Milk fatty acids predict the foraging locations of the New Zealand fur seal: continental shelf versus oceanic waters

Abstract

Lactating New Zealand fur seals Arctocephalus forsteri utilise 2 ecological regions: con- tinental shelf habitats and oceanic habitats associated with the Subtropical Front. Using milk fatty acids (FA) obtained from 29 satellite-tracked fur seals, we characterised the FA composition of seals that foraged on the continental shelf, and those that foraged in oceanic waters. Seals that foraged within oceanic waters were characterised by milk being comparatively high in monounsaturated fatty acids (MUFA; 47.4 ± 4.4%, mean ± SD), and lower in polyunsaturated fatty acids (PUFA; 23.8 ± 4.0%) when compared to seals that foraged in continental shelf waters (MUFA 36.7 ± 5.4 and PUFA 31.4 ± 5.5%). Based on FA compositions, we predicted the likelihood that milk samples collected at random (n = 131) represented individual seals having foraged either on the continental shelf or in dis- tant oceanic waters. Results indicated that 74% (n = 97) of seals were likely to have foraged in oceanic waters, with 26% (n = 34) likely to have foraged within continental shelf waters. These results were supported by the small sub-sample of 29 satellite-tracked seals, which indicated that 62% of seals had foraged in oceanic waters. FA analysis and satellite-tracking results contrasted with scat analyses, from which only 7% of scats contained prey remains from oceanic waters. The results suggest scats were biased toward females foraging on the continental shelf. To further understand the diet of New Zealand fur seals, additional information on potential prey species that inhabit waters associated with the Subtropical Front south of Australia is required, as well as the continued devel- opment and application of alternative dietary techniques.