Krill moult roughly every two to four weeks through their lives, and the moult determines size. In extended food shortage, particularly through the Antarctic winter when phytoplankton is scarce, krill emerge from a moult smaller than they went in. Laboratory and field observations both record this negative growth, and it can continue over many moults.
The strategy trades reproduction for survival. Shrinking reduces metabolic demand and allows the animal to persist for months on little food, and krill also draw on stored lipid and graze on ice algae attached to the underside of sea ice. Reproductive tissue regresses in the process, so a shrunken krill can superficially resemble a juvenile.
The practical consequence is that body length is unreliable for ageing. Because size can go down as well as up, length-frequency methods that work for fish misread krill populations, and this is one reason estimates of krill lifespan and biomass have been revised repeatedly. Antarctic krill are also a fished species, with catches concentrated in the south-west Atlantic sector.