The Researches talk about a "Balance" within the Ecosystem that Exists within the
Stream Ecosystem. With what you know, and saw in the Video, What is the
"Balance" they are talking about when introducing the Grayling back into the
Ecosystem?

the balance is everything must have a process of living and dying in an ecosystem. such as, an animal killing another animal for prey to eat it, that is a process. in a water ecosystem, fish lay eggs and come out and usually get eaten by a bigger animal, as they grow up, usually by bigger fish itself such as the Grayling.

Explanation:

climate change and pollution

only about ten percent of the energy at one trophic level is passed on to organisms at the next trophic level. the remainder of the energy is used in life processes or lost as heat.