Like here with the sacramento river. The upper river by oroville has a dam. He who controls the flow of water will control some species. The flow of water is needed for certain fishes to spawn and migrate. There is a decline in the salmon population and other fishes that depended on the sacramento river to flow at its peak like the American Shad. By slowing the flow of water, a species of fish thrive as it produces the most optimal conditions for their own population blooming growth, the invasive sacramento pike minnow.
Even in my country, without conservation, the nets are getting bigger and the catches are getting smaller.