Abstract The aim of this study was to determine the effects of different mesh sizes and structure on catch efficiency in fyke net codends that are widely used for crayfish fishing in Turkey. The catch efficiency of experimental codends — 26 mm diamond, 34 mm hexagonal, and 42 mm diamond — was investigated as alternatives to the 34 mm diamond codend (control group), which is widely used in commercial fishing. Field studies were conducted in Lake Eğirdir (Isparta, Turkey) through nine fishing operations in 2015. Statistically significant differences were found between the mean lengths of crayfish caught by all experimental groups (p<0.05), except for the comparison between the 34 mm hexagonal and 34 mm diamond nets. Catch comparison results revealed that the 26 mm diamond codend was more effective than the control codend in retaining individuals below the minimum conservation reference sizes (MCRS). The 34 mm hexagonal codend performed similarly to the control group, while the 42 mm diamond codend was less effective for individuals under the MCRS. In conclusion, this study determined that the fyke net codend rigged with a 34 mm diamond mesh captures undersized crayfish at a very high rate (approx. 80%). This rate can be reduced to 61.81% by utilizing a 42 mm diamond mesh. Future research should address the structural factors compromising length selectivity in fyke net codends, while regulatory measures should mandate larger mesh sizes to reduce undersized catch.