New I'm curious why can't they just put the fish into a suitable river??
So you want to buy juveniles of large species of fish and grow them up, and when they are too big to care for, release them into the local river?
Introducing new species and or foreign species into an ecosystem can destroy that ecosystem. The introduced animal/ fish can eat all the native fishes and destroy the fishing industry (eg: redtail catfish, arowana, big predatory cichlids). They can cause an imbalance in the system and this can allow some species to thrive and other species to struggle. The native species usually suffer and become extinct.
They can dig up the substrate and destroy plants and cause massive erosion of river banks (eg: carp, plecostomus). This can lead to flooding in areas that have never been flooded, or drought in areas that use to have lots of water.
They can attack people (piranha, snakeheads, pike). People have released predatory fishes into rivers and those fish have survived and bred. When local residents go swimming at their creek, they get attacked by the introduced species.
They can introduce new diseases into that ecosystem that can wipe out the endemic species. In the south-west of Western Australia is the Goodga River. It was home to Galaxias truttaceus and a couple of other species of fish and freshwater crayfish. This river is miles from anywhere and the fish have never been exposed to common fish diseases. Over the last 20 years, people have released introduced species (goldfish, rosy barbs, some cichlids, etc) into this river and the native fishes are all dying from the new diseases they are being exposed to. The native fish could become extinct in the next 10 years, because of introduced species bringing in new diseases.
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If we look at other unwanted introductions, there are numerous species that have been introduced into new areas and they have done considerable damage to the environment. Cane Toads were brought into Australia about 60 years ago. They were meant to control cane beetles. It turns out they don't like cane beetles but do like native frogs and other native organisms. None of the native animals have ever been exposed to Cane Toads so they just think it is a big frog and try to eat it. The Cane Toad secretes a poison, which kills the native animal. Australia has lost millions of native animals just to Cane Toads. We even lose domestic pets who grab the toads and die from the poison.
Cats are another introduced species. In Australia there is an estimated 80 million feral cats living in the bush. There are only 25 million people here. If each cat kills and eats 1 native animal per day, we are losing 80 million native birds and animals every single day of the year. It works out to about 29,200,000,000 animals and birds killed every year just in Australia. I don't normally deal with numbers that have that many 0s, but I think that number is 29 billion. That's an astonishing amount of animals to lose and explains why so many native animals here are threatened with extinction.
Rabbits were introduced here 200 years ago and have wiped out native plants and caused the deserts to spread by eating all the native vegetation and digging up the soil. They breed prolifically when there is rain and after the food runs out, millions of rabbits starve to death.
In Tasmania (island off the south-east coast of Australia) is a small native bird called a Swift Parrot. About 20 years ago someone introduced sugar gliders to Tasmania. The sugar gliders are going into the parrot nests at night and killing the adults and baby parrots. The parrots are on the endangered species list.
There is an introduced plant here called the African daisy. It has spread through the bushland and is smothering native orchids, many of which are on the endangered species list due to habitat destruction caused by people digging up the bushland. When the orchids disappear, the native bees that rely on them will disappear too. People also spray herbicide on the native plants and kill them.
In Papua New Guinea, Tilapia were introduced into Lake Wanam for the locals to eat. The locals hate Tilapia and won't eat it. The Tilapia have destroyed the native fish populations in the lake and most of the species from Lake Wanam are now either extinct or on the endangered species list.
My big hate is humans. Everywhere people go they destroy everything. The human race has relocated itself around the world and is destroying the environment. Yes I am human, at least the doctor says I am, but people are introducing themselves to new areas and decimating those new areas. Papua New Guinea is being deforested and dug up, and thousands of species are being wiped out. Some of these species are known to science and others are undescribed species. People are destroying plants and animals that could potentially be a cure to cancer, or any other disease, and people just don't care.
Introduced species can directly compete against native species for food, breeding sites, habitat and other resources. They can introduce new diseases into that environment. Many have no natural predators in the new environment and can breed out of control and destroy everything in the area. Introducing new species to an ecosystem should never be done.