Which of these conditions most likely explains the sudden death of fish in a river on a hot summer day?

• A. Reduced dissolved oxygen
• B. Increased predators
• C. Increased algae growth
• D. Reduced food supply

Answer: (A)

The key idea is dissolved oxygen in water and how temperature and fish demand interact. On a hot summer day, water warms and can hold less oxygen in solution, so the available oxygen in the river drops. At the same time, fish become more active and metabolically need more oxygen to fuel their tissues. That combination—less oxygen present and greater oxygen demand—can create a hypoxic environment in which fish can’t get enough oxygen to survive, leading to sudden mass die-offs.

The other scenarios don’t fit as well. More predators would take a toll through predation, not cause a widespread, rapid die-off tied to the water’s oxygen content. Algae blooms can influence oxygen levels, especially as blooms decay or at night, but the immediate, most direct cause on a hot day is the reduced dissolved oxygen. A reduced food supply would lead to starvation over time, not an abrupt death spike on a single hot afternoon.