Hatchery Technology for High Quality Juvenile Production

Benedenia disease caused by the ectoparasite Benedenia seriolae is a serious parasitic disease in yellowtail aquaculture, leading to secondary viral or bacterial infections. Benedenia disease is difficult to prevent in marine aquaculture systems. To evaluate the genetic basis of Benedenia disease resistance in yellowtail, a genome-wide and chromosome-wide linkage analysis was initiated using F1 yellowtail families. Two major quantitative trait loci (QTL) regions on linkage groups Squ2 and Squ20 were identified and then confirmed in F1 families. These QTL regions explained 32.9– 35.5% of the phenotypic variance. This is the first genetic evidence detailing phenotypic resistance to Benedenia disease, and the results will help resolve the mechanism of resistance to this important disease of yellowtail. INTRODUCTION In Japan, the name yellowtail is commonly used for three species, Seriola quinqueradiata, S. lalandi and S. dumerili. The combined production of cultured yellowtail in Japan was approximately 152,800 tons in 2009, which accounted for 59% of marine finfish aquaculture in Japan (MAFF of Japan 2009). Yellowtail have been cultured in the southern areas of Japan using juveniles caught in natural waters and sometimes imported from other countries as seedlings. Harvest of wild juveniles has varied from year to year, but in recent years it has declined, likely due to decreasing wild populations (Nakada 2008). Capturebased aquaculture (CBA) is practiced worldwide on a variety of marine species, with important environmental, social and economic implications. A problem of CBA is that it has a negative impact on wild stocks of both targeted species and non-targeted species. Therefore, there has been an increased interest in artificial seedling production. Advances have been made in artificial seedling production of yellowtail (Mushiake et al. 1994; 1998), and it is expected that artificially produced seedlings will eventually replace seedlings caught from wild sources. Although research on disease, nutrition and pond management has supported the development of the yellowtail aquaculture industry, genetic improvement programs leading to improved yellowtail lines are still nascent. Genetic linkage maps play a prominent role in many areas of genetics, including quantitative trait locus (QTL) analysis, marker-assisted selection (MAS), positional candidate or positional cloning of genes approach, and comparative genomics. Linkage maps are essential tools to study QTL. Low resolution genetic linkage maps were first constructed for yellowtail by Ohara et al. (2005), and recently, the second generation maps that span the genome at a higher resolution have been completed (Fuji et al. 2012). The map contains several hundred markers with microsatellites associated with candidate genes. The map will facilitate the genome mapping efforts in S. quinqueradiata, and other related species. The mapping data could be compared to reference species and utilized for QTL analyses and further MAS breeding programs of yellowtail. Benedenia disease, caused by Benedenia seriolae, is a serious parasitic disease for yellowtail in aquaculture, and often leads to secondary infection due to viral or bacterial disease. Because fish rub their bodies against the fish cage to remove the parasite, the mortality can be quite high, particularly with juveniles. Although the method of removing the parasite is generally to soak affected fish in a freshwater bath, this method takes a lot of time, cost, and effort. Thus, Benedenia disease is difficult and costly to treat in marine aquaculture systems. In addition, for wildlife conservation, yellowtail aquaculture is considered a potential vector of parasite transmission (Hutson