Increased foraging effort and reduced chick condition of razorbills under lower prey biomass in coastal Newfoundland, Canada

Abstract

When resources are finite, individuals optimize energy allocation between survival and reproduction to maximize fitness. As food becomes more limited within foraging ranges of breeding colonies, long-lived seabirds are predicted to increase time spent foraging, and chick growth, breeding success and ultimately adult survival, will be reduced. Across 3 years (2017, 2019, 2020), we investigated responses of breeding razorbills Alca torda in coastal Newfoundland, Canada, to inter- and intra-annual variation in biomass of the primary forage fish, capelin Mallotus villosus , which migrate from offshore into coastal regions during the summer to spawn. We combined capelin biomass (hydroacoustic surveys) with chick-rearing razorbill foraging effort (GPS, temperature-depth recorders), dietary proportions (blood δ 15 N and δ 13 C) and adult/chick body condition. Peak capelin biomass was near the long-term average for the study area in 2017 (0.109 g m -2 ) and 2019 (0.098 g m -2 ) and the lowest on record in 2020 (0.002 g m -2 ). In 2017 and 2019, razorbill foraging effort decreased after spawning capelin arrived inshore, but this decrease was not observed in 2020 (mean ± SE trip distance: 12.8 ± 2.8 km in 2017, 14.3 ± 1.7 km in 2019, 150.4 ± 25.2 km in 2020; number of dives per trip: 48.4 ± 11.8 in 2017, 385.8 ± 67.4 in 2020). Increased foraging effort during 2020 was associated with lower chick and adult body condition, likely due to parental birds flying farther and diving more to capture capelin. Findings suggest that lower capelin biomass observed in recent years may have long-term population-level consequences for razorbills and other seabirds in this region.