Is the seasonal growth of Strangomera bentincki (Clupeidae) and Engraulis ringens (Engraulidae) a consequence of their reproductive strategy to adapt to the seasonal upwelling ecosystem they inhabit? This question is addressed by analysing monthly length-frequency data, gonadosomatic index and condition factor of the species in relation with the seasonal patterns of environmental variables. Modal progression analysis of mean length-at-age of cohorts along the time axis was used to study the growth in the period 1990–1997. A seasonally oscillating growth curve was estimated for both species, with the slowest growth rate occurring between April and May, a few months before the higher reproductive activity occurring in August–September. The reproductive strategy is to spawn when environmental conditions are related with onshore transport in winter (August), favouring the concentration and retention of eggs and larvae. One month later, a moderate upwelling determines an enrichment in food particles and the spawning area is transformed in a nursery area for juveniles. The reproductive strategy is combined with an ‘energy storage strategy’ during the period of upwelling. The energy stored is used for reproduction several months later, affecting the growth process of the species. It is concluded that the regularity in the seasonal growth in both species is a response, from an evolutionary point of view, of a long-term reproductive adaptation to the seasonal upwelling ecosystem of the central southern area off Chile.