The Garden, in partnership with local koi clubs, hold an auction each and every year to reduce the bioload in the pond. It is an event that is happily attended by koi hobbiests from all over the region.
Also, last year they had a confirmed outbreak of KHV, which is deadly, highly communicable between koi and incurable, in the pond. All of the koi that were in the pond had to be destroyed and the pond was sterilized. The source of the KHV is unknown, but may have been from donated koi that were quarantined but not KHV tested or koi that were "dumped" in the pond by irresponsible koi owners. New fish were introduced after testing confirmed them to be KHV-free - some were just fingerlings and some were larger koi that were purchased from Japan. The Garden has instituted a process of testing all donated koi while during the quarantine timeframe. They are fortunate to have the Long Beach Aquarium of the Pacific labs to conduct testing for them.
This is clearly an example or the importance of testing and quarantine for all koi owners! One infected koi can wipe out your entire collection - and a carrier can be completely symptom-free.