Population dynamics and production of Themisto gaudichaudii (Amphipoda, Hyperiidae) at South Georgia, Antarctica

The population dynamics, individual growth rates and productivity of the hyperiid amphipod Themisto gaudichaudii were studied in Cumberland Bay, a fjord on the north-eastern coast of South Georgia. The study involved the weekly sampling of the population and its potential food with a Rectangular Midwater Trawl (RMT1) at weekly intervals between May 2005 and October 2006. Offshore samples were taken the previous austral summer using an RMT8 net. T. gaudichaudii produced one main cohort in the spring (September/October) and a secondary weaker cohort in the late summer (January/February). The main cohort appeared at the time of phytoplankton increase, about 3 weeks before an increase in mesozooplankton. Larger individuals (20-25 mm) resided offshore but not within the Bay during the main recruitment period. T. gaudichaudii grew at an average rate of 0.1 mm d−1 during the first four months after release and then by 0.07 mm d−1 for the subsequent four months. At these rates, it is possible that the main cohort reaches reproductive size in time to spawn the autumn cohort, although the relative weakness of the autumn cohort suggests that few manage this. T. gaudichaudii biomass reached its highest levels in November at a mean value of 5.4 mg DW m−3 while daily production was also highest in November at a mean value of 0.17 mg DW m−3 d−1. The P/B d−1 ratio varied between 0.03 d−1 in the summer to below 0.01 d−1 in the winter. Daily mortality rate was initially 0.12 d−1 during the first two months post-recruitment, falling to 0.03 d−1 for the remainder of the year. Integrating production over the year gave a value of 10.04 mg DW y−1, equivalent to 3.62 mg C m−3 y−1, making T. gaudichaudii a relatively productive component of the pelagic community. During the summer, the species was estimated to consume around 1-2% of available mesozooplankton biomass and a mean of 15% of mesozooplankton daily productivity. T. gaudichaudii may have a controlling influence on mesozooplankton dynamics as well as on the recruitment of the larval stages of important commercially fished species such as Antarctic krill (Euphausia superba) and Mackerel icefish (Champsocephalus gunnari).