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Pursuit-diving seabird endures regime shift involving a three-decade decline in forage fish mass and abundance

William A. Montevecchi*, Kara Gerrow, Alejandro D. Buren, Gail Davoren, Keith Lewis, Marina W. Montevecchi, Paul M. Regular

*Corresponding author:

ABSTRACT: Forage species availability is a key determinant of seabird success, survival and population change. In the Northwest Atlantic Ocean capelin Mallotus villosus, the keystone forage species, experienced a stock collapse in the early 1990s, which was a pivotal component of a regional regime shift. Since then, capelin have exhibited delayed protracted spawning, younger spawning age, distribution shifts and smaller size. As capelin specialists, pursuit-diving common murres Uria aalge at the species’ largest colony have had to adjust to these changes. We show that the masses of capelin they provisioned to chicks declined steadily from 1990 to 2017. We predicted that the parental provisioning of lower quality prey would reduce offspring condition, lower parental body mass and increase foraging effort. Offspring condition declined, and while no negative effects were found on adult body mass, parental murres worked substantially harder when (2016) capelin were dispersed and availability was low and when offspring and parental mass were the lowest in the time-series. These circumstances suggest that the murres neared a behavioral tolerance of parental effort. Despite the multi-decadal order of magnitude reduction in the regional capelin stock, parental murres coped by exploiting local prey availability at persistent shallow-water spawning sites and by increasing foraging effort. Even while the keystone forage fish stock remained at extremely low levels, the murre population increased, a likely consequence of enhanced adult survival due to amelioration of anthropogenic risk factors.