The phsyiological and locomotive reaction to factors that influence environmental behaviour of Nordic krill from the Gullmarfjorden were studied in terms of swimming energetics, predator avoidence and food utilization. In a newly developed experimental approach, individuals were maintained under defined conditions in flow through chambers and continuously monitored for swimming activity and oxygen consumption. Chemical, physical and biological parameters were applied and the reaction of the krill determined. Stress levels, defined this way, will serve as a reference for unfavourable conditions in the field. Thermal characteristics of digestive enzymes from the midgut gland were furthermore identify the optimum conditions for nutrient assimilation. The results will contribute to the understanding of diel vertical migration, dispersion and aggregation of krill which, in turn is essential for the interpretation of ecosystem dynamics and trophic interactions.
The experimental approach of this study was to fix the krill within a comparatively small respiration chamber of about 20ml and to apply a continuous flow through of air saturated seawater. The dimensions of the chamber and the flow through rate were suitable to detect a strong difference between of 100% oxygen saturation at the inflow vs. 70-90% at the outflow. Variations of respiratory activity appeared within few minutes and thus could be related to different, even short termed, activity levels. Although fixing the krill inside the chamber by gluing seems not adequate for planktonic animals, it did not restrict the abdominal motility and the breathing of the pleopods, and thus the swimming behaviour. The results clearly show that the span between minimum and maximum activity in M. norvegica is high. Fast swimming can double the energy expenditure compared to slow swimming or resting. This observation is in contrast to the concept of low energy expenditure for swimming in zooplankton species (Klyashtorin & Yarzhombek 1973, Foulds & Roff 1976). Kils (1981) showed, that in the case of the Antarctic krill, Euphausia superba, animals may swim at speeds between 0 and 15 cm s-1 without affecting their "standard metabolism" due to optimal execution of the pleopod stroke at this speed. The present study, however, shows on the basis of direct measurements that the surplus of swimming can amount up 100% of the standard metabolism and thus constitute a significant demand of energy. This estimation is in agreement with studies on the euphausiid Euphausia pacifica (Torres & Childress 1983) and the mysid Mysidium columbiae (Buskey 1998). In either species the rates for active metabolism amounted to about the double of the basic rates.