SIZE GRADING MAY ALTER SEX-RATIOS OF FINGERLING CHANNEL CATFISH

The influence of size grading on sex ratios and growth was evaluated f or small (0.2-4.0 g) channel catfish (Ictalurus punctatus). Channel ca tfish 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 fibe rglass tanks. Weight, length, and sex were then determined on up to 10 0 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% ma les, the frequencies were not different from the expected 1:1 male: fe male ratio. Males usually were preferentially selected (mean +/- SD, 6 5.1 +/- 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 di d not consistently affect the sex distribution. Harvest size of finger lings 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, a nd in those instances males were always larger than females. Variances for weight and length between sexes were different in less than 10% o f the tanks, and the magnitude of the variance differences was signifi cantly influenced by families. These results suggest that grading fing erlings as small as 3 g can affect sex ratios and that channel catfish families may be selected to enhance or decrease sex-related growth di fferences.