SIZE GRADING MAY ALTER SEX-RATIOS OF FINGERLING CHANNEL CATFISH

The influence of size grading on sex ratios and growth was evaluated for small (0.2-4.0 g) channel catfish (Ictalurus punctatus). Channel catfish from 15 families were ungraded or were graded into two or three size-groups by mean of bar graders with 0.40-0.99-cm slot widths. The 57 groups obtained were reared separately for 3-4 months in 150-L fiberglass tanks. Weight, length, and sex were then determined on up to 100 fish per group. The frequency of males from all families was 51.5%; although progeny from individual families varied from 45.1 to 56.0% males, the frequencies were not different from the expected 1:1 male: female ratio. Males usually were preferentially selected (mean +/- SD, 65.1 +/- 3.5%) by a grader with a slot width of 0.91 cm, which retained the largest fish in a population; graders with smaller slot widths did not consistently affect the sex distribution. Harvest size of fingerlings was variable and was influenced by initial weights and densities; family-by-grader interactions were significant (P less-than-or-equal-to 0.05). Sexually dimorphic growth was observed in 19 of 57 tanks, and in those instances males were always larger than females. Variances for weight and length between sexes were different in less than 10% of the tanks, and the magnitude of the variance differences was significantly influenced by families. These results suggest that grading fingerlings as small as 3 g can affect sex ratios and that channel catfish families may be selected to enhance or decrease sex-related growth differences.