I couldn't say for certain, not being a fish farmer, but using hybrids is common in agriculture. You'll maintain multiple germ lines and cross them to produce

I couldn't say for certain, not being a fish farmer, but using hybrids is common in agriculture. You'll maintain multiple germ lines and cross them to produce more desirable offspring which either don't breed true or aren't viable adults or are infertile. Happens with chickens, corn, mules. Edit: just read other reply; yup, not a fish farmer.