If people don't log properly, dirt and mud can erode into streams, rivers, lakes, and oceans. This harms fish because they can't breath when there's a lot of soil in the water. Now most loggers are careful when they log, and they do not harm fish. Now loggers don't let logs roll down into rivers and streams. They cut only a few trees at a time and they replant new trees so the soil won't wash away.
Dams can also be a problem to fish because they stop the fish from migrating up stream.
But people have built fish ladders so the fish can get up stream. They also raise fish in hatcheries to replace the fish populations that have been lost through dams. Cattle grazing can cause problems, too. Cattle are a problem when they graze too close to a stream. They can cause disease in the water and trample vegetation.