Distribution, abundance and population demographics of Salpa thompsoni on the Kerguelen Plateau

The salp, Salpa thompsoni, is an important grazer in the Southern Ocean. It often occurs in dense swarms in the warmer waters of the Antarctic Polar Frontal (APF) zone, however, has been found in increasing densities near the Antarctic continent in recent decades. Micronekton, including salps, were sampled during late summer to early autumn (26^th February – 15^th March 2017) at contrasting hydrological stations on the Kergeluen Plateau (Indian sector). Studies on S. thompsoni in the Indian sector of the Southern Ocean are lacking, and these are the first estimates of S. thompsoni population dynamics and growth rates on the Kerguelen Plateau. S. thompsoni abundances (2901 individuals 1000 m^-3) and population structures were within previous estimates for populations in the APF. The impact of S. thompsoni on the local Kerguelen ecosystem is likely to be great in late summer, as at two stations, S. thompsoni made up almost half of the micronekton population. Prior environmental conditions were an important factor when determining the success and development of the S. thompsoni population, however, growth rates were more variable and were indicative of current conditions. Growth rates (0.45 – 7.01% d^-1) were higher than previously calculated ranges from the Antarctic Peninsula (0.3 – 4.6% d^-1) but lower than the APF (3.7 – 20.7% d^-1). Despite warm surface waters (4 – 5ºC), low chlorophyll a concentrations at the time of sampling may have restricted the salp populations from growing as fast as populations from the APF. This study highlights that environmental conditions in the Kerguelen Plateau region benefit large populations of S. thompsoni. As the Kerguelen Plateau region deflects a branch of warm water southward into the Cooperation Sea, more studies of S. thompsoni population dynamics across multiple seasons are needed to fully understand their dynamics on the Kerguelen Plateau and their invasion potential into the Cooperation Sea and higher latitudes.