Culling & Selective Breeding ~ 篩選與改良

It is particularly incumbent on those who never change their opinion, to be secure of judging properly at first.
~ quote from "Pride & Prejudice" by Jane Austen.

Wouldn't it be great if we can judge and select our friends just from their phenotypic traits? We know it is not that straightforward as our intrinsic virtues should be valued above our extrinsic traits (ideally speaking). The fact is, it takes time to know someone and see who are our true friends :)) In the parallel context for selective breeding of livestocks, we tend to breed only animals with favourable physical traits, but in truth, genetics is far more than meets the eyes and it takes time for breeders to truly understand their bloodlines. 

One of my new creations - Pearl, Pompom or Dragon eyes?
the onus is on me to create a bloodline that is both healthy and
has good ornamental value...

In nature, culling is a definitive refinement process - by the fundamental law of nature, the weaker ones shall perish and the stronger ones will propagate their genes to ensure a better chance of survival for future generations. 

Big, broad and round lobed caudal fins, prevalent
in most Thai-bred Orandas is evidence in this
Chinese bred Oranda - a result of selective
breeding and outcrossing in the Thai Oranda bloodline

Selective breeding and culling of livestock by man is also a refinement process, but usually with the aim to enhance certain phenotypic traits. As such, the strongest ones may not necessarily be those we want to keep. Fancy Goldfish, like many pedigree animals bred for ornamental appreciation are "freaks of nature" that would not have been existed without the intervention of man, simply because their fanciful traits hinder their chances of survival in nature

One of my Okayama bloodline Female Tosai
with good colour depth and head wen
features.

In other words, selective breeding is about man meddling with mother nature, and so we can expect things to go awry. It is all too easy for breeders focusing on physical traits to neglect genotypic attributes like resistance to diseases, fertility and other genetical problems which are not easy to envisage, especially if the brood stocks are too young. Common examples in the human world are genetic heart disorders and the male pattern hair loss that manifest at a certain age. Hence, there is much wisdom in the age old adage that breeders should try to select their brood stocks from fairly matured individuals because their genetic attributes are more "stable".

Broad frame and big bone structure, rarely found
in the males of my bloodline makes this fish
one of the most favourable brood stock
candidates for next year's breeding

Back to the context of Goldfish breeding: besides resistance to diseases and fertility, it is important for the brood stock to process strong characteristic traits pertaining to its variety, having stable and good colouration as well as strong bones and finnage to balance and support its structure throughout its lifespan. Some very experienced Japanese Ranchu masters working for decades on their bloodlines know their bloodline so well that they could strategize the grooming techniques and growth for individual fishes for breeding or competitions. 

Sometimes, the breeder may hit a dead end and have to decide if it would be better off to outcross or restart from scratch than to delve in a vicious circle. Having said that, it should never become too easy to conclude that our bloodlines need major revamp without thorough analysis.