Effect of Size Sorting on Growth Performance of Juvenile Spinefoot Rabbitfish, Siganus rivulatus

Marbled spinefoot rabbitfish, Siganus rivulatus , is an herbivorous euryhaline marine fish with established market demand. Interest in aquaculture of the marbled spinefoot is increasing in Eastern Mediterranean, Red Sea, and Arabian Gulf nations. However, culture protocols for commercial production are incomplete. In this work, we evaluated the benefits of size sorting S. rivulatus juveniles before stocking. Two experiments were performed. In the first experiment, juveniles were size sorted into three different groups: small (5.5 g), medium (8.7 g), and large (12.1 g), and 15 fish were stocked per tank in a recirculating system of 180‐L tanks with three replicates per treatment. Two additional treatments were established: a mixed treatment consisting of five fish from each size category stocked together and a small treatment offered as much feed daily as the medium treatment. Feed was offered at 4% body weight daily for 8 wk. Survival was greater than 95% in all treatments. Average final weights of the fish at harvest were 52.2, 38.1, 28.3, 39.6, and 30.5 g for large, medium, small, mixed, and small with high‐ratio treatments, respectively. Fish in the large treatments grew faster than fish in all other treatments. In the second experiment, fish were trapped off the Beirut beach in April, June, and August 2007. Individual weight and length and condition index of each fish were determined. Males and females were differentiated by identifying testicular and ovarian tissue, respectively. Gender identification was performed at 10× magnification. There were no differences in length and weight between males and females in all three samples collected. Results suggest that size grading of juvenile rabbitfish will maximize culture production by selecting faster growing fish with better condition indices but will not select for gender because S. rivulatus juveniles (10.6–12.1 cm) do not exhibit sexual size dimorphism.