The gulf menhaden commercial fishery in the Gulf of Mexico is the largest by weight in the United States. Spawning takes place on the continental shelf during fall and winter and the pelagic larvae are transported into estuarine nursery areas. Quantitative information on a transport mechanism had previously been lacking. Knowledge of the coupling between continental shelf and estuaries is necessary to understand the causes of high natural variability in estuarine recruitment and to develop and evaluate spawner-recruit and environment-survival relationships. Analysis of a variety of biological and physical data led to the development of a testable transport hypothesis. The hypothesis suggests that west-northwest longshore advection within the horizontally stratified coastal boundary layer is the primary mechanism transporting gulf menhaden larvae to the Calcasieu River estuary, the major estuarine system in western Louisiana.