Increasing goose populations in the northern hemisphere during the past 50 years have generated human-goose conflicts regarding agricultural damages and various ecosystem effects. The population increase, and the marked southward breeding range expansion of Barnacle Geese Branta leucopsis into temperate regions in northwestern Europe, motivated the adoption of an AEWA International Single Species Management Plan for the species. To protect the population at internationally important key sites and, if necessary, grant derogations to prevent damage, the management plan emphasised the need for updated estimates of the size of different population segments, data on demographic parameters, and assessments of the role of predators in regulating populations. In this study, the increase, and subsequent 98% decrease, of the breeding population of Barnacle Geese on the islands of Gotland and Öland, Sweden, in the central Baltic Sea during the 55-year period from 1971 to 2025 is analysed. Previous analyses of demographic parameters performed during the increase phase are supplemented by analyses of reproductive success and survival during the decline phase. The first breeding pair in the Baltic Sea region was recorded on Gotland in 1971. Thereafter, the total number of breeding pairs on Gotland and Öland increased rapidly to c.5,290 breeding pairs in 2002. At that point these birds constituted the major part of the species Baltic population. Thereafter, the number declined rapidly to only c.100 breeding pairs in 2025. Reproductive parameters such as clutch size and production of fledged young declined or remained low during the decline phase, but the lower reproductive output could not alone explain the decline of breeding pairs. Instead, declining adult survival rates due to intensive predation by increasing numbers of White-tailed Eagles Haliaeetus albicilla, in combination with increased natal dispersal rates, were found to be the major cause of the steep decline.