If fish decline in a river system, why can water birds be affected?

Correct answer: Because fish can be part of their food in the food chain

The direct answer is C. Fish can be part of the food chain on which water birds depend. A food chain shows how energy and food move between organisms: aquatic plants or small organisms may support small fish, larger fish may be eaten by birds, and birds may also eat other aquatic animals. If fish decline, birds that eat them may have less food, and the wider aquatic ecosystem may be disturbed. Option A is wrong because water birds do not eat only rocks; rocks are not their normal food. Option B is wrong because a fall in fish numbers does not mean a river always becomes a desert. Option C is correct because fish may be prey for birds and are an important link in some aquatic food chains. Option D is wrong because birds do need water for drinking, bathing, habitat and food, even though their exact needs vary. The effect is not necessarily identical for every bird: some species can change food, but serious fish loss can still reduce survival or breeding. Memory cue: remove a food-chain link, and dependent organisms may suffer.