Evaluating the nutritional condition of individual whitefish (Coregonus spp.) larvae by the RNA/DNA ratio

Larvae of Coregonus spp. were reared in the laboratory under different temperature (4, 6 and 8°C) and feeding conditions ( ad libitum , limited, with starvation intervals). Their RNA/DNA ratios were determined with a highly sensitive fluorescence technique. After resorption of their yolk reserves (about 2 weeks after hatching), well fed larvae (RNA/DNA >2.5) could be significantly distinguished from larvae reared under limited food supply (RNA/DNA < 2.5), both at the 6 and 8°C levels. At 4°C, no differences due to the feeding regime were found. During a second series of experiments, larvae were affected by an intestinal disease, which was caused by the ingestion of unsuitable copepod plankton. This disease provoked high mortality, decreased growth and RNA/DNA ratios which were almost as low as in temporarily starved larvae from the first series. Coregonid larvae sampled in Lake Constance during spring 1990 showed RNA/DNA ratios which were unexpectedly low when judged on the basis of mean body length and average ambient temperature. It was obvious from macroscopic observations that some of these wild larvae were severely attacked by the intestinal disease. The low RNA/DNA ratios in field samples are, therefore, interpreted as a sublethal result of this disease.