Evaluating the Benefits of Size-Sorting Tilapia Fry Before Stocking

Abstract

Size-grading is routinely practiced in fish hatcheries for reasons such as increased growth among small fish, reduction of cannibalism, decrease in size variability among harvested fish, and facilitated feeding; yet there is little information with regards to growth potential of sorted groups. Two growth trials were conducted with 30-day-old, size-sorted tilapia, to evaluate growth depensation and the influence of social interactions. In the first experiment, sibling juvenile tilapia were sorted into three size classes, small (2.6 g), medium (3.7 g), and large (4.9 g), and 15 fish were stocked into 60-L aquaria with three replicate aquaria per treatment. A fourth treatment (mixed) consisted of five fish from each size category stocked together. Diet was offered at 5% body weight per day over a 7-week growth trial. Average final weights of the fish at harvest were 13.3, 18.8, 27.0, and 24.7 g for small, medium, large and mixed treatments, respectively. Fish in the large treatment grew faster than fish in the medium treatment, which in turn grew faster than fish in the small treatment. Condition index of fish in the large treatment was significantly greater than in the small treatment (P < 0.05). In the second experiment sex-reversed juvenile tilapia were sorted and divided into small (0.9 g), medium (1.2 g), and large (1.5 g). Methods were as previously described except after eight weeks of culture, the tilapia were harvested and then transferred into larger tanks (150 L) to accommodate for their increased size. Average final weights after 13 weeks were 61.3, 78.8, 95.0, and 75.8 g for small, medium, large, and mixed treatments, respectively. Fish in the large treatment grew significantly faster (P < 0.05) than fish in the medium, small, and mixed treatments. Results suggest that size grading before stocking would improve commercial returns for farmers.