Energy budget of the Japanese flounder Paralichthys olivaceus (Temminck & Schlegel) larvae fed HUFA-enriched and non-enriched Artemia nauplii

The energy budget of the Japanese flounder Paralichthys olivaceus (Temminck & Schlegel) larvae fed enriched (EA) and non-enriched (NEA) Artemia nauplii was determined by equating energy intake (EI) with the summation of energy channelled to faeces (F), metabolism (M), excretion (U) and growth (G). Larvae (21 days post hatching, 2.2 mg mean wet wt) were reared in six 80-L circular tanks with three replicates of 160 larvae per tank and fed EA and NEA for 20 days. EI was calculated from the energy content of consumed nauplii, M from the summation of energy for routine, feeding and active metabolisms, U from ammonia excretion and G from energy gained based on weight gain, while F was the difference between EI and the total of other components. The heat increment of larvae was calculated from the difference of O2 consumption at post-prandial and routine conditions. Except for G and F, variables were correlated to the dry body weight (W) of larvae in a power function: Y=aWb. Coefficients a and b were estimated by regression after a logarithmic transformation of the raw data. Overall, growth and survival rates of the larvae fed EA were higher than those fed NEA. For a larval flounder growing from 2 to 20 mg wet wt, the ingested energy was partitioned as follows: 22.8% to faecal loss, 38.3% to metabolism, 1.5% to urinary loss and 37.4% to growth for the EA group, whereas 35.4% to faecal loss, 28.4% to metabolism, 1.3% to urinary loss and 34.9% to growth for the NEA group. Gross conversion and assimilation efficiencies were higher, but the net conversion efficiency was lower in EA-fed larvae than NEA-fed larvae. This study suggests that the higher growth and survival rates of the EA-fed group compared with the NEA-fed group were attributed to their higher intake of essential fatty acids, higher metabolism and lower energy loss of faeces.