Studies of larval krill

 

Bettina Meyer1, Ulrich Freier1, So Kawaguchi2, Rob King2, Kerrie Swadling3, Pattie Virtue3, Zhongnan Jia3

  1. Alfred Wegener Institute for Polar and Marine Research, Germany (AWI)
  2. Australian Antarctic Division (AAD)
  3. IMAS University of Tasmania (UTAS)

The distribution and abundance of krill have been intensively studied for nearly 90 years. An evaluation of net surveys between the 1920s and present documented a reduction in krill abundance for the Scotia Sea sector of the SW Atlantic during the period of 1976-2003. The krill stock declined in concert with decreasing winter sea ice cover, suggesting that the declining trend in krill biomass is driven by recruitment, rather than by predation pressure on adult krill. Larval krill develop during summer and winter to juveniles in the forthcoming spring. A vital step in successful krill recruitment is larval krill survival through the first winter. However, the conditions that foster larval krill development and survival  in the winter sea ice are largely unknown . Therefore our main objective is to investigate the  habitat quality (physical and biological) that promotes larval krill development.

Hypothesis

Over-rafted ice floes are favourable regions for the promotion of larval krill survival

Unlike adults, larvae have to find food continuously to meet their energetic needs. High abundances of larvae have regularly been observed associated with the sea ice in winter and in regions with over-rafted ice floes in particular. Due to under-ice topography and its influence on current speed, larvae as well as their planktonic prey can aggregate and rest in these rafted ice refuges. Sea ice biota can be released by floe movements within these refuges, resulting in favourable feeding condition for larval krill. SCUBA observations in the Lazarev Sea saw the majority of larvae on the “floors” of the rafted ice refuges (see Figure below). In an environment with high current speeds, ice refuges might be essential for resting and feeding of krill larvae in seasons of low food availability in the water column.

Adult krill (left) compared to larval krill:  Furcilia stage

 

Sampling approach on sea ice stations

For sampling of krill larvae we have two different pump systems. One pump system called MASMA (MAnguera SubMArina) is a sled based pump that is capable of filtering up to 400 litres per minute. This pump sucks from the discharge side of a filter screen so the krill and plankton that are caught do not pass through the pump itself and hence in perfect condition for further physiological experiments on board ship. A remotely operated vehicle (ROV) will be used to search for areas of high krill density under the ice.  Holes will be drilled through the ice into the chambers within the ice in which krill have been detected previously by the ROV. 

The other pump system is a large fish pump that has been mounted in a 20 foot container that can be lowered onto the ice beside the ship by the ship’s crane.  A hole is cut in the ice and water is then sucked through the pump at about 1300 litres per minute.  The water is passed through a sieve, which removes any krill that are present which are then transferred to the laboratories for experimentation. Krill will pass through the pump without injury due to the nature of the pump impellor, which was originally designed for pumping live Atlantic salmon for aquaculture. This operation can run continuously and is likely to be conducted during the late afternoon and night when ROV operations have been completed for the day.

The freshly caught larvae collected by both pumping systems will be transferred to the ship. The larvae will be examined (staged and sized) under stereo microscope before being frozen at -80°C for analysis of dry weight (DW), elemental (carbon: C, nitrogen: N), biochemical composition (total body lipid, protein, and fatty acid (FA) composition), stomach and gut contents, and molecular as well as stable isotopic (δ15N) analysis. Another subsample will be taken for measurements of larval growth and metabolic rates (oxygen uptake and ammonium production rates).

We will relate the condition of the larvae with the microbial sea ice community by taking sea ice, water and zooplankton samples below sea ice from the same location where the pumps collected the larvae to determine the energy content (C, N, lipid, FA, protein) and species composition as well as their δ15N signature.

Regional Approach

The same approach used here will also be conducted on the German winter voyage WISKY (Winter study on sea ice and key species), which will take place from 14th August – .16th October. 2013 in the Southern Scotia, northern Weddell Sea. A direct comparison of data will be assessed with respect to anthropogenic warming in both regions. (Fig.y: Map Antarctica with both study regions).

Dr. Bettina Meyer, bettina.meyer@awi.de.

Dr. Ulrich Freier, Ulrich.freier@gmx.net

Rob King rob.king@aad.gov.au

Patti Virtue, p.virtue@utas.edu.au

Zhongnan (Molly) Jia, Zhongnan.Jia@utas.edu.au