Biology of Salpa thompsoni at the Chatham Rise, New Zealand: Reproduction, diel vertical migration and demography

The pelagic tunicate, Salpa thompsoni, is the circum-Antarctic species with the highest abundances found between the Subtropical Convergence and the Antarctic Polar Front. The Chatham Rise, New Zealand, is a northernmost boundary of this species distribution range in the Southwest Pacific. While S. thompsoni is conspicuous and known to create large blooms, its demography and fundamental biology are often studied only opportunistically. The primary aim of the present study was to investigate the stage composition of salp populations and derive salp growth and development rates. Second, size-depended diel vertical migrations in the epipelagic (upper 200 m) were explored. During a 30 day-long research voyage onboard R/V Tangaroa, Salpa thompsoni were encountered in high abundances in three out of five study areas in October and November 2018. Considering that the environmental conditions (temperature, salinity, chlorophyll a concentration) were similar in these study areas, it was possible to substitute space by time and to derive growth and development rates for aggregate and solitary forms. Growth and development of S. thompsoni at the Chatham Rise were within the range reported from the Atlantic and Indian Ocean sectors of the Southern Ocean. Size-dependent differences in epipelagic S. thompsoni abundances during day and night aid to better understand diel vertical migrations in this species.