We investigated the relationships between stock biomass, distribution area and mean density of sardine and anchovy populations off California, Peru, South Africa and Japan. Our objective was to elucidate whether their ecological responses to habitat availability and use would support the possibility of them developing synchronic, alternating biomass fluctuations. Results indicate that as populations of both species grow in size, both the area they occupy and their packing density increase, generally consistent with the basin model. The relationship between distribution area and stock biomass is allometric, which implies that there is a limit to the stock expansion to new areas. Patterns of space occupation appear to differ between sardine and anchovy in some regions. In South Africa and Japan anchovy occupies larger area per unit biomass than sardine, consistent with habitat requirements as determined by their feeding ecology. Off California and Peru results confirm that in general species expand their distribution area with stock size, but specific patterns are less clear. Reasons for this are discussed, including sampling biases, the role of upwelling in limiting anchovy's habitat off Peru, the ability of sardine to use offshore habitats off California, as well as possible differential space occupation patterns during population growth and decline. It is suggested that habitat availability may not be a pre-requisite for sardine growth in some areas, while anchovy may require habitat to become available for populations to grow. The differential habitat dependency and use of space between both species suggests that they cannot be considered to ecologically replace each other. While differences in space utilization may provide opportunities for diverging population paths, the ecological mechanisms behind out-of-phase fluctuations may be much more complex than a simple replacement. The results provide the first comparative, quantitative estimates of biomass/area relationships for pelagic fish, of use in models investigating the potential for expansion and contraction of anchovy and sardine populations worldwide based on climate change-driven habitat predictions. © 2009 Elsevier Ltd.