F-1 



6. KRILL 



6.1 Introduction and General Biolop;y 



There are in the Southern Ocean several commonly occurring species of euphausiid crusta- 

 cean. These are listed below and their approximate latitudinal ranges shown in Fig. 6.1. 



Euphausia superba 

 Euphausia vallentini 

 Euphauaia triacantha ' 

 Euphaujja frifj.da 

 Euphaunia crystallarophias 

 Thysanoesaa macrura 



The distribution of these species along with an identification key are described by 

 Mauchline and Fisher (1969) and more detailed information is available in John (193?) and 

 Baker (1965) (Euphausia species), in Baker (1959) and Itorr (1962) (Euphausia triacantha )? 

 Marr (I962) (Euphausia superba ) and in Nemoto and Nasu (I958) ( Thr/sanoSssa ). 



Of these species Ej_ superba (Fig. 6.2) is the largest and most abundant and is the spec- 

 ies generally considered as being synonymous with the term "Antarctic Krill". It is this 

 species which is of greatest interest from the fisheries point of view since its habit of 

 congregating into dense swanns makes it an ideal target for modem midwater trawlers. The 

 bulk of this section is therefore in the main restricted to infonnation on Ej_ superba althou^ 

 other species have been occasionally found to be more abundant in some areas of the Southern 

 Ocean (Nemoto, Doi and Nasu, in press, found Ej_ vallentini and Thy canoe ssa macrura predomin- 

 ated in six out of 310 samples). 



Before reviewing in detail the information on E. superba , a brief outline of some of the 

 essential features of its general biology will be given in order to assist the reader in 

 gaining a clear vmderstanding of the processes which are described in the sections which fol- 

 low. 



The main part of the spawning season for _E^ superba occurs from January to March. The 

 eggs, about 0.7 mm in diameter, are released at the surface and then sink to considerable 

 depth, almost certainly to the lov/est levels of the warm deep water. During the weeks which 

 follow the eggs hatch into nauplii ajid, whilst rising throu^ the water column, develop 

 throu^ a metanaupliar stage to arrive at the surface as first calyptopes. They then develop 

 throu£^ two further calyptopis and six furcilia stages before being classed as adolescents. 

 During the subsequent two or more years the krill increase in size to a maximum length of 

 about 6 cm. and are to be found, for the most part, in the top 200 m of the water column. 

 Any diurnal vertical migration that does occur is thou^t to be within the Antarctic surface 

 water so that in the West V/ind Drift the krill are in vrater masses having a slight northerly 

 component. 



The more or less total restriction of ^ superba to the surface water may well be associ- 

 ated with the fact that it is the only species of euphausiid which at least during the summer 

 feeds predominantly on phytoplankton. Other food types are however taken depending upon 

 availability. 



The habit of Ej_ superba of congregating in dense swarms has been well known for a long 

 time. The swarms themselves are often several hundred metres across and within thorn the krill 

 may be as concentrated as several kilograms in a cubic metro of water. Thus, within a region 

 of above average abundance the distribution is extremely patchy. 



