Availability of blue mussels as supplements in the diet of Japanese flounder
Feeding experiments were conducted to examine the availability of blue mussel as supplements in the diet of juvenile Japanese flounder Paralichthys olivaceus. Fish of less than 10 g initial body weight were fed the diets twice daily to apparent satiation each 6 days per week for 6 weeks at 20℃. Prior to the feeding experiments, enzymatic dissolution of the mussel meat was compared to detach the mussel meat effectively from the shell. Among papain, trypsin, pancreatin, carboxypeptidase, pepsin and driselase, papain and trypsin showed significantly higher activity than that of the other enzymes at 50 to 70℃. In trial 1, 10 to 20% of the control diet was exchanged with freeze-dried meat of blue mussel, mussel meats treated with papain and trypsin. The control diet mainly consisted of fish meal, potato starch, and pollack liver oil. The final body weight and weight gain of fish fed diets containing the mussel were higher than those of fish fed the control diet, although significant differences were not found except for weight gain of fish fed the diet containing trypsin treated mussel meat. The growth of fish was independent of the inclusion level of papain treated mussel meat(10 and 20%). Feed efficiency and protein efficiency ratio were similar among dietary groups tested. In trial 2, 5, 10, and 20% (weight/weight) of the control diet was exchanged with aqueous extracts of blue mussel. The final body weight, weight gain, and feed efficiency of fish fed diets containing the extracts were significantly higher than those of fish fed the control diet. However, these parameters were not different among diets containing the extracts independent of the inclusion level. A similar trend was shown in protein efficiency ratio as fish fed the control diet had a significantly lower value than the other dietary groups. In both trials, triglyceride level of fish fed diets with enzymatic dissolutions and aqueous extracts of the mussel meats was lower than that of fish fed the control diet, while other blood constituents were relatively similar for the dietary groups tested. Whole body crude lipid content and lipid retention of fish fed the mussel diets were higher than those of fish fed the control diet. Whole body crude protein was identical regardless of the dietary composition, however, protein retention of fish showed a similar trend to lipid retention. Dietary levels of free amino acids, especially alanine and glycine, increased with the inclusion of the mussels, and these amino acids are considered to the most plausible feeding attractants in blue mussels for Japanese flounder.