Diets and distributions of Leach's storm-petrel (Oceanodroma leucorhoa) before and after an ecosystem shift in the Northwest Atlantic

The Grand Bank ecosystem has undergone significant shifts during the past two decades owing to oceanographic and fishing effects. Effects on upper trophic level seabirds (dietary shifts, reduced reproductive performance) have been mediated through changes in the biology and behaviour of capelin (Mallotus villosus (Muller, 1776)), the focal forage species. To explore for effects at lower trophic levels, we combine dietary (1987-1988, 2003-2006) and distributional (1966-1990, 1998-1999) data for Leach's storm-petrel (Oceanodroma leucorhoa (Vieillot, 1818)), a small, abundant, and highly pelagic seabird. Fish and crustaceans formed the bulk of nestling diet at two colonies, with fish dominating in all sampling periods and years (occurrence >70%, reconstructed mass >75%). Five families were represented, but mature myctophids (glacier lanternfish (Benthosema glaciale (Reinhardt, 1837)), homed lanternfish (Ceratoscopelus maderensis (Lowe, 1839)), Protomyctophum arcticum (Lutken, 1892)) and sandlance (genus Ammodytes L., 1758) dominated. Crustaceans occurred frequently but typically comprised <= 10% by mass; Hyperia galba (Montagu, 1813) dominated this prey class. General diet composition was similar through time with birds relying heavily on myctophid fishes in 1987-1988 and 2003-2006. Crustacean diversity, however, declined with fewer species of hyperiid amphipods and no small euphausiids (genus Thysanoessa Brandt, 1851) consumed in 2003-2006. The latter parallels changes in spring diets of capelin and winter diets of murres (genus Uria Brisson, 1760) in the region. Associations of storm-petrels with deep water are consistent with the predominance of mesopelagic prey in their diets.