Using stable isotope ratios as a tracer of feeding adaptation in released Japanese flounder Paralichthys olivaceus

Release-recapture experiments were conducted to examine temporal changes of the carbon and nitrogen stable isotope (δ¹³C and δ¹⁵N) ratios in the muscle tissue of artificially produced Japanese flounder Paralichthys olivaceus, juveniles. About 9000 juveniles (mean ± s.d. 43.3 ± 5.2 mm in standard length and 1.07 ± 0.37 g, n = 15) were released in each of three coastal areas: Chojagasaki, Arasaki and Jogashima with different geographical conditions, along Sagami Bay, Pacific coast of central Japan. Recapture efforts were made on 4, 11, 18, 40 and 55 days after the release. The stable isotope ratios, RNA:DNA ratio, stomach content mass (per body mass M_sc) and condition factor (K) of recaptured individuals were measured. The mean ± s.d. δ¹³C and δ¹⁵N values (n = 15) were -18.3 ± 0.2‰ and 12.2 ± 0.2‰, respectively at the release. Wild Japanese flounder juveniles were captured only in Chojagasaki, and the δ¹³C and δ¹⁵N values (n = 6) were -14.0 ± 0.4‰ and 13.2 ± 0.7‰, respectively; these values were considered to represent the wild diet. Nutritional conditions of the released and recaptured juveniles as determined by the RNA:DNA ratio, M_SC and K were indicated to be the best in Chojagasaki, in which the stable isotope ratios gradually shifted towards and reached the wild values within 40 days. This result along with stomach content analyses suggested that the released juveniles had acquired a wild feeding habit. In Arasaki and Jogashima, nutritional conditions of the recaptured juveniles were poorer, with no clear changes in the stable isotope ratios. Greatly varied stable isotope ratio values were observed in the juveniles recaptured in Chojagasaki 11 days after the release, ranging from the release levels to the wild levels. The extent of changes in the stable isotope ratios had a positive correlation to the RNA:DNA ratio and K of these juveniles (r = 0.87, n = 10 and r = 0.83, n = 18, respectively). The analyses of stable isotope ratios coupled with nutritional condition were considered to be an effective tool to examine post-release feeding adaptation of Japanese flounder juveniles.