Effect of feed portion size on growth of rainbow trout, Oncorhynchus mykiss (Walbaum), reared at different temperatures

Groups of 125 rainbow trout, Oncorhynchus mykiss (Walbaum) were fed identical daily feed rations using various portion sizes (0.23–4.29 pellets fish − 1 portion − 1 ). The procedure was repeated for rearing temperatures of 5 °C, 10 °C, and 15 °C. Throughout the study, growth rates and feed conversion ratios (FCR) were best when a portion size of 0.52 pellets per fish was used. As temperature increased, good growth rates and FCR were observed over a broader range of portion sizes (0.52–3.11 pellets per fish) than previously seen at 5 °C, indicating a high level of plasticity in the trout's ability to adjust to different portion sizes at higher temperatures. This study suggests that the temperature of the water should be considered when feed portion sizes are decided upon as the rate at which trout are able to catch pellets appears to be influenced by temperature.