Spatiotemporal variability in Antarctic krill (Euphausia superba) around South Georgia from Antarctic fur seal (Arctocephalus gazella) diets at two long term monitoring sites

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Coleman, Jamie
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University College Cork
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Despite its biological, biogeochemical, and economic importance, there is much about Antarctic krill (Euphausia superba Dana, 1852) that is still poorly understood. This is especially true for its recruitment into South Georgia’s waters, which is highly variable and difficult to predict. Krill fluctuations exert bottom-up pressures on the entire ecosystem which can be reflected in changes in the breeding success and diet of dependent higher predators, such as Antarctic fur seals - Arctocephalus gazella (Peters, 1875). This study compares two long-term datasets (2008-2021) of Antarctic fur seal diet from Bird Island and King Edward Point, South Georgia, to investigate year-round krill dynamics. Foraging tracks from breeding female fur seals demonstrated that seals from the two sites forage in distinct regions. Krill length-frequency distributions generally followed a synchronous pattern across both sites with summer increases in length associated with growth, and winter decreases associated with ongoing senescence, mortality and possibly krill shrinkage, coupled with recent recruits with smaller body lengths entering the population. Using a multivariate approach, mixed layer depth, sea surface temperature, chlorophyll a, and salinity together explained 43% of the dissimilarities in krill length-frequency distributions predated by fur seals between seasons. Bird Island seals took significantly larger krill than those at KEP, possibly a result of continued krill growth during advection between the two areas, or differences in local oceanography. Krill cohorts visibly developed across years with large-scale recruitment of smaller krill occurring every 4-5 years. An anomalous period between 2017 and 2020 occurred when synchrony between sites broke down and later re-established. The beginning and end of this anomalous period appears to coincide with switches in the El Niño Southern-Oscillation, however more data are needed to formally test this link. This study highlights the importance of having multiple spatially separated long-term monitoring sites; this is particularly important in this ecological system as it can vary over a range of scales, including local scales, and it is also recognised to be experiencing significant changes.
Krill , South Georgia , Southern Ocean , Antarctic fur seal , Diet analysis
Coleman, J. E. P. 2022. Spatiotemporal variability in Antarctic krill (Euphausia superba) around South Georgia from Antarctic fur seal (Arctocephalus gazella) diets at two long term monitoring sites. MSc Thesis, University College Cork.