The present study was carried out at the Turkman catch station of Golestan province in March and May 2002 to examine suitable mesh size of gill nets used for Persian sturgeon catch in order to decrease possibility of immature Persian sturgeon and increase likelihood of catch of large sized specimens. The Holt model (1963) was used to determine suitable length for catch and also selection of mature Persian sturgeon females. Following preliminary studies some 920 experimental gill net (mesh size 170mm from knot to the next) similar to those generally used by Iranian fisheries except for mesh size (150 mm) were set up in the Caspian Sea. During two months experimental period some 128 and 110 sturgeon fish were caught in witness and experimental nets respectively. Out of those fishes caught by witness nets 63 specimens were Persian sturgeon (49.2%), 57 were stellate sturgeon (44.5%) while in the experimental nets 95 specimens (86.4%) were Persian sturgeon and 7 specimens (6.4%) were stellate sturgeon. Considering maturity status of fishes 42 Persian sturgeon caught by witness net were mature females while this figure was 83 (87.4%) in experimental nets. Mean fork length of Persian sturgeon was 152.9± 13.8cm and 162.4± 13.6cm in witness and experimental nets respectively. The optimum calculated fork length to begin catch of female Persian sturgeon in witness net was 132.5cm and in experimental net 150.2 cm. The calculated selection factor was 0.9. Considering the reported mean fork length of Persian sturgeon females in statistical and biological study of sturgeons the mesh size of 162mm is recommended as suitable mesh size for this species. The amount of caviar and mean caviar yield of Persian sturgeon in witness net was 26.7 and 6.4 ±1.7 kg and in experimental nets 610.6 and 7.4± 1.9kg. One way ANOVA and Tukey s test showed that there are significant differences between factors like age, mean caviar yield and fork length of Persian sturgeon caught by witness and experimental nets at 95% confidence level. The results obtained in this study shows that increasing mesh size of gill nets used for Persian sturgeon catch to 170mm may guarantee objectives of stock managements including decrease in catch of immature and increase in mature and larger sturgeon catches to secure optimum and sustainable stock deployment.

Iranian Fisheries Science Research Institute

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