1. Using stable isotope ratios to explore the trophic ecology of freshwater animals requires knowledge about effects of food quality on isotopic incorporation dynamics. The aim of this experimental study was to: (1) estimate carbon and nitrogen isotopic incorporation rates and trophic discrimination factors (TDFs) of a freshwater first‐feeding fish (i.e. salmonid fry) fed three diets that differed only in protein quality (animal or plant or a blend of both); (2) investigate effects of fasting and; (3) evaluate the proportion of each source assimilated when fry were fed a 50:50 animal:plant‐based diet.
2. For each diet, incorporation rates of δ¹³C and δ¹⁵N values were estimated using a time or growth‐dependent isotopic incorporation model. Effects of fasting on isotope ratio values were measured regularly until the death of fry. Bayesian stable‐isotope mixing models were used to estimate the contribution of animal and plant material to fish fed a blend of both food types.
3. Our results show that incorporation rates were lower for fry fed a plant‐based diet than for those fed an animal‐based diet as growth rate decreased. Time‐ and growth‐dependent models indicated that growth was solely responsible for isotopic incorporation in fry fed an animal‐based diet, whereas catabolism increased in fry fed a plant‐based diet. After lipid extraction, carbon TDFs were similar regardless of the diet, whereas nitrogen TDFs increased for fry fed a plant‐based diet. Long‐term fasting induced an increase of 0.63‰ in δ¹³C values of fry in 23 days, whereas δ¹⁵N values did not vary significantly. Proportions of food sources assimilated by fry fed an animal:plant‐based diet were similar to those consumed when using a mixing model with the estimated TDFs, while proportions were unrealistic when using mean TDFs extrapolated from the literature.
4. The results of our study indicate that the quality of food must be considered to use an appropriate timescale to detect changes in fry diets in the field. Moreover, we recommend using different carbon and nitrogen TDFs, one for animal‐derived sources and one for plant‐derived sources, to increase the accuracy of mixing models.

中文翻译：

---

蛋白质质量如何驱动淡水首次摄食鱼类的掺入率和碳氮稳定同位素比的营养区分

1. 使用稳定的同位素比率探索淡水动物的营养生态需要了解食品质量对同位素掺入动力学的影响。这项实验研究的目的是：（1）估算仅饲喂三种蛋白质质量（动物或动物）的日粮的淡水首次摄食鱼（即鲑鱼苗）的碳和氮同位素掺入率和营养歧视因子（TDF）。植物或两者的混合物）；（2）研究禁食的效果；（3）评估以动物：植物为基础的饮食以50:50喂鱼苗时每种来源的同化比例。
2. 对于每种饮食，δ的掺入率¹³ C和δ ^15个N个值使用时间或生长依赖性同位素掺入模型估计。定期测量禁食对同位素比值的影响，直至鱼苗死亡。贝叶斯稳定同位素混合模型用于估计动植物材料对两种食物混合饲料对鱼类的贡献。
3. 我们的结果表明，随着动物生长速度的降低，以植物为基础的鱼苗的掺入率要低于以动物为基础的鱼苗的掺入率。基于时间和生长的模型表明，以动物为食的鱼苗中同位素的掺入是生长的唯一原因，而以植物为食的鱼苗中分解代谢增加。脂质提取后，无论何种饮食，碳TDF均相似，而以植物性饮食喂养的鱼苗，其氮TDF升高。长期禁食诱导δ同比增长0.63‰ ¹³在23天鱼苗的C值，而δ ¹⁵N值变化不大。用动物：植物性饲料喂养的鱼苗所吸收的食物来源比例与使用带有估计的TDF的混合模型时所消耗的食物来源相似，而当使用从文献中推断的平均TDF时所占比例则是不现实的。
4. 我们的研究结果表明，必须考虑食品质量，以使用适当的时间范围来检测田间鱼苗饮食的变化。此外，我们建议使用不同的碳和氮TDF，一种用于动物源，另一种用于植物源，以提高混合模型的准确性。