Question

If someone adds millions of small fish to a lake, how would the number of big fish change

Answer 1

It will throw the food chain/food web off to where the the small fish population will out numbered the big fish population and I know certain animals eat the big fish like bears and if they get out numbered then the bears won't really have anything else to eat and will eat other organisms like berries and other animals. It will completely throw off the food chain. 

Hope this helped :)
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Answer 2

The big fish population will increase because they would have a lot of small fish to eat