The impact of human exploitation depends mostly on the size of the catch and the species targeted. The value of a species is an important explanatory variable in understanding human impact. Co-management of resources should take into account these different resource values, when evaluating exploitation strategies. The prey choice and foraging behaviour of women and children searching for crabs and shells on the intertidal area at Inhaca Island, South Mozambique, were investigated using optimal foraging theory. This theoretical framework offers the possibility to understand the reasoning of an exploitation strategy and the preference for certain prey species. The number of people was registered, catches were analysed, and timing and substrate choice were recorded. The value of species was estimated using contingency tables. Women were more efficient than children, as their catch was heavier, and the mean weight/animal was larger. The density of women and their timing were positively correlated to prey availability. During neap tide, they spread their visit over more of the low water period and collected crabs by digging in the mangrove forests. No digging occurred during spring tide when a larger area was exposed, the total abundance of species increased, and more species became available. Women then switched to a second strategy, targeting swimming crabs in the tidal channel. Mean neap and spring tide catches were equal (133 g ash-free dry weight per person), but spring catches comprised significantly fewer animals per catch (42 against 123 per person), and mean animal weight was larger (5.4 against 3.0 g ash-free dry-weight per person). Diet breadth was narrower during spring tide, and decreased significantly with increased catch weight. Species with profitabilities (energy intake/handling time) lower than the mean intake rate of 0.024–0.028 g ash-free dry weight s−1 were generally excluded from the diet. The prey preference was positively related to the relative value ranks of the prey species, as measured by ranking of species by women. Women maximized the cumulative relative value ranks during spring tide, instead of total weight. Using this analysis, differences in prey choice and spatial differences in exploitation can be understood as a strategy aimed at maximizing intake and the relative value of a prey species.