Pelagic fish recruitment success and reproductive strategy in upwelling areas: environmental compromises

Past work has shown that indices of recruitment or abundance of pelagic fish in eastern boundary currents can be both negatively and positively related to upwelling intensity. Such results question the existence of a unified theory relating recruitment with the environment in upwelling areas. However, positive and negative correlations may both be valid if the relationship between recruitment and upwelling intensity is dome-shaped, as suggested by the "optimal environmental window" hypothesis. New evidence for such a relationship is presented for the Moroccan sardine Sardina pilchardus and the northern anchovy Engraulis mordax. Recruitment success increases with upwelling intensity when wind speed is low to moderate. In areas or periods of strong wind, recruitment success decreases with upwelling intensity. Using a comparative approach, the spatial and temporal reproductive dynamics of West African coastal pelagic fish are investigated. In some areas, the spawning season coincides with the upwelling season, but in other areas spawning and upwelling are out of phase. High reproductive activity coincides with the seasonal occurrence of wind speed of about 5–6 m·s−1, which is the value defined as the "optimal environmental window". This pattern is hypothesized to be the response, from an evolutionary point of view, of a long-term reproductive adaptation to the environment that maximizes recruitment success.