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This study reveals the correlation between operational sex ratio (OSR), body characteristics and pairing in one-year-old reproductively active St. Peter's fish, Sarotherodon galilaeus, a mouth brooding tilapia that shows a wide variety in mating strategies including uni- vs biparental brood care, and monogamy vs polygamy. In this study individually marked fish were exposed to different OSR regimes: male-biased, balanced, and female-biased. Larger individuals formed a pair more rapidly than smaller ones. A bias in the OSR caused a longer delay in pairing for individuals of the more abundant sex. The same patterns were found in males and females, supporting the theory that assortative mating patterns emerge due to both male and female choice. Furthermore, we found that pairs that spawned during the experiment were more closely matched in body size than pairs that were not successful in spawning, suggesting that size assortative mating has benefits for reproductive success in S. galilaeus. [ABSTRACT FROM AUTHOR]