Northern Gannets (Morus bassanus) breeding at their southern limit struggle with prey shortages as a result of warming waters

Abstract

AbstractNorthern Gannet (Morus bassanus) colonies near the species’ southernmost limits are experiencing plateaued or declining population growth and prolonged poor productivity. These trends have been linked to reductions in the availability of the species’ key prey, the Atlantic mackerel (Scomber scombrus). Declines in mackerel availability have been associated with warming ocean temperatures and over-fishing. Here, we assessed the influence of prey availability, abundance, and sea surface temperature (SST) during the breeding season on Northern Gannet reproductive success over a multi-decadal time span at their southernmost colony at Cape St. Mary's, NL, Canada. We demonstrate that warming SST affects reproductive success differently in early vs. late chick-rearing, but that overall, declining mackerel availability (landings and biomass) due to warming SST and over-exploitation has resulted in poor productivity of Northern Gannets at their southernmost limit. Our study is consistent with previous findings in other colonies in Atlantic Canada and France, and contrasts with findings in more northern colonies where mackerel population increases and range expansion are coinciding with gannet population growth. This implies that warming SST is having opposing influences on Northern Gannets and mackerel at the different extremes of the gannets’ breeding range.