Model experiment for practical application of inseminated sperm method for production of interspecific hybrids (Clarias gariepinus x Heterobranchus longifilis)

In this study, the inseminated sperm method (sperm injection into the ovarian lobe in externally fertilised fish species) combined with hormonal induction was conducted as a novel method to produce hybrid catfish by induced/tank spawning. In the treated group, four Clarias gariepinus female were inseminated equally with Heter-obranchus longifilis sperm in the ovaries at a dosage of 1 mL/fish and then introduced into four spawning cages together with four C. gariepinus male. Ovulation was induced by extracted carp pituitary (CPE) hormonal admin-istration (female; 5 mg CPE/BW kg). The hybridization rate was determined on the basis of morphological markers to distinguish between pure African catfish and hybrid in juvenile stage. A control experiment was also designed to determine this and to explore possible interactions between the two genotypes. In the control group, in vitro fertilisation (IVF) was used to produce pure progeny of C. gariepinus and the hybrid progeny between C. gar-iepinus female x H. longifilis male. Exogenously fed larvae from each treatment were raised and compared after 28 days in four replications; the mixed group contained 50 C. gariepinus and 50 hybrids from IVF grown together, while every 100 larvae from each cage spawning pair were grown separately. The hybridisation rate determined based on morphological traits was 98.11 +/- 1.59 (mean +/- S.D.) for 28-day-old reared offspring. There were no sig-nificant differences (P > 0.05) in survival rate (%) between C. gariepinus (45.79 +/- 6.82 %) and the hybrid (54.21 +/- 6.82 %) in the control treatment; therefore, neither genotype had a negative effect on the other during the rearing period. From this result, we supposed that the hybridisation rate was also over 95 % at the moment of fertilisation. This approach could be an alternative method for producing hybrid catfish or other fish species without in vitro fertilisation.