Optimum lysine requirement of juvenile Pacific white shrimp, Litopenaeus vannamei when fermented soybean meal partially replacing fish meal in the feed

The fish meal remains a high price with the worldwide shortage. Seeking different plant protein sources replacing fish meal to reduce the cost of feed has become one of the hotspots in shrimp nutrition research. However, most plant protein sources face the problem of amino acid imbalance after substituting fish meal, which affects the growth performance and feed utilization of the shrimps. Supplementing crystalline amino acids in feed is still the most effective and practical method to achieve amino acid balance of feeds. This study intended to explore the effects of different levels of lysine in feed on the growth performance, body composition and blood indices of Pacific white shrimp L. vannamei after replacing fish meal with fermented soybean meal at an optimal level, and to estimate the optimal dietary lysine requirement of juvenile L. vannamei. Five isonitrogenous and isolipidic practical diets(containing 41.5% crude protein and 7.5% crude lipid) were formulated with dietary supplemental lysine levels of 0%, 0.25%, 0.50%, 0.75% and 1.00%, and the measured values of dietary lysine contents were 2.43%, 2.65%, 2.87%, 3.06% and 3.24%, which were recorded as Lys0, Lys25, Lys50, Lys75 and Lys100. The L. vannamei with an average initial body weight about (2.0±0.1) g was randomly divided into 5 groups with 3 replicates per group and 50 shrimp per replicate. The feeding experiment lasted for 56 days in indoor cement pools. The results showed that, with the increase of feed lysine level, the weight gain rate and specific growth rate increased first and then decreased, reaching the highest in the Lys75 group with the lowest feed conversion ratio. The survival rate was not affected by feed lysine level. Muscle in Lys75 group had the highest crude protein and crude fat content, while crude ash and moisture content were not affected by feed lysine level. A variety of essential and non-essential amino acids in the muscle showed a trend of first increasing and then decreasing with the increase of feed lysine level. The turning point was in the Lys50 or Lys75 group. With the increase of feed lysine level, the activities of alanine aminotransferase and aspartate aminotransferase in serum increased first and then decreased, while the triglyceride content gradually increased, and the superoxide dismutase activity of Lys75 group was significantly lower than the other four groups. Based on the quadratic regression model of SGR and FCR with dietary lysine content, The optimal lysine requirements of L. vannamei were 2.88% (6.94% of dietary crude protein) and 2.89% (6.96% of dietary crude protein), respectively.