats used in research (years 2019-2023): (a) WLA-187, (b) WLA-220, and (c) WLA-71. Source: courtesy of Necfish Co.  
for cod fishery (T90 codons), a herring codon, and between themselves. They were named by the following acronyms:  
1. UC + JD + JK: a UC with innovative devices reducing the speed of water flow, i.e., one tarpaulin diffuser--JD and one net confusor--JK,
2. UC + DD + DK: a UC with innovative devices reducing the speed of water flow, i.e., two tarpaulin diffusers--DD and two net confusors--DK (see the photos in Figure 2). Additionally, in 2019, several single bottom trawls were conducted with an extra-fine mesh codend cover installed for the collection of escaping fish. The acronym of the cover was set to DOD. Measurements of the geometry of the codons were made by means of a specifically designed hydroacoustic system, which measured the distance between 10 pairs of ultrasonic transducers fixed to the codend's net fabric. The measurement range of the system was from 0.1 m to 10 m, with a measurement accuracy of 1 cm. The measurement frequency was 1 Hz. An example of how the transducers were installed is shown in the photo presented in Figure 3, which includes an explanation of the fastenings. The extra DOD for a tested codend (see Figure 4), its rigging, the trawl doors, and the trawl lines (warps, sweeps, and bridles) were arranged to be released and retrieved from  
Figure 3: The net of a tested codend with ultrasonic transducers installed (black transmitter, red receiver) with (**A**,**C**)—permanent fastenings to the net fabric; (**B**)—pass-through fastening. Figure 2: UC + DD + DK codend stretched out before sea trials and while hauling. the cutter's stern by a trawl winch. The sweeps, bridles, and the body of the trawl, along with elements of its armament, were wound onto the net winch. Due to the use of large hoops for spreading the codend's cover, the cover was retrieved manually. Emptying the codend, the divided cover, and its inner sleeve of caught cod was carried out by packing. \"Packages\" of caught cod were hoisted via a lifting strap (halving becket), using a rope wound on the sliding drum (capstan) of the trawl winch.