A stable isotopic investigation into the causes of decline in a sub‐Antarctic predator, the rockhopper penguin Eudyptes chrysocome

Abstract

The rockhopper penguin (Eudyptes chrysocome) is a conspicuous apex marine predator that has experienced marked population declines throughout most of its circumpolar breeding distribution. The cause(s) for the declines remain elusive, but the relatively large spatio-temporal scale over which population decreases have occurred implies that ecosystem-scale, at-sea factors are likely to be involved. We employ stable isotope analyses of carbon ( 13 C/ 12 C, expressed as d 13 C) and nitrogen ( 15 N/ 14 N, d 15 N) in timeseries of rockhopper penguin feather samples, dating back to 1861, in order to reconstruct the species' ecological history. Specifically, we examine whether rockhopper penguin population decline has been associated with a shift towards lower primary productivity in the ecosystem in which they feed, or with a shift to a diet of lower trophic status and lower quality, and we use long-term temperature records to evaluate whether shifts in isotope ratios are associated with annual variations in sea surface temperature. Having controlled temporally for the Suess Effect and for increases in CO 2 concentrations in seawater, we found that overall, d 13 C signatures decreased significantly over time in rockhopper penguins from seven breeding sites, supporting the hypothesis that decreases in primary productivity, and hence, carrying capacity, for which d 13 C signature is a proxy, have been associated with the decline of penguin populations. There was some evidence of a long-term decline in d 15 N at some sites, and strong evidence that d 15 N signatures were negatively related to sea surface temperatures across sites, indicative of a shift in diet to prey of lower trophic status over time and in warm years. However, a siteby-site analysis revealed divergent isotopic trends among sites: five of seven sites exhibited significant temporal or temperature-related trends in isotope signatures. This study highlights the utility of stable isotope analyses when applied over relatively long timescales to apex predators.

Rapidly increasing methyl mercury in endangered ivory gull ( Pagophila eburnea ) feathers over a 130 year record

“…This contrasts with other seabirds over similar time frames in the North Atlantic [38,51], North Pacific [18,52], and Indian and southern Oceans [53][54][55]. While we found that ivory gull stable-isotope values remained constant, their diet could have changed to different prey with similar isotopic values.…”

Plasticity in the foraging behavior of male Southern Rockhopper Penguins (Eudyptes chrysocome) during incubation in the Falkland/Malvinas Islands

“…In turn, the latter resulted in earlier egg-laying dates with increased clutch masses (Dehnhard et al 2015b) and an overall higher breeding success (Crawford et al 2008). However, over the past decades, the global Rockhopper Penguin population has significantly decreased (Pütz et al 2013), with declines related to both decreasing (Guinard et al 1998) or increasing SST (Cunningham and Moors 1994;Thompson and Sagar 2002;Hilton et al 2006;Raya Rey et al 2007), indicating that these birds are highly sensitive to changes in SST in either direction. As a specific example, in 2016, a mass mortality occurred in SRP in the Falklands during the molting period, which was attributed to pre-molt starvation following much colder than usual sea temperatures, which in turn was assumed to have resulted in reduced food availability (Crofts and Stanworth 2017;Morgenthaler et al 2018).…”

Seabird year-round and historical feeding ecology: blood and feather δ13C and δ15N values document foraging plasticity of small sympatric petrels

“…Secondly, fossil carbon introduced into the atmosphere has a lower δ 13 C value than background carbon, thus inducing an accelerating decrease in δ 13 C in the biosphere (the Suess effect; Keeling 1979). Since the Suess effect decreases with increasing latitudes (Hilton et al 2006), the raw δ …”

“…When long time series of historical and recent samples of δ 13 C are analysed, the raw δ 13 C values have to be adjusted according to the increase in atmospheric CO 2 in response to human fossil fuel burning (Hilton et al 2006). Firstly, the resulting rise in aqueous CO 2 increases, in turn, the phytoplankton fractionation, thus reducing its δ 13 C isotopic values (Rau et al 1992).…”