Long-term trends in albatross diets in relation to prey availability and breeding success

Abstract

Diet analyses can reveal important changes in seabird foraging ecology and, by inference, resource availability and predator-prey dynamics within the wider marine ecosystem. Here, we analysed stomach contents of 1544 grey-headed albatross Thalassarche chrysostoma (GHA) and black-browed albatross T. melanophris (BBA) chicks from Bird Island, South Georgia. We describe dietary shifts (1996-2017), and link those to annual prey availability indices and breeding success. Annual variability in diet was high, and long-term trends in the main components were broadly similar in both albatrosses. Fish consumption (by mass) generally increased over time. Mackerel icefish Champsocephalus gunnari occurrence increased in GHA diets, but was unrelated to local densities derived from fisheries/research cruises. Cephalopod consumption declined until the early 2000s, then plateaued, and the occurrence of the ommastrephid squid Martialia hyadesi declined over time in both albatrosses. In BBAs, Antarctic krill Euphausia superba consumption decreased over time. Conversely, Antarctic krill consumption by GHAs increased until the early 2000s, decreased until the mid-2010s, and increased again in 2017. Antarctic krill consumption was unrelated to local densities based on acoustic surveys, and did not correlate with breeding success. Remotely sensed chlorophyll-a within core foraging areas showed a positive relationship with Antarctic krill in GHA diets, but a negative relationship with M. hyadesi occurrence in both albatross diets. Dietary shifts had consequences for GHA breeding success, which was negatively related to the importance of the cranchiid Galiteuthis glacialis and positively related to M. hyadesi importance. These results highlight the complex mechanisms linking prey availability, diet and breeding success in albatrosses.

Mercury exposure in an endangered seabird: long-term changes and relationships with trophic ecology and breeding success

“…Firstly, grey-headed albatrosses may have shifted their diets or foraging habitats to more contaminated prey or regions. Analyses of stomach contents of chicks have revealed a major dietary shift in breeding grey-headed albatrosses at South Georgia since the mid-1990s, including a reduction in the occurrence of the seven-star flying squid ( Martialia hyadesi ) and an increase in mackerel icefish ( Champsocephalus gunnari ) [ 34 ]. However, adults may not consume the same prey that they provision to chicks and, given the difficulties in obtaining samples, the only conventional diet information for grey-headed albatrosses outside of the breeding period is for the cephalopod component [ 56 ].…”

“…The relationship between δ 15 N and Hg exposure is often apparent when comparing mean values for different species within seabird communities [ 10 , 15 ], but is less frequently observed within a single species [ 8 ]. That it was apparent in our study population is probably because grey-headed albatrosses at South Georgia consume a wide range of prey from multiple trophic levels [ 34 ], and the variation in δ 15 N values among individuals was very high (range: 8.8–15.2‰).…”

“…50%), but has been in long-term decline and is currently listed as endangered by the World Conservation Union [ 30 , 31 ]. This species is extremely long-lived [ 32 ], and forages at mid- to high trophic levels—predominantly consuming cephalopods, but also fish and Antarctic krill ( Euphausia superba ) [ 33 , 34 ]. During the non-breeding period, free from the constraints of central-place foraging, birds disperse across a wide range of oceanic habitats though mainly targeting the Subantarctic Zone (SAZ) between the APF and the Subtropical Front (STF) [ 35 – 37 ].…”

Long-term changes in habitat and trophic level of Southern Ocean squid in relation to environmental conditions

“…Many studies have examined responses to changing oceanographic conditions of organisms at different trophic levels, or sought to predict the impacts on distributions 22 , 23 . In the Southern Ocean, documented effects of climate variability on ecological processes, include changes in diet, foraging areas, breeding success or abundance, particularly at higher trophic levels, such as in penguins and albatrosses 24 – 27 . The only such studies on key pelagic zooplankton or nekton are those on krill 28 , 29 .…”

Stable isotopes demonstrate intraspecific variation in habitat use and trophic level of non‐breeding albatrosses

“…2006) and is a population‐level generalist (Mills et al . 2020). Two studies analysing geolocator data from a small number of non‐breeding birds revealed that most target the Sub‐Antarctic Zone (SAZ: between the Antarctic Polar Front (APF) and the Subtropical Front (STF)), with evidence for small‐scale sexual segregation and some consistency in habitat use within the same non‐breeding period (Croxall et al .…”