The relative benefts of group foraging change as animals grow. Metabolic requirements, competitive abilities and predation risk are often allometric and infuenced by group size. How individuals optimise costs and benefts as they grow can strongly infuence consumption patterns. The shoaling fsh Sarpa salpa is the principal herbivore of temperate Posidonia oceanica seagrass meadows. We used in-situ observations to describe how ontogeny infuenced S. salpa individual feeding behaviour, shoaling behaviour and group foraging strategies, and its potential consequences to seagrass meadows. Shoaling was strongly infuenced by body length: shoals were highly length-assorted and there was a clear positive relationship between body length and shoal size. Foraging strategies changed dramatically with shoal size. Small shoals foraged simultaneously and scattered over large areas. In contrast, larger shoals (made of larger individuals) employed a potentially cooperative strategy where individuals fed rotationally and focused in smaller areas for longer times (spot feeding). Thus, as individuals grew, they increased their potential impact as well, not merely because they consumed more, but because they formed larger shoals capable of considerably concentrating their grazing within the landscape. Our results indicate that ontogenetic shifts in group foraging strategies can have large ecosystem-wide consequences when the species is an important ecosystem modifer.