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A lack of methods for sex control (i.e., controlling the differentiation/development of the gonads) is a major impediment to robust growth and sustainability of marine aquaculture and is important in two ways: 1) Sterile fish are highly desirable due to biosafety concerns associated with escapement of farmed fish from net pens where they could genetically contaminate wild stocks. 2) All-female aquaculture production of species such as sablefish should improve growth rates because females grow larger than males. The purpose of this project is to determine when sablefish gonads normally differentiate into ovaries or testes and to establish methods to control this process. This research is being conducted by NWFSC FTEs, a contractor, and a University of Washington undergraduate student. The process of normal gonadal differentiation is being studied at the morphological level with histology and at the molecular level using a cutting-edge technique known as Next-Generation Sequencing. Cultured sablefish are also being treated with hormones (estrogen and testosterone) delivered via the diet during early development to determine the window when sex can be controlled in this species. After maturation, fish treated with hormones will also enable us to reveal for the first time the sablefish sex-chromosome system (e.g., XX/XY as in humans?). Some of these fish will also be useful in strategies to produce all-female stocks in a natural manner for aquaculture. This research is expected to produce a description of early sexual development in sablefish, including criteria for distinguishing the sexes with histology, data on the timing of sexually-dimorphic growth, sequences for genes expressed in differentiating ovaries and testes of sablefish, sex-specific molecular markers, and ultimately all-female and/or sterile stocks of sablefish to test in the aquaculture setting. It is important to note that research on sex control in sablefish would apply in many ways to other candidate marine aquaculture species. This ongoing project is critical to the development of sustainable aquaculture in the United States. Furthermore, information related to basic reproductive biology (age, size and timing of sexually dimorphic growth and maturation) and genetics is important to stock assessment and management of valuable, wild sablefish stocks in the North Pacific. Fish in experiments are often PIT tagged and regularly checked for growth in fork length and body weight.