Estimation of gillnet mesh size for Narrow-barred Spanish mackerel (Scomberomorus commerson Lacépède, 1800) using girth measurements, Northwest Persian Gulf

This research was conducted in the fishing ground of Bushehr Province in the northern Persian Gulf from October 2018 to June 2019. Scomberomorus commerson specimens were caught using common multifilament drift gillnet with mesh sizes of 130, 140, 150 mm (STR). The main purpose was to determine the proper gillnet mesh size for the prohibition of catching non-adult fish using girth measurements. Fork length, girths were measured at the preopercle (POG), opercula (OG), and first (DF1G) and second dorsal (DF2G) fins. Overall, 1230 specimens were caught during the autumn, winter, and spring seasons. The percentages of immature Narrow-barred Spanish mackerel caught by gillnets with the mesh sizes of 130, 140, and 150 mm (STR) were 69.2% , 66.5% , and 60.3 %, respectively. The estimated equations between fork length and girths were POG=0.2285FL+5.4836 (R2=0.8366), OG=0.2477FL+6.8582 (R2=0.8006), DF1G=0.2851FL+7.0874 (R2=0.8566), and DF2G= 0.3191FL+8.1031 (R2=0.8316). Pearson correlation analysis also showed a positive correlation between the measured girths and fork length (p<0.01). Two-way ANOVA results showed that the effect of mesh size on fork length and girths were significant (p<0.05). Based on the fork length of first sexual maturity (83.6 cm), the use of the estimated mesh size based on the DF2G (17.39 cm) will be essential to reduce the catch of non-adult fish and having sustainable fishing in the future.