Fishery practice versus experimental design: Preliminary results of the introduction of protective sieves in the eel fyke-net fishery of the Vistula Lagoon, Poland

In response to the high by-catch of juvenile commercial fish species in fyke-nets used in targeted eel catches in the Vistula Lagoon, a regulation came into force requiring gear to be fitted with protective sieves with openings 20 mm × 65 mm in diameter. The protective sieves replace material with 16 mm mesh bar lengths in the codend. This paper presents the quantitative composition and length distribution of fish from commercial catches prior to and following the obligatory mounting of sieves in the gear (1999) as well as after they had come into widespread use (2004). The application of the sieves limited the retention of juvenile bream, perch, and roach. They also significantly limited the numbers of fish of the smallest length classes that were retained. The total number of pikeperch retained in the fyke-net was reduced, although the length distribution of this species did not vary as much as it did with other species. A difference in the protection effect was noted in the catches from the current study in comparison with experimental catches conducted in the same month in 1998-1999. Selectivity curves for pikeperch and bream were calculated based on samples collected with gear fitted with protective sieves accompanied by an additional net sewn to the outside of the sieve to retain all the fish that passed through the fyke-nets. L50% was 17.6 and 12.4 cm TL, respectively. In the commercial catches made in 2004, only a slight limitation in the retention of pikeperch of the 11-14 cm length class was noted, and bream specimens smaller than 17 cm TL were all but eliminated. The authors are of the opinion that behavior and factors that affect mortality following contact with the fishing gear are significant. The variability of bream regarding the dependence of body height on length might have influenced the results.