Surveys have shown theat a square yard of healthy Alaska stream gravel may support 5000 benthic macroinvertebrates. Energy from the sun travels through a stream from dead leaves or algae, through the insects to the fish. Basically, if you have a stream that cannot produce insects it also cannot produce fish. Invertebrates also need relatively silt free water. If a stream carries enough silt to fill in the crevices between rocks, then the insects have nowhere to escape from the current of the stream.
Water can hold less dissolved oxygen as temperature increases.
This can be critical for returning salmon. Elevated water temperatures can lower the dissolved oxygen level so far that there may not be enough oxygen in the water to support the number of fish present. Temperature is also critical to the hatching of fish eggs. The warmer the water, the less time it takes fish eggs to mature and hatch. Over tens of thousands of years, runs of salmon have adapted to specific stream or river temperatures to assure that their eggs hatch at a time when feed is plentiful in the watershed.