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A 2 × 3 factorial design was used to reevaluate the dietary protein requirements and to determine the optimum dietary protein to energy (P/E) ratios in Japanese eel, Anguilla japonica, reared in the recirculating system. For each of two experiments, six experimental diets (45P16, 45P17, 45P19, 50P16, 50P17, and 50P19) were formulated and prepared to contain two protein levels (45 and 50%) and three energy levels (16, 17, and 19 kJ/g diet) at each protein level. In the first experiment, glass eel of initial weight 0.1 ± 0.02 g (mean ± SD) were used, while the second experiment was conducted with juvenile eel of initial weight 15.0 ± 3 g (mean ± SD). The first and second experimental periods were 6 and 16 wk for the glass and juvenile eel, respectively. At the end of the first experiment, there were no protein, energy, and their interaction effects. Also, there were no significant differences in weight gain (WG), specific growth rate (SGR), feed efficiency (FE), and protein efficiency ratio (PER) for glass Japanese eel fed all diets. Although there were no significant differences in growth parameters of glass eel fed all experimental diets, these parameters were higher for fish fed 50P16 than for fish fed the other diets. For the second experiment, there were significant protein effects on WG, SGR, and PER (P < 0.05). However, there were neither significant energy effects nor protein and energy interaction effects on WG, SGR, FE, and PER. Fish fed 45P19 had a higher WG, SGR, and PER than did fish fed 45P16, 50P16, and 50P19 (P < 0.05). However, there were no significant differences in growth parameters among fish fed 45P16, 45P17, 50P16, 50P17, and 50P19 and among those fed 45P17, 45P19, and 50P17. These results may indicate that the optimum dietary protein requirement and the P/E ratio could be 44.3% and 24.1 mg protein/kJ (45P19), respectively, in juvenile Japanese eel, based on WG, SGR, and PER.

A 14‐wk feeding trial was carried out to evaluate the optimum dietary ascorbic acid (AA) level in juvenile sea cucumber, Apostichopus japonicus. Sea cucumbers averaging 1.49 ± 0.07 g (mean ± SD) were randomly distributed into 18 rectangular plastic tanks of 20 L capacity in a recirculating system (20 animals per tank). Six semipurified experimental diets were formulated to contain 0 (l‐ascorbyl‐2‐monophosphate [AMP]; AMP0), 30 (AMP24), 60 (AMP48), 120 (AMP100), 240 (AMP206), and 1200 (AMP1045) mg AA/kg diet in the form of AMP using casein as the main protein source. Sea cucumbers were fed each of the six experimental diets in triplicate groups. At the end of 14 wk of feeding trial, weight gain (WG), specific growth rate (SGR), and feed efficiency (FE) of sea cucumbers fed AMP100, AMP206, and AMP1045 were significantly (P < 0.05) higher than those of animals fed AMP0, AMP24, and AMP48. However, there were no significant differences in WG, SGR, and FE among sea cucumbers fed AMP100, AMP206, and AMP1045 and among animals fed AMP0, AMP24, and AMP48. Whole‐body vitamin C concentration increased with AA content of the diets. Broken‐line analysis of WG showed an optimum dietary AA level of 105.3 mg AA/kg diet in sea cucumber. These results indicated that the optimum dietary vitamin C level in sea cucumber in the form of AMP could be greater than 100 mg AA/kg diet but less than 105.3 mg AA/kg diet.

An 8‐wk feeding trial was conducted to estimate the optimum dietary protein level and protein‐to‐energy (P/E) ratio in juvenile Korean rockfish Sebastes schlegeli. Twenty experimental diets were formulated with four energy levels and five protein levels at each energy level. Four gross energy levels of 14.2, 16.5, 18.6, and 20.9 kJ/g diet were included at various crude protein (CP) levels. Diets containing CP at 30, 40, 45, 50, and 55% had either 14.2 or 16.5 kJ/g energy; those with CP levels of 35, 40, 45, 50, and 60% had either 18.6 or 20.9 kJ/ g energy. After 2 wk of conditioning, fish initially averaging 7.3 ± 0.04 g (means ± SD) were randomly distributed into net cages as groups of 20 fish. Each diet was fed to fish in three randomly selected net cages for 8 wk. After 8 wk of the feeding trial, weight gain (WG) of fish fed 50% and 55% CP with 14.2 kJ/g diet was significantly higher than those of fish fed 30% and 40% CP diets (P 0.05). WG of fish fed 45, 50, and 55% CP with 16.5 kJ/g diet was significantly higher than those of fish fed 30% and 40% CP diets (P < 0.05). WG of fish fed 60% CP with 18.6 kJ/g diet was significantly higher than those of fish fed 35, 40, and 45% CP diets. WG of fish fed 45% CP with 20.9 kJ/g diet was significantly higher than those of fish fed 35, 40, and 60% CP diets. Generally, feed efficiency (FE) and specific growth rate (SGR) showed a similar trend as WG. However, protein efficiency ratio (PER) was negatively related to dietary protein levels. WG of fish did not always increase with increasing dietary protein and energy levels. Comprehensive comparison among diets containing 40, 45, and 50% CP with different energy levels indicated that the increase in protein from 40 to 45% significantly increased WG (P < 0.05), but such effect was not significant when protein increased from 45 to 50% at all energy levels. Increasing dietary energy significantly increased WG of fish fed 40% and 45% CP at each energy level; however, there was no difference in WG of fish fed 50% CP with energy levels of 18.6 and 2.9 kJ/g diet. There was no significant difference in WG of fish fed 50% CP with 18.6 kJ/g or 45 and 50% CP with 20.9 kJ/g diet. Broken‐line analysis of weight gain indicated that the optimum dietary protein level was 50.9 ± 1.1% and PIE ratio was 35.4 ± 0.8 mg/kJ with 14.2 kJ/g diet; the optimum dietary protein level was 49.3 ± 5.0% and P/E ratio was 30.2 ± 1.0 mg/kJ with 16.5 kJ/g diet; the optimum dietary protein level was 46.2 ± 9.2% and P/E ratio was 24.7 ± 4.9 mg/kJ with 18.6 kJ/g diet; and the optimum dietary protein level was 45.1 ± 1.8% and P/E ratio was 21.5 ±0.7 with 20.9 kJ/g diet. Therefore, these data indicated that the concept of P/E ratio must be restricted to diets containing adequate protein and energy levels. Based on WG, the optimum P/E ratio was between 21.5 and 35.4 mg protein/kJ gross energy in juvenile Korean rockfish when gross energy ranged from 14.2 to 20.9 kJ/g diet.