It all depends on which type(s) of Carp you released. There are a few different ones. Some depend mostly on vegetation while others use the same resources that native fish fry use which is why they suffer from them being there. The Carp are a hardier fish and out compete the other fish for the food.
You can read this article that explains it.
Asian Carp Overview - Mississippi National River & Recreation Area (U.S. National Park Service)
Hope this helps
I'm not truly sure that's the reason. Some types of aquacultured fish are sterile to begin with and I believe that's the reason your Sister-in-law's fish have not reproduced. The following is an excerpt from the Warnell school of Forest Resources ( in GA, USA) in regard to using sterilized Carp for weed control in lakes. The grass carp (Cteno pharyngodon idella) occurs naturally in large rivers of the eastern USSR and China. It was introduced into the United States in 1963 by the United States Bureau of Sport Fisheries and Wildlife in cooperation with Auburn University. The feeding habits of the grass carp were well known and it was thought to have great potential as a biological weed control agent. However, there was concern that the grass carp could reproduce in the wild and become an environmental nuisance, destroying valuable areas such as wetlands, swamps and waterfowl feeding grounds.
Yes del, lionfish are saltwater but the concept is what's being compared. Lionfish were released into the Atlantic Ocean by hobbyists and since they had no natural predators in the Atlantic, they flourished and now are found in many areas of the Atlantic Ocean and seem to be uncontrolable at this point. They have damaged the natural ecosystems by eating the foods ( fish) that other native species need to survive. This is similar to what's happening with the goldfish.
And there seems to be no way to eradicate them as well. I recently saw a program where someone in a mini sub found one a few hundred feet down ( maybe even a mile down, I forget) so they are living in many areas where noone goes as well. This is what could also happen with any introduced species that can take over an ecosystem.
While this may be an encouraging sign, there are questions that I didn't see answers to. Such as What was the after effect to the grouper from eating the venom of the lionfish?
Second point: It may be that the groupers are now eating the Lions because there is no other food fish available anymore so "desperate times call for desperate measures" but if the grouper had any ill effects digesting the fish, I doubt it would eat another one.
Third point: These groupers are native to the areas where Lionfish first were introduced. If they were the answer, why did the lions get so out of control in the first place?
Just some thoughts...
See what happens when there are no natural predators?
