In Portugal, Mesolithic shellfisher/gatherers persisted in estuary environments, long after agricultural economies were established elsewhere. The co-existence of foragers and farmers makes Portugal an interesting region in which to study whether resource depression is a common factor in the adoption of agriculture. It is difficult to generalize about Mesolithic and Neolithic resource use, since faunal remains at archaeological sites reflect the variation in terrestrial and marine species available to inhabitants of rocky shores, sandy estuary environments, and interior zones. On the Costa Vicentina, where the transition to agriculture occurred quite early, assemblages from a variety of Mesolithic and Neolithic sites show a trend toward lower energetic returns from wild resources, both in shifting species compositions, and in shellfish diminution. The most significant decrease in foraging efficiency occurred across the Mesolithic/Neolithic transition, suggesting a causal relationship between resource stress and the emergence of farming.