 Complete the following phrase:  Tonelli took up swimming due to his asthma, and quickly came to prominence. Selected to represent Australia at the 1973 World Championships, he came sixth in the 200 m backstroke at the age of 16. He won his first Australian titles in 1974 in the 100 m backstroke and 200 m butterfly and went on to the 1974 Commonwealth Games in Christchurch, where he won his first major international race, the 100 m backstroke, and took silver in the 200 m backstroke. In 1975, Tonelli won his only individual medal at global level, a silver in the 200 m backstroke at the 