MEPS 495:105-118 (2014)  -  DOI: https://doi.org/10.3354/meps10553

ABSTRACT: Although Thysanoessa macrura is one of the most abundant and widely distributed euphausiid species in the Southern Ocean, knowledge about its seasonal distribution and overwintering strategy remains sparse. We describe the variation in abundance, demography, and distribution of T. macrura during 3 seasons in the Lazarev Sea, located in the south-east Atlantic sector of Antarctica. Adult and larval T. macrura were widely distributed across the Lazarev Sea in all seasons, but with large seasonal variability in the adult population. Mean summer densities (4.9 and 7.0 ind. m^-2 in 2005/06 and 2007/08, respectively) were 20-fold higher than densities recorded during autumn (0.4 ind. m^-2) and winter (0.2 ind. m^-2). Comparison of length-frequency distributions and sex ratios, particularly between shallow (0-200 m) and deep (0-2000 m) samples taken during winter, suggest a seasonal vertical migration. A large proportion of the adult population seems to migrate to greater depth during winter, possibly as an overwintering strategy. Further, the demographic data and a set of growth models suggest that females and males have deviating life history traits. Females grow to larger size but their growth rate is slower, and they may mature later than males. The presence of mature males and females with spermatophores attached during late June to August indicated that spawning had already started during mid-winter conditions. These data, which were uniquely sampled in the same region from summer to winter, provide a new perspective on the seasonal behavior and general life history of T. macrura.

KEY WORDS: Euphausiid · Seasonality · Demography · Seasonal vertical migration · Overwintering · Southern Ocean