Overwintering Strategies and Predation on Antarctic Krill

Kendra Daly

College of Marine Science, University of South Florida

Larval Euphausia superba were abundant in the upper 50 m throughout the study area during autumn (April-May) 2001 and occurred in many stages of development (Calyptopis III to Furcilia 6), suggesting that the previous summer was a strong year for krill reproduction, reproduction extended over a relatively long period, and that circulation was favorable for retaining larvae on the shelf. In contrast, juveniles were generally absent. Adult krill were rare in the open waters of Marguerite Bay, but were relatively abundant in adjacent coastal fjords and embayments where upper trophic level predators also were aggregated. Sea ice was just beginning to form as we left the study area in June.

During winter (July-August) 2001 when the study area was covered with sea ice, most larvae had developed to Furcilia 6. Some individuals remained at the F6 stage after molting, while others recruited to the juvenile stage. Chlorophyll concentrations in the water column (0.03-0.06 ug l-1) and at the undersurface of sea ice (0.01-0.5 ug l-1) indicated that autotrophic food availability was extremely low. Gut contents, however, verified that larvae and adults continued to feed during winter. Experimental results suggested that larvae likely fed on microzooplankton and detritus and adults fed on copepods. Winter molting rates were similar to those during autumn, but growth rates were negligible for all stages. Even though larval krill were observed at the undersurface of sea ice, preliminary results suggest that sea ice formed late in the year at these latitudes may not allow sufficient growth of sea ice communities to provide a significant food source for overwintering krill. Several alternative food sources may have supported the population at a maintenance level.

Fish and marine birds and mammals are known to be important predators of adult krill, but little is know about predation on larval krill. Gelatinous zooplankton were relatively common in the study area and therefore potentially an important predator of larval krill. During July and August, diver observations indicated high densities of ctenophores (> 1 ctenophore m-3 just under the pack ice (top 10 meters of the water column) where larval krill aggregated. Gut analysis of mertensiid ctenophores (n = 30) collected by divers and in net tows showed that about 62% of the ctenophores containing recognizable prey had remnants of krill larvae in their digestive system. However, 57% of the ctenophores sampled did not have recognizable prey item in their guts. Ctenophore and larval krill abundances from ROV images and net tows will be assessed.

Krill rate measurements from field experiments:

Molting, growth, feeding (gut fluorescence), feeding on microzooplankton w/ Scott Gallager et al., adult feeding on copepods, egestion, fecal pellet production, assimilation efficiency, algal lipid assimilation w/ Rodger Harvey(?).
Other data sets being analyzed: