Krill meal enhances performance of feed pellets through concentration-dependent prolongation of consumption by Pacific white shrimp, Litopenaeus vannamei

Pacific white shrimp, Litopenaeus vannamei, is often grown on feed containing 30–40% soybean meal or other sources of plant protein because of its favorable cost and availability compared to marine animal protein. However, plant material is relatively less attractive and palatable to shrimp compared to animal material. Consequently, the current practice is to include some marine animal meal to enhance the attractability and palatability of feeds containing high levels of plant protein. Our study examined mechanisms underlying krill meal's effects on feed consumption by L. vannamei. The first step required behavioral experiments to quantify in a concentration-dependent manner the effect of krill meal on attractability and palatability of feed using assays of groups of animals. We showed that an aqueous extract of krill meal is highly attractive, causing shrimp to alert, move toward, probe, and grab an airstone releasing it in a concentration-dependent manner and at concentrations as low as 13.3μg/ml. We then showed that krill meal added to feed pellets (with a base diet of 30% soybean meal, 27% poultry protein concentrate, 34% wheat flour, and additional minor components) enhanced the palatability (i.e., consumption) of pellets in a concentration-dependent fashion, with 6% krill meal being significantly effective. The second step required performing two experiments to characterize the mechanisms underlying the enhanced performance of krill-containing pellets. The first experiment tested the hypothesis that krill meal-containing pellets are more attractive than control pellets. Our results did not support this hypothesis for trials simulating short (5-min) feeding trials but did for long (60-min) trials. The second experiment tested the hypothesis that krill meal enhances the shrimps' rate of feeding on pellets, by examining the behavior of individual shrimp in a palatability assay in which individual animals were fed single pellets sequentially until it stopped feeding (defined as not completely consuming a pellet within 60min). This experiment showed that krill meal did not significantly increase the rate of feeding on a pellet, but rather it increased in a concentration-dependent manner the number of pellets eaten and the total time spent eating before shrimp stopped eating. Taken together, our results support the idea that krill meal is a chemostimulant whose major effect when added to feed pellets is to increase the pellets' palatability by prolonging t