Samantekt:
High mortality rates commonly observed during the early life stages of intensively reared Atlantic halibut have among other things been related to high bacterial numbers and an unfavourable bacterial community. The study describes the effects of two different methods for environmental shading on larval survival and numbers of cultivable bacteria in the culture water and the gastrointestinal tract of first feeding larvae at Fiskey Ltd. Larval survival was not affected by the method used for environmental shading. Lower bacterial numbers were observed in the tank water with environmental shading provided by inorganic clay as compared with marine algae, primarily during the first days of exogenous feeding. Gram negative, fermentative bacteria dominated the cultivable community in the gastrointestinal tract of larvae during the first weeks in feeding. The use of inorganic clay has clear economic advantages as compared to the use of marine microalgae, and the commercial producer has used the product exclusively for environmental shading during first feeding of halibut larvae since 2003.