Developing seabird bioindicators for the Gulf of Maine: a demographic study of an Arctic Tern (Sterna paradisaea) metapopulation

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University of New Brunswick
As climate change proceeds, ecosystems are changing rapidly. Marine ecosystems are complex and difficult to monitor, making the prediction of future changes a daunting task. Seabirds are often suggested as potential bioindicators, yet the development of their data for general predictive use is rare. I, using the data of many collaborators, examined the potential use of Arctic Tern (Sterna paradisaea) diet and demographic data as bioindicators in the rapidly warming Gulf of Maine (GOM) ecosystem. The collapse of the breeding colony at Machias Seal Island, formerly the largest in North America, was primarily driven by unmitigated egg predation by large Larus gulls, not a decline in Atlantic herring (Clupea harengus). At the metapopulation scale, reproductive success of Arctic Terns was subject to multiple environmental interactions. The proximity of a colony to shore was associated with higher diet quality, but also greater predator pressures. Interactions between diet quality, weather, and predator behaviour strongly influenced the reproductive success of terns. At the ecosystem scale, the diets of Arctic and Common Terns (S. hirundo) could be combined with environmental data to describe and track ecosystem states. Arctic Terns were more likely to consume white hake (Urophycis tenuis) and small marine invertebrates, but their dietary trends also tracked measures of the herring stock and fishery. Common Terns were more likely to target high-lipid fish, but they showed stronger spatial trends that limited regional inferences. Finally, I estimated rates of survival, dispersal, and return rates. Arctic Terns have strong dispersal behaviour and regularly leave the major colonies of the GOM, but adult survival has not changed, despite major warming. Multiple types of tern data indicated that the weakest period of the past 30 years was between 2004-2012, when salinity was reduced and small copepods like Centropages typicus were less abundant. Arctic Terns were able to react to major declines in habitat quality, and have adjusted well to recent warming. Although Arctic Terns are affected by both top-down and bottom-up forcing, their diet and demographic data are useful as indicators. Terns could function well as ecosystem, guild-, or site-specific indicators, depending on the desired use.