REPLACEMENT OF SULFUR, CARBON, AND NITROGEN IN TISSUE OF GROWING BROAD WHITEFISH (COREGONUS-NASUS) IN RESPONSE TO A CHANGE IN DIET TRACED BY DELTA-S-34, DELTA-CL-13 AND DELTA-N-15

We monitored the change in the isotope composition of sulfur, carbon, and nitrogen in broad whitefish (Coregonus nasus) tissues in response to a change in the isotope composition of their food. One of two batch es of 2.5-yr-old fish raised in the laboratory were given a new food s ource with different delta(34)S, delta(13)C, and delta(15)N, which wer e monitored in muscle and liver tissue for 1 yr. A model including cha nge due to tissue accumulation (growth) and metabolic replacement was developed. For all three isotopes, most of the change could be attribu ted to growth. Metabolic replacement expressed as a turnover rate was only 0.1-0.2%.d(-1) and was similar for the three isotopes. Although l iver tissue was -4.4 and -4.1 parts per thousand, respectively, for de lta(34)S and delta(13)C relative to muscle tissue, the response over t ime to the new food was the same as for muscle. We expect that the com plete change in the isotope composition of fish tissue in response to a change in food could take years in slow-growing wild populations. Th e stable isotope composition would represent a long-term average of th e food. In fast-growing fish the rate of change would directly reflect the growth rate.