Abstract
Populations of marine top predators are exhibiting pronounced demographic changes due to alterations in prey availability and quality. Changes in diet composition is a key potential mechanism whereby alterations in prey availability can affect predator demography. Studies of long-term trends in diet have focused on the breeding season. However, long-term changes in non-breeding season diet is an important knowledge gap, since this is generally the most critical period of the year for the demography of marine top predators. In this study, we analysed 495,239 otoliths from 5888 regurgitated pellets collected throughout the annual cycle over three decades (1985–2014) from European shags Phalacrocorax aristotelis on the Isle of May, Scotland (56°11′N, 02°33′W). We identified dramatic reductions in the frequency of lesser sandeel Ammodytes marinus occurrence over the study, which was more pronounced during the non-breeding period (96% in 1988 to 45% in 2014), than the breeding period (91–67%). The relative numerical abundance of sandeel per pellet also reduced markedly (100–13% of all otoliths), with similar trends apparent during breeding and non-breeding periods. In contrast, the frequencies of Gadidae, Cottidae, Pleuronectidae and Gobiidae all increased, resulting in a doubling in annual prey richness from 6 prey types per year in 1988 to 12 in 2014. Our study demonstrates that the declining importance of the previously most prominent prey and marked increase in diet diversity is apparent throughout the annual cycle, suggesting that substantial temporal changes in prey populations have occurred, which may have important implications for seabird population dynamics.
Highlights
Marine environments are changing rapidly across the globe due to a range of anthropogenic activities, including pollution, overfishing and climate change
Due to the large number of prey types and models, we only report those models within 10 AICc points of the top model, which is shown in bold
We identified dramatic changes in the diet composition of full-grown European shags Phalacrocorax aristotelis
Summary
Marine environments are changing rapidly across the globe due to a range of anthropogenic activities, including pollution, overfishing and climate change Our understanding of seabird diet outside the breeding period is largely based on indirect methods such as stable isotopes and fatty acid analysis (Owen et al 2013; Kowalczyk et al 2014) or samples from shot/dead birds (Blake 1984; Harris et al 2015) Such studies have produced valuable insights into non-breeding diet, demonstrating marked differences from the breeding season, owing to a combination of altered prey availability (Kowalczyk et al 2015), energetic constraints (Markones et al 2010), habitat association (Ainley et al 1996) and, in migratory species, altered locations (Ronconi et al 2010). Our specific aims were to: (1) quantify year round diet composition of shags over three decades; and (2) test whether dietary trends differ between the nonbreeding and breeding period
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