Chapter 5 Use of Yeasts as Probiotics in Fish Aquaculture Paola Navarrete and Dariel Tovar-Ramírez Additional information is available at the end of the chapter http://dx.doi.org/10.5772/57196 1. Introduction According to the UN Food and Agriculture Organization (FAO), “aquaculture, probably the fastest growing food-producing sector, now accounts for nearly 50 percent of the world's food fish” [1]. However, production is hampered by unpredictable mortalities that may be due to the negative interactions between fish and pathogenic bacteria. Intensive fish farming has resulted in a problematic growth in bacterial diseases, prompting the necessary and intensive use of antimicrobials for their treatment. Because of the rapid expansion of aquaculture, a limited supply of fishmeal has the potential to impede the future growth of this industry. Consequently, much effort has been given to studying other protein and oil sources, but finding a suitable alternative has proved to be challenging. Among the alternatives, plant-based formulations are the least expensive, and many such formulations have a suitable protein profile and long-term availability. Oilseeds, in particular soybean and grain products, have great potential as alternative sources of fish feed. Soybeans are rich in protein and represent the most commonly used plant protein source on the world market. Soybean meal (SBM) has already become an important protein source in fish feed. However, the inclusion of some vegetable proteins, such as SBM, in the diets of fish at levels of >20% may induce intestinal disorders including pathomorphological changes in the distal intestinal epithelium accompanied by diarrhea [2, 3], sometimes caused by the anti- nutritional factors that are present in SBM. The addition of probiotics (acid lactic bacteria) to starter diets appeared to improve SBM utilization in first feeding rainbow trout [4]. In this context, two of the major challenges in fish aquaculture facilities are 1) the control of diseases, especially during the earliest life stages, and 2) the improvement of nutrition by optimizing food utilization, especially for new fish species. It is well recognized that the bacterial microbiota of fish is beneficial to the host and affects important biological processes, including nutrient processing and absorption, the develop‐ © 2014 Navarrete and Tovar-Ramírez; licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.