Growth and behavior of larval krill (Euphausia superba) under the ice in late winter 2001 west of the Antarctic Peninsula

In situ growth rates are one measure of the physiological condition of larval Antarctic krill in winter. During the southern ocean global ecosystem dynamics (SO GLOBEC) research cruise in July/August 2001 aboard the ARSV Laurence M. Gould west of Adelaide Island and Marguerite Bay, larval krill were observed in the under-ice habitat. Ten instantaneous growth rate experiments were conducted with larvae collected from the under-ice habitat on the outer shelf, mid-shelf, and just south of Adelaide Island in the mouth of Marguerite Bay. For all experiments, average growth increments for larval krill were about −1.6% per intermolt period, with a median intermolt period of 30.6 d. Larvae were observed feeding on the ice, even though no color was visible in the ice. Pigment (chlorophyll a equivalents) content of larvae from three of the sites was measured as an indicator of ingestion of sea-ice microalgae. Pigment values were above those found in starved controls, but only about 20% of maximum possible values. Although feeding, the data suggested that the larval krill were unable to ingest enough food to maintain growth. The results from this first SO GLOBEC winter process cruise were compared to growth rates of larval krill during previous winter cruises that covered all months from late May through September, except for the month of August.