A large amount of untreated sewage entered the river. Many fish died. Untreated sewage contains organic matter and bacteria. Explain why many fish died. (5)

Sewage entering the water caused plants to die.Bacteria then decayed this dead organic plant matter (1) through digestion (1).These bacteria will carry out aerobic respiration, using oxygen (1).This therefore lowered the oxygen concentration in the water (1).Therefore this meant fish could no longer carry out sufficient aerobic respiration, reducing their energy production and therefore they died (1).