In Satya-yuga, Lord Narayana was worshiped by meditation (krte yad dhyayato visnum). Indeed, everyone always meditated upon Lord Visnu, Narayana, and achieved every success by this process of meditation. In the next yuga, Treta-yuga, the performance of yajna began (tretayam yajato mukhaih). Therefore this verse says, trayi treta-mukhe. Ritualistic ceremonies are generally called fruitive activities. Srila Visvanatha Cakravarti Thakura says that in Treta-yuga, beginning in the Svayambhuva-manvantara, ritualistic fruitive activities were similarly manifested from Priyavrata, etc.