Age groups of antarctic krill,Euphausia superba dana, in the Prydz Bay region

Abstract

Age groups of Antarctic krill (Euphausia superba Dana) in the Prydz Bay region were studied by distribution mixture analysis based on length/frequency data collected by R/VJidi during the 1989/1990 and 1990/1991 austral summer. Five age groups were determined, i.e. 1+, 2+, 3+, 4+, and 5+, or six age groups in all, if the 0+ larvae were included. The mean body length of 1+ to 5+ age groups was 25.70 mm, 40.47 mm, 45.52 mm, 50.52 mm and 54.52 mm respectively. Supposing the difference in body length between successive age groups is a reflection of the early growth, the maximum growth rate occurred during the period from 1+ juveniles to 2+ subadults (14.77 mm/a). From 2+ subadults to 3+ adults the growth rate dropped steeply (5.05 mm/a) because at this stage, increase of body length was substituted, to a great extent, by the growth of sexual products. From 3+ onwards the growth rate was maintained at a relatively low level and decreased slowly with age. The relative abundance of age groups 1+ and 2+, in our sample must be much lower than that in the real population owing to both the large mesh size we used and the distribution difference between juveniles and adults. If we left aside 1+ and 2+ age groups and just looked at the relative abundance of adults, we found that age group 3+ dominated the adult population and that the relative abundance decreased sharply with increasing age. If this situation is normal, one can expect an extremely high mortality rate in adults, 82.6% from 3+ to 4+ and 94.0% from 4+ to 5+. This is reasonably expectable for the Prydz Bay region.