Fishmeal is typically the main protein ingredient when formulating a diet for carnivorous fish due to its high protein content, good digestibility and balanced essential amino acid profile . Issues around the sustainability and high cost of fishmeal have driven research to reduce its inclusion and replacement us ing alternative animal protein sources. Two feeding trials were conducted to determine the effect of dietary protein and fishmeal replacement on survival, growth and feed conversion ratio (FCR) of juvenile yellowtail, Seriola lalandi in a RAS. The first experiment investigated four formulated iso-calorific diets containing different dietary protein levels (38%, 44%, 50% and 56%) for 49 days. Dietary protein level did not have an effect on survival of juvenile yellowtail. In terms of growth performance, specific growth rate (SGR) (ANOVA: F (3,8) =10, p=0.003) and weight gain (F (3,8) =10, p=0.003) was greater in fish fed 56% protein compared to fish fed 38% protein but there were no differences between the 56%, 50% and 44% protein diets. There were no differences in protein efficiency ratio (PER) (1.28±0.32- 1.6±0.5) between the four diets. FCR decreased with increasing protein level with fish being fed the 56% protein yielding the lowest FCR (1.26±0.09) compared to fish fed 38% protein (1.76±0.2) (F (3,6) =13, p=0.003) however there was no difference between the 44%, 50% and 56% protein levels.
A second experiment investigated alternative animal protein sources using the 44% protein diet from Experiment 1. Four experimental diets were formulated to be iso-nitrogenous (44% protein) and iso-calorific (19.50 Mg/Kg). Diets included (i ) a control diet with 100% fishmeal as the only protein source (the same formulation as used in Experiment 1), (ii) 67% fishmeal, (iii) 33% fishmeal and (iv) 0% fishmeal with the balance of protein being made up in diets 2-4 of chicken meal, blood meal and soyabean meal in similar proportions. Different fishmeal levels in the diets did not have an effect on the survival on juvenile yellowtail. Specific growth rate was however greater (0.50 ±0.05-0.54±0.10) for fish fed the 67% and 100% fishmeal diets (One-Way ANOVA, F= 6.27 df=3, p= 0.017) compared to fish fed 0% fishmeal (0.16±0.16). There were however no differences in SGR between fish fed 33%, 67% and 100% fishmeal (0.46±0.17-0.54 ±0.10). Diets containing fishmeal had FCR’s which were significantly lower than the diet with no fishmeal at all (p= 0.001). Fish fed diets having 67% and 100% fishmeal had low PER (0.44±0.11-0.45±0.11) (p= 0.026) compared to fish fed 0% fishmeal (0.11±0.10). The results suggest that a 44% protein diet is optimal and that full replacement of fishmeal (0% fishmeal diet) is not recommended as it produced the worst SGR, FCR and PER. The remaining diets were statistically similar suggesting that 33% fishmeal diet produces equivalent growth, FCR and PER as the 67% and 100% fishmeal diets. However, as there were similarities in SGR and PER for 0% and 33% fishmeal diets it is recommended that future experiments possibly focus on the 10% - 50% fishmeal component to find the optimal breakpoint for the various growth and feeding indicators while reducing diet cost and the reliance on fishmeal.