Feeding strategies are needed for reducing the impact of microbial diseases on the production performance of whiteleg shrimp (Litopenaeus vannamei). The present experiment studied the effect of dietary yeast hydrolysate (YH) on the performance, survival rate and immunity of whiteleg shrimp in three conditions: a) without an intentional challenge factor, b) with Vibrio parahaemolyticus (Vp) challenge, and c) with white spot syndrome virus (WSSW) challenge.
The study was started in 27 aquariums of 120 liters with 15 ppt saline water, and 30 juvenile shrimp with a mean initial weight of 1.54 g. Commercial-type shrimp feed was amended with YH (Progut® Extra, Hankkija Oy, Finland) at 0.0, 0.5, and 1.0 kg/ton for the dietary treatments T1-T3, respectively (9 replicate aquariums/diet). Feed was given at 3-5% of body weight 3 times a day. Uneaten feed was siphoned out 1 hour after feeding, dried and weighed. Shrimp weight, feed intake, feed conversion ratio (FCR) and mortality were recorded every 2 weeks. Six replicate aquariums/diet were allocated to the 8-week performance test without a challenge factor. At the 4-week time point, shrimp from the remaining 3 replicate aquariums/diet were allocated into 100-litre challenge aquariums, 10 shrimp/aquarium and 3 aquariums/diet for both challenges. For the Vp challenge test, the highly virulent EMS/AHPND strain was used as an immersion treatment. Shrimp mortality was monitored for 21 days post-challenge. For the WSSV challenge test, the virus was introduced by subcutaneous injection to each shrimp. Six hours post-infection haemolymph was collected for confirming the infection, and mortality was monitored for 7 days post-challenge. After each test, samples of haemolymph, hepatopancreas, and intestinal contents were collected for analyses of microbiological and immunological parameters. The study was conducted in a completely randomized design. Data was analysed with one-way ANOVA, followed by Duncan’s multiple range test, using p<0.05 as a limit for statistical significance.
In the performance test without intentional challenge, dietary YH improved shrimp weight gain between weeks 4 and 6, and FCR for periods 0-4 weeks, 0-6 weeks, and 0-8 weeks. The 8-week survival rate was over 80% for all treatments (NS). Density of Vibrio (background challenge with low virulence) in hepatopancreas and intestine was lower for T2 and T3 than T1. Haemolymph protein concentration and the activities of lysozyme and superoxide dismutase were higher for treatments T2 and T3 than T1. In the Vp challenge test, the 21-day post-challenge survival rate was 66.7%, 73.3%, and 83.3% for T1-T3, respectively (p<0.05). Dietary YH decreased Vibrio counts in hepatopancreas and intestine. Hemocyte count, haemolymph protein concentration, and activities of phenoloxidase and lysozyme were higher for T2 and T3 than T1. In the WSSV challenge test, the 7-day post-challenge survival rate was 23.3%, 66.7%, and 80.0% for T1-T3, respectively (p<0.05). Haemolymph protein, and activities of phenoloxidase, lysozyme and superoxide dismutase were higher for T2 and T3 than T1. Moreover, dietary YH decreased the virus counts in haemolymph.
According to the results, dietary YH improved the performance, immunity, and antioxidative capacity of whiteleg shrimp in the unchallenged condition, and increased shrimp survival when challenged with virulent strains of Vp or WSSV. In conclusion, dietary YH amendment may be a suitable strategy for improving the production performance and disease resistance of whiteleg shrimp.