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

Abstract.

During 1 tidal cycle in September 2001, ca. 2.5 million Themisto libellula (Mandt) were washed ashore along 600 m of the inner Adventfjorden, Svalbard (78°13′N; 15°38′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 fjord. 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 1 year in the fjord.