. What is different about them. They both have the same genes. They were born out of a native at some time in their lives.
explain it to me in terms that I can understand. Being born in a cement pond doesn't change their genes. SO why are they different.
The problem is that hatchery reproduction narrows the gene pool, and selects for different traits. In natural reproduction, native fish return over a period of weeks or months, depending on conditions, and their own built in clock. This creates diversity in offspring, and passes on a wide range of traits that promote survival in the wild. The RUN of fish is therefor more resilient, and will survive small challenges to the population, like mudslides, because more fish spawned both before and after an event.
In a hatchery, humans may produce the same quantity of fry from eggs, but the fish are all collected over a short period of time because it's more efficient and less labor intensive than collecting a few fish for eggs and sperm throughout the return. This practice narrows the population dramatically, and abruptly. What's more, the fish produced are those best adapted to being produced under factory conditions, which can include traits like being docile enough to not be damaged by banging around in cement ponds. In each generation, the gene pool is narrowed yet again by this practice. Eventually, they're screwing their sisters.
Another factor one of the guys mentioned, is that the stock introduced to a river may come from a completely different area, and not have the genes required to flourish in the new drainage, due to return timing, and other factors. If allowed onto the redds, these fish may be able to mate with wild stock, but do not pass on the the best genes for long term health of the run.
I realize that there are a lot of guys on the forum with more expertise than me on these issues, but this is my simplified version of some of the main issues with hatchery supplementation, and leaves out the expense, vulnerability to disease in the factory, and a lot of other reasons that hatcheries are an artificial, short term "solution" to declining populations.
Jim, this is they way it's been explained to me, so forgive me if I'm over simplifying, I don't want tot sound like I'm talking down to anyone on the forum, but this is how it biologists explained the issue to a PE major so I could understand.