Population dynamics of the sympagic amphipods Gammarus wilkitzkii and Apherusa glacialis in sea ice north of Svalbard

The population structure and production of the two autochthonous sympagic amphipods, Gammarus wlkitzkii and Apherusa glacialis, collected in different categories of sea ice north of Svalbard and in the Fram Strait, were examined. A longevity of 6 years for male G. wilkitzkii and 5-6 years for females was indicated, using a Mixture distribution analysis technique. Growth is best described by the Gompertz growth function. The theoretical maximum body length was estimated to be 53.2 and 45.3 mm, and the corresponding maximum age was estimated to be 7.5 and 6.3 years for males and females, respectively. Juveniles of age classes 0+ and 1+ of G. wilkitzkii were most abundant in first-year ice and were regarded as predominant colonizers of new ice habitats. It is unclear whether juveniles have colonized the ice independently or been released there by females colonizing the new ice. Age classes 2+ and 3+ were numerous in multi-year ice. The growth rate for G. wilkitzkii was low, and lower for females than for males. Highest growth rates were found in an area dominated by old ice. A mean production of 0.0237 g ash-free dry weight year(-1) m(-2) and a somatic P/B ratio (productivity) of 0.255 year(-1) were calculated for the entire set of data. Individuals of age class 4+ were the most productive within the population, while juveniles have little influence on the annual productivity. A. glacialis reached a maximum age of 2 years and age class 0+ was about tenfold more abundant compared to age class 1+, indicating a high mortality rate during the 1st year of life. The population structure seems to be more affected by ice conditions (e.g. floe size) than by the age of the ice.