Effect of water temperature on the protein digestibility of formula feed and mainly-raw-fish diet of young yellowtail

The effects of protein digestibility of formula feed (extruded pellet; EP a nd single moist pellet; SMP) and mainly-raw-fish diet (OMP) of young yellow tail Seriola quinqueradiata on the post-feeding changes of apparent protein digestibility (APD) and gastric digesta contents during several seasons we re examined, APD of OMP were constantly about 85%, irrespective of the season and post-f eeding time. APD of EP and SMP were 60-80% at the earlier time of post-feed ing and 80-90% at the later time. In these two feeds, the lowering of APD w as observed in low temperature season. Gastric evacuation rates were marked ly slower in fish fed EP than in fish fed SMP or OMP, during all seasons. A ccumulative protein digestibility values calculated based on post-feeding c hanges of gastric digesta contents and APD were about 85% in fish fed OMP, during all seasons. On the other hand, those values declined from 83% to 77 % in fish fed EP, and from 82% to 68% in fish fed SMP, as water temperature declined. These results indicate that the protein digestibility of formula feeds such as EP and SMP are lower than OMP for young yellowtail during th e low water temperature periods.