1. Marine Protected Areas (MPAs) can be effective tools for marine resource management. However, despite evidence of the positive effects of MPAs, such as increases in body sizes of organisms targeted by fisheries, there is often heterogeneity in biological outcomes among them. 2. Because fishing can drastically impact fish populations, the goal of this study was to determine if the levels of exploitation prior to protection could predict variation in the magnitude of MPA effects. 3. Using a diver operated stereo-video camera system, we compared sizes of fishes targeted by anglers within seven MPAs spanning the Southern California Bight to nearby comparison areas (non-MPAs). We used fine-scale data on pre-closure fishing pressure to test responses of fishes to protection along a gradient of exploitation. 4. Fish size responded to protection in proportion to pre-closure fishing pressure, with MPAs in areas with high pre-closure fishing having greater responses in average lengths and size distributions than those in areas with low fishing pressure. 5. This response was evident in species heavily targeted by recreational fishing, but not in species that were not targeted in fisheries. 6. Synthesis and applications: Pre-closure fishing pressure of an area can impact the efficacy of MPAs, and, when available, should be considered when predicting and evaluating MPA performance. Prioritizing heavily exploited areas for protection when implementing spatial management tools could maximize ecological outcomes.