Foraging behaviour of female Weddell seals (Leptonychotes weddellii) during lactation: new insights from dietary biomarkers

Despite extensive studies on Weddell seals (Leptonychotes weddellii) in McMurdo Sound since the 1960s, uncertainty still remains regarding female foraging habits during the lactation period. Based on their large body mass at the start of lactation and large relative mass loss at the end, the current hypothesis is that Weddell seals fast or feed to a neglible extent during lactation. However, this hypothesis has not been fully tested to date, as evidence for foraging is indirect and is based primarily on dive behaviour. The work presented in this thesis describes the development of a new dietary method, the biomarker method, and its application for studying the foraging behaviour of female Weddell seals during lactation. Biomarkers were used to (1) monitor the onset of feeding in individual animals, and (2) determine what prey females were feeding on using characteristic/taxon-specific biomarker patterns. Proton nuclear magnetic resonance spectroscopy (1H NMR) and liquid chromatography-tandem mass spectrometry (LC-MS/MS) assays were developed to detect and quantify dietary biomarkers in biological samples, mainly tissues, serum and plasma. Trimethylamine N-oxide, arsenobetaine, dimethylsulfoniopropionate, homarine and glycine betaine were first measured in thirty-three prey and potential prey species of Weddell seals collected from the Ross Sea and McMurdo Sound regions of Antarctica. These same compounds were then measured in the plasma of twelve female Weddell seals over the lactation period at the Hutton Cliffs seal colony, McMurdo Sound in 2006. Time-depth recorders monitored seal dive activity over the same period. The data obtained from both NMR and LC-MS/MS assays showed that biomarkers in Antarctic species varied both in content and concentration. The compound homarine, which occurs primarily in cephalopods, is suitable for distinguishing between major food groups of known prey of Weddell seals (i.e., fishes versus cephalopods). DMSP, a compound that occurs primarily in fish common in McMurdo Sound (e.g., Trematomus bernacchii and Pagothenia borchgrevinki) but not in significant amounts in Dissostichus mawsoni or Pleuragramma antarcticum, two main prey items for Weddell seals, may also be a suitable biomarker for distinguishing between major and minor prey types. The detection of plasma TMAO, AsB and homarine indicated that 75% of Weddell seals studied fed during lactation. The presence of these three compounds indicates the seals were preying upon a combination of fish and cephalopods. Two lactating females started foraging as early as 9 to 12 days postpartum and elevated biomarker levels were concurrent with increased dive activity. The onset of foraging and dive behaviour amongst individuals was highly variable; however, the results suggests that the number of females who feed during lactation may be more prevalent and initiated at an earlier stage than previously thought. This may have implications for future reproductive success given effects of climate change on sea ice abundance and resource availability. Overall, the work presented in this thesis provides new insights into the foraging behaviour of female Weddell seals during lactation and has added to the current knowledge of the biomarker distribution within the Antarctic ecosystem