Comparison of condition metrics and lipid content between Euphausia pacifica and Thysanoessa spinifera in the northern California Current, USA

Abstract Krill are a key component of pelagic food webs where they are vital to the transfer of energy from phytoplankton to higher trophic levels. Krill have a high lipid content compared to other zooplankton and form dense aggregations, making them an important prey source for higher trophic level fish, seabirds, and marine mammals. The two dominant euphausiid species in the northern California Current (NCC) are Euphausia pacifica and Thysanoessa spinifera. E. pacifica is the most abundant species of euphausiid in the NCC, but T. spinifera has a higher potential energetic content due to its larger body size and higher lipid density. Most studies have inferred differences in lipid content and body condition between the two species, but few studies have quantified these differences in the NCC. Here, we report on the body condition, carbon and nitrogen content, as well as lipid and fatty acid composition of these two krill species, and the extent to which these metrics vary across season, year, and reproductive status. Body condition, elemental composition, and total lipids strongly differed between the species. T. spinifera had higher length-weight, Fulton’s K, hepato-somatic index, carbon to nitrogen ratio, total lipid per wet weight, and storage lipid compared to E. pacifica, indicating that T. spinifera has a higher energetic value for predators. However, there were strong seasonal differences in the energetics of T. spinifera. Carbon and lipids were highest in non-reproductive T. spinifera from August through October. Although there were strong ontogenetic and inter-specific differences, the lipid and fatty acid compositions of both species followed a seasonal progression characterized by low lipids during the pre-upwelling period, an increase in lipids, triacylglycerols, and diatom markers during upwelling, and increased proportions of dinoflagellate and bacterial diet markers during the fall post-upwelling period.