True...but what are the odds that someone can sucessfully get a female Wels and a male Wels of sexual maturity and two separate people just both so happen to release them into the same body of water....don't know about you...but once your dedicated and invested in the hobby and spend $100s of dollars on these fish the thought of someday releasing that fish you own never even crosses your mind...it would be more likely that the a plane carrying the "invasive species" from overseas crashes into a body of water and introduces the fish that way than an actual hobbyist releasing a single specimen into the wild....people who keep the wels,the perch and others don't tend to be the type to release the fish into the wild...people who tend to release fish into the wild tend to be inexperienced novices who don't know what they are doing and don't want to clean little Johnnys fish bowl any more