By Marilyn Smith Yeager
[94K] We measured length and observed developmental stages of Antarctic krill to determine if they were randomly mixed within aggregations or if internal structures related to biological factors existed in their distribution. In order to evaluate effects of physical factors on Euphausia superba distributions, the depth, temperature and salinity of water filtered were recorded to determine characteristics of water from each sample.
During our study a major hydrographic front marking the mixing of Antarctic Surface Water (AASW) with Weddell Scotia Confluence Water (WSC) crossed the study area. Euphausia superba length frequency distributions showed distinct separations by length and developmental stage between these surface water masses. On the north shelf of Elephant Island, predominantly juvenile krill resided in cooler, denser WSC water. On the west Elephant Island shelf and slope, predominantly adult krill were collected in warmer, fresher Transition Type II water, a type formed by mixed AASW and WSC water. These data suggest that mesoscale (10s of kilometers) distribution of water masses exerted the greatest influence on the distribution of Antarctic krill, although further analysis revealed that the degree of water stratification and diel period of sample collection (day, night or twilight) also affected distribution.
Antarctic krill are commonly reported to engage in diel vertical migration although a consistent pattern is seldom observed. During this study, Antarctic krill showed a diel vertical migration apparently influenced by light and water stratification. Generally the krill migrated to deeper depths by day and shallower depths at night, but in well-mixed water they migrated to the deepest depths (above 150 m), while in strongly-stratified water with a well-defined thermocline they stayed in shallower water. Under latter conditions most of the krill resided in the upper 60 meters in all diel periods. Strongly stratified waters also had highest concentrations of chlorophyll a in the mixed layer. These data suggest that the degree of water stratification influenced phytoplankton population growth and the depth of Antarctic krill's vertical migration.
On a fine scale (less than 1 km), biological factors may exert larger influence on Antarctic krill distributions. Within individual tows, Antarctic krill formed vertical layers in aggregations according to length and developmental stage. In the upper 32 meters of water, as many as four vertical layers showed increased length and later developmental stage with increasing depth. This vertical layering was observed above the thermocline during a single diel period, stratification level and within one water mass. Krills' swimming abilities increase with developmental maturity, so individuals in a layer of similarly-sized krill would have swimming abilities like their companions. The vertical layering of similarly-sized krill may constitute an adaptation to forage for food more efficiently or escape from predators.
These data will be used to compare biomass measurements from net samples with acoustic estimates. Biomass is estimated from the acoustic data using a model which includes a value for acoustic target strength, which depends on the length, orientation and species composition of organisms in the water. Data from zooplankton trawls provide information on species composition and length which can be used to ground-truth acoustic measures of target strength. A major goal of AMLR's program includes use of acoustic tools to estimate krill biomass on large scales to enable determination of harvest limits and development of policies to protect Antarctic marine living resources.
Editor's Note: Marilyn graduated with her master's degree in August and now works part-time with Assistant Professor Niall Slowey researching glacial to interglacial climate changes in the North Atlantic Ocean.
Oceanography, Texas A&M University
Updated July 24, 1995