The Kemp's ridley sea turtle is an endangered species with limited distribution in the Gulf of Mexico due to near extinction in the 1970s. Intensive conservation efforts on the nesting beach and fishing gear improvements that reduced at sea mortality appeared to have a strong positive effect on the population over the past 20 years. Population growth rates were exponential from 1995 to 2009, with annual rates of 15-18%. This recovery came to an abrupt halt in 2010, when the Deep Water Horizon oil spill coincided with the nesting and migration season of the species. I am updating an age-structured model that I developed for the species' Recovery Plan to determine the projected number of turtles that would have nested if the recovery trend had persisted, and testing a variety of hypotheses for why the population dropped abruptly and has failed to rebound. Importantly, population momentum from 20 years of increasing cohorts limits the number of possible scenarios that can fit the available time series, even when density dependence is incorporated into the model.