Trophodynamics of krill and its potential role in blue whale feeding in the Perth Canyon, south-east Indian Ocean

Migrating blue whales along the Western Australian coast exhibit feeding behaviour within the Perth Canyon, which is an area of high krill abundance, particularly for Euphausia recurva. The importance of krill in marine food webs has led to a number of trophodynamic studies investigating their fatty acid and stable isotope compositions. In the south-east Indian Ocean, the suppression of upwelling by the dominant Leeuwin Current results in relatively oligotrophic waters, particularly during autumn and winter. Oligotrophic waters tend to be dominated by small autotrophic flagellates (i. e. dinoflagellates) and cyanobacteria. We relate biochemical data obtained for E. recurva, as well as Stylocheiron carinatum and Pseudeuphausia latifrons with their potential food source, phytoplankton, and one of their potential predators, the endangered pygmy blue whale (Balaenoptera musculus brevicauda) sampled in the Perth Canyon. Fatty acids of all three krill species were dominated by polyunsaturated fatty acids (PUFA; ~50%) largely comprised of omega-3 PUFA, which is typical for krill. The high docosahexaenoic acid (DHA) to eicosapentaenoic acid (EPA) ratio reflects a dinoflagellate, rather than a diatom diet, and the high oleic acid (18: 1 9) to vaccenic acid (18: 1 7) ratio is indicative of an omnivorous diet. Stable isotope analysis positions E. recurva as a first, possibly second order consumer (5. 8 - 8. 4 15N)and phytoplankton as the likely source of carbon (-18 to -24 13C) .The fatty acid composition of krill did not match that of the surface phytoplankton sampled, which was low in PUFA and more reflective of degraded and detrital material. This suggests that krill are not feeding at the surface, and may feed closer to the deep chlorophyll maximum. The outer blubber layer sampled from the pygmy blue whale was high in monounsaturated fatty acids (MUFA, 58%) rather than PUFA, and did not reflect the krill fatty acid composition. However, the high DHA to EPA ratio in the blubber indicated a diet originating from dinoflagellates, as found for krill. Stratification of fatty acids across blubber layers is common for marine mammals and the outer blubber layer for some species has been found to not accurately reflect the diet of the animal.