New codend designs to improve the size selectivity of fyke net for narrow-clawed crayfish (Pontastacus leptodactylus)

Abstract

The narrow-clawed crayfish (Pontastacus leptodactylus) is one of the most ecologically and economically important freshwater species in Turkey. However, the harvest of this species has declined from 7937 t in 1984 to 696 t in 2019. One of primary reasons for this decrease in stock is the use of fishing gears with poor size selectivity. In this study, new codend designs were investigated to improve the size selectivity of fyke nets forP. leptodactylus. Seven codends of three different designs were tested: (1) a commercially used standard codend with 34 mm mesh size (Com34); (2) sorting grids with 20 (SG20), 25 (SG25) and 30 mm (SG30) bar spacing attached to the last circle of the net; and (3) a stiff rigged net, the last part of the fyke nets (codend) with 34 (SRN34), 42 (SRN42) and 50 mm (SRN50) mesh size. The average length at 50% retention probability (L50) and selection range (SR) (L75–L25) values ofCom34were 9.4 cm and 3.1 cm, respectively. For theSG20,SG25andSG30grids, theL50values were 11.3 cm, 11.6 cm and 12.0 cm, while theSRvalues were 1.4 cm, 1.3 cm and 0.6 cm, respectively. For theSRN34,SRN42andSRN50codends, theL50values were 10.9 cm, 11.6 cm and 11.6 cm, while theSRvalues were 2.1 cm, 2.1cm and 1.1 cm, respectively. Overall, the commercial codend resulted in lowerL50values when the minimum conservation reference length of 10 cm was considered. However, all tested new codend designs showed improved selectivity compared with the standard, with optimum results obtained withSG20.SG20grid decreased discarding by 15.7% compared to the classic commercial fyke net; thus, this is a very important result for the sustainability of naturalP. leptodactylusstocks.