The aim of our visit to Kristineberg was to study the stable carbon and nitrogen isotope fractionation of Meganyctiphanes norvegica in response to different food supply, and to evaluate the importance of physiological processes (assimilation and growth) in generating the new stable isotope pattern. This calibration will contribute to the evaluation of the stable isotope method as an approach to the study of food sources of animals in the field.
Furcilia and juvenile krill were caught with an Isaac-Kidd Midwater Trawl (mesh size 1.5mm) in the deep basin of the Gullmar-Fjord on August 31 and on September 5. After the first catch we trained in handling of the animals, chacked their survival in the incubation bottles and tested their feeding on the two food sources (Artemia and commersial fish food/Aquafauna Bio-Marine). Before starting the experiments all healthy looking individuals were weighed gently, to obtain their initial weight. The water for incubation was <1 microm filtered seawater taken from the high salinity stratum of the fjord. The experiments were carried out at 8 degrC and dim light, with krill maintained individually in 1L bottles. About 80 krill were selected for the Artemia food, which was either offered at saturating concemtrations (300 individuals per bottle) or unsaturating concentrations (100 individuals per bottle). Fish food was offered in saturated concentration to about 100 krill. Every day, molts were collected, the uneaten Artemia were counted, and the krill were transferred to new bottles with fresh food. Assimilation rate of the fish food was estimated once (comparing the C and N concentration in experimental and control bottles). To examine individual and daily variation in the feeding-defecation activity, as well as to measure assimilation efficiency, fecal pellets have been collected for seven individuals over a period of two weeks. The metabolic activity (respiration and NH4 excretion rates) has been measured 2 to 3 times for each individual. For the fish food treatment, 5 individuals of different size classes were frozen every week, after measuring their final wet weight. For the Artemia treatment, animals were frozen only at the end of the experiment.