SIZE SELECTIVITY OF TRAPS (FYKE-NETS) OF DIFFERENT MESH SIZE ON THE NARROW-CLAWED CRAYFISH, ASTACUS LEPTODACTYLUS (ESCHSCHOLTZ, 1823) (DECAPODA, ASTACIDAE) IN ESCHSCHOLTZ LAKE, TURKEY

The present study was carried out to evaluate the effects of selectivity of fyke-nets of different mesh size on Astacus leptodactylus (Eschscholtz, 1823), in lake Egirdir in Turkey. Crayfish were sampled monthly using fyke-nets of 26 mm (control), 34 mm (traditional), and 42 mm (increased) mesh size from July to December 2007. Differences in the mean size, weight, and number of crayfish caught with each mesh size were determined. The average length and weight were found to increase proportionally to mesh size. The highest mean weight (g) (+/- SD) (41.07 +/- 4.61) was achieved at 42 mm mesh size compared with mean weights of 32.76 +/- 4.53 and 29.29 +/- 4.37 at sizes of 34 and 26 mm, respectively. The same trend was observed in mean TL of crayfish caught at 42, 34, and 26 mm size, but no significant difference was detected in mean length and weight for male and female crayfish. The weight of crayfish caught in 42 mm mesh was 8.3% higher than that in 34 mm mesh size. The lowest number of crayfish was caught with the 42 mm mesh size, while the highest number was captured with 34 mm mesh. This study shows that increasing mesh size has a positive effect on selectivity, because the mean total length L-50 in the catch approximates the minimum landing size for this species in Turkey. L-50 values (total length) for female, male, and combined sexes were 9.93 +/- 0.41 cm, 9.73 +/- 0.34, and 9.93 +/- 0.34 cm for 42 mm, and 6.54 +/- 0.60 cm, 7.28 +/- 0.30, and 7.09 +/- 0.3 cm for 34 mm, respectively. It was presumed that 34 mm mesh size, which is commercially used, has poor selectivity. On the other hand, 34 mm mesh size caused a loss in marketable crayfish among the three tested mesh sizes. The 42 mm mesh size provided the most appropriate selection of the crayfish, compared with sizes of crayfish at 34, and 26 mm mesh, respectively. Meanwhile, as mesh size increases, the female escape ratio becomes higher than the male escape ratio. Thus, increase in mesh size of nets will have a positive impact not only on preserving a sustainable stock, but also sparing labour and time to be spent on fishing operations.