It has long been established, indeed it is almost axiomatic, that annual variability in births of new fish (“recruitment”) is the most important reason why the total mass of fish populations varies from year to year. The rate of which individual fish grow (“body growth”) is also known to vary from year to year, but is generally considered to be fairly unimportant in explaining population variability. Now, though, a new study examines the contribution that recruitment and body growth play in fish population variability, finding that for 2 of the 8 fish populations examined, fish biomass varied more because of body growth, while for 5 of the 8 fish populations, fish biomass varied more because of recruitment. Thus more attention should be paid to measuring annual changes in body growth, since this is potentially a key part of understanding why fish biomass varies so much from year to year. The new work by SAFS graduate student Christine Stawitz and SAFS professor Timothy Essington, appears in the Journal of Animal Ecology.