I always try to allow for extra time to sort or examine the fish that I buy from a hatchery. OR I watch them count the fish into a bag/bucket. I have seen too many hatchery workers do their job. Some workers are not too particular at how they do their job and some just flat out make mistakes.

Almost all hatcheries have some contamination by other fish species in a single specie pond. Contamination can come from many sources. Some fish farms raise several species together - polyculture.

If a worker is sorting out fish and drops one wrong type of fish into a tub of several thousand fish do you think they take the time to resort all of them? Rarely would this resorting happen. Some hatcheries will catch this error when counting out fish for the customer. Sometimes fish are sold in large quantities by the pound and the amount of sorting is minimal.

When I am sorting a net full of fish containing several different species, often one jumps out of the net into a wrong container. Oops! I try to get it on the second or third resort. Rarely does a hatchery have the time to do a resort. Or the next worker is just a counter and not a very good sorter. Some workers don't care - a fish is a fish and an extra species is just considered a bonus or surprise. Bad busines in my opinion.

Expect some fish contaminaton when buying fish which is why I have learned to always sort my fish that I buy from a hatchery. Contamination is more likely with large orders, when buying smaller sized fish or from from certain hatcheries with a reputation. This style of business is how reputations are made. The hatchery can always use the excuse "the birds brought in those extra fish".

aka Pond Doctor & Dr. Perca Read Pond Boss Magazine -
America's Journal of Pond Management