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Marine Ecology Progress Series

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MEPS 698:155-170 (2022)  -  DOI:

Intra- and inter-annual shifts in foraging tactics by parental northern gannets Morus bassanus indicate changing prey fields

Kyle J. N. d’Entremont1,*, Gail K. Davoren2, Carolyn J. Walsh1, Sabina I. Wilhelm3, William A. Montevecchi1

1Cognitive and Behavioural Ecology Program, Psychology Department, Memorial University of Newfoundland, St. John’s, Newfoundland and Labrador A1C 5X7, Canada
2Department of Biology, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada
3Canadian Wildlife Service, Environment and Climate Change Canada, Mount Pearl, Newfoundland and Labrador A1N 4T3, Canada
*Corresponding author:

ABSTRACT: Seabirds are constrained by central-place foraging during breeding, when the energy obtained from prey must outweigh the costs of travel, search, capture and transport. The distribution and phenology of the cold-blooded marine fishes they exploit are heavily influenced by oceanic climate. Northern gannets, the largest breeding seabird in the North Atlantic, use a generalist foraging strategy, preying on a wide array of pelagic fishes. They employ different foraging tactics for different prey types, with rapid, shallow V-shaped dives used for large, powerful prey such as mackerel, and U-shaped dives for smaller forage fishes like capelin. Here we assess intra- and inter-annual differences in foraging effort and influences of prey availability at the southernmost colony of the species at Cape St. Mary’s, Newfoundland, Canada. We compared foraging trip characteristics (total and maximum distance, directness, duration and number of dives) of parental gannets during the breeding seasons of 2019 (n = 10) and 2020 (n = 7) using GPS/time-depth recorders. Individual gannets shifted away from using U-shaped dives in early chick-rearing to primarily V-shaped dives in late chick-rearing. Shifts were abrupt and occurred in mid-August in 2019 and 2020. Maximum and total foraging trip distance and duration were significantly greater during early chick-rearing in 2020 than in 2019. Kernel density 50% utilization distributions were larger and expanded further from the colony during early chick-rearing in 2020 (7297 ± 1419 km2; mean ± SE) than 2019 (2382 ±797 km2). Increased foraging effort during early chick-rearing in 2020 was likely due to decreased capelin availability, resulting from earlier spawning, and greater variation in the timing of spawning among sites, which may have been influenced by warmer waters.

KEY WORDS: Northern gannet · Foraging behaviour · Prey availability · Prey switching · Dive profiles · Forage fish

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Cite this article as: d’Entremont KJN, Davoren GK, Walsh CJ, Wilhelm SI, Montevecchi WA (2022) Intra- and inter-annual shifts in foraging tactics by parental northern gannets Morus bassanus indicate changing prey fields. Mar Ecol Prog Ser 698:155-170.

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