Anchovies and sardines are some of the most economically and ecologically important and well-studied fishes on Earth, but there is still uncertainty regarding how distributions and abundances change through time and space. We bring together larval abundance data for northern anchovy ( Engraulis mordax) and Pacific sardine ( Sardinops sagax) collected by United States and Mexican scientists over 50 years (1963–2015) to test the Basin and Asynchrony hypotheses. The Basin hypothesis states that a species’ geographic range and spawning area ( R) increase with overall abundance ( A) according to a power law, R =  aAb , where the exponent ( b) is less than ∼0.5 when the rate of increasing area occupied saturates as population size increases. The Asynchrony hypothesis postulates that anchovy and sardine abundances are negatively correlated through time. We found that the Basin hypothesis was supported for both species but the Asynchrony hypothesis was not during this 53-year period. Due to collaboration between US and Mexican scientists, we were able to better understand how two important fishes utilize their environment.