Standardized sight-resight data of Humpback Whales () in Glacier Bay and Icy Strait, Alaska, have been collected since 1985. We applied closed robust design capture-recapture models to these data to provide inferences about: (1) population size; (2) population growth rate; (3) apparent survival rate; and (4) temporary emigration rate in the region during 1985–2009, while accounting for imperfect detection probability. Population size estimates ranged from a low of 49.8 (95% CI: 44.3–64.4) in 1986 to a high of 181.1 (95% CI: 173.5–196.2) in 2009, and the geometric mean of year-to-year changes in abundance suggested a rate of population growth over the 25-y period of about 4.4%/y. We estimated the annual survival rate to be 0.948 (95% CI: 0.936–0.957). We estimated the probability of temporarily emigrating from the study area to be 0.106 (95% CI: 0.086–0.128), and the probability of a temporary emigrant remaining outside the study area as 0.777 (95% CI: 0.712–0.830). Our results provide new insights into the status and dynamics of this endangered species in and around a large marine protected area and highlight the value of intensive long-term population monitoring efforts.