The correct answer is option 3: depletion of oxygen. During summer, the water temperature increases, which reduces the amount of dissolved oxygen in the water. Fish rely on oxygen to survive, so when there is a depletion of oxygen, it can lead to a higher mortality rate among fish. As the water warms up, the metabolic rate of fish also increases, causing them to require more oxygen. If the oxygen levels in the water are already low or become depleted due to factors such as algal blooms or excessive organic matter decomposition, the fish may not be able to get enough oxygen to survive. This can result in mass death of fish during the summer months.