Observations of mass mortality of Themisto libellula (Amphipoda, Hyperidae)

During 1 tidal cycle in September 21001, ca. 2.5 million Themisto libellula (Mandt) were washed ashore along 600 m of the inner Adventfjorden, Svalbard (78degrees13'N; 15degrees38'E). This corresponds to ca. 840 kg dry weight, Density of apparently weakened T. libellula in waters immediately adjacent to the shore was ca. 136 ind. m(-3). The incident occurred during a period with strong wind shear directed out of the fjord. It is likely that the mortality was due to osmotic stress after advection of organisms into the low-saline shallow waters at the inner parts of the f-Jord. Size structure of caught organisms (average length = 17.7 mm, range 12.4-23.4 mm) indicates the bulk of individuals to be juveniles, and accords with the view that T. libellula reaches sexual maturity within I year in the fjord.