Overfishing has led to declines in many populations of Caribbean queen conch Strombus gigas, leading to its listing on CITES Appendix II. No‐take marine protected areas (MPA) are increasingly being used as a conservation tool for protecting stocks and sustaining fisheries. A reaction–diffusion model was developed to study queen conch population responses to various sizes of MPAs, differing spatial and temporal fishing mortalities, and fluctuating recruitment rates. Movement behaviour and demographic data from a protected conch population at Glover's Reef, Belize, were used to parameterize the model and assess simulation results. The model predicted increases in density ranging from 300% in an MPA encompassing one‐tenth of the total habitat area to 450% in an MPA encompassing half of the habitat, agreeing with field observations. Queen conch stock in the fishing zone was predicted to decline 6‐40% with MPAs encompassing proportions of 0.1–0.5 of habitat area. Spillover estimated from conch movement was expected to reduce these losses and supplement the fishery by 2–4%. Sensitivity analysis of fluctuating recruitment showed that larger reserves can sustain population buildup even with lower recruitment rates. Relatively small increases in recruitment rates can potentially reduce fishing losses to 0 from MPA closures. Opening MPAs to fishing periodically or permanently is expected to rapidly deplete the conch population in a fraction of the time necessary for the initial buildup at the current fishing mortality rate. These results support those of previous modelling studies but also showed complex dynamics for this case study. While MPAs reduce the stock available to the fishery, spillover can supplement the fishery, and increases in recruitment could increase fishery yield beyond pre‐MPA levels. With high fishing mortality, it would not be prudent to open MPAs to fishing for queen conch in any situation. Copyright © 2012 John Wiley & Sons, Ltd.