Influence of diet shift from formulated feed to live mysids on the carbon and nitrogen stable isotope ratio (δ 13C and δ 15N) in dorsal muscles of juvenile Japanese flounders, Paralichthys olivaceus

To estimate the accumulated food consumption of released fish, we investigated the temporal change in the stable isotope ratio of carbon (delta C-13 ) and nitrogen (delta N-15) in dorsal muscles when the diet of Japanese flounder juveniles was switched from a formulated feed to live mysids. The average delta N-15 of fish that fed on mysids ad libitum did not change during the experiment but the average delta C-13 gradually increased from -20.70parts per thousand to -19.19parts per thousand after the diet shift. The delta C-13 values at the start and at the end of the experiment were almost equivalent to the delta C-13 of the formulated feed and mysids, respectively. The relations between time after diet shift and the delta C-13 of fish fed mysids at 2.2% BW/day (2.2% group) and 4.4% BW/day (4.4% group) were as follows: delta C-13 = - 17.25 - 2.03 exp(- 0.04 x t) and delta C-13 = - 17.18 - 2.09 exp(- 0.05 x t), respectively. The change in delta C-13 of the 4.4% group was significantly greater than that of the 2.2% group (likelihood ratio test). Since the growth rate of Japanese flounder juveniles was extremely high and the delta C-13 of the control group that was fed the same formulated feed during the experiment did not change, it was suggested that the change in delta C-13 was mainly attributed to the growth rate of the fish. The combination of stomach content analysis with the stable isotope study makes it possible to estimate the growth rate in terms of body weight and consequently, cumulative food consumption of individual fish after release using the rate of change in delta C-13. (C) 2003 Elsevier Science B.V. All rights reserved.