Temperature regulates sex determination and growth in the paralichthid flatfish California halibut

California halibut (Paralichthys californicus) is a candidate species for aquaculture and stock enhancement. These applications rely on sex control, either to maximize the production of faster growing females or to match sex ratios in the wild. Other paralichthids exhibit temperature-dependent sex determination (TSD), but the presence and pattern of TSD is not well defined in California halibut. Juvenile California halibut were cultured at three distinct temperatures (15°C, 19°C, and 23°C) through the developmental period presumed to be thermosensitive based on findings from congeners. Sex ratios were quantified in each treatment using phenotypic sex identification techniques applied early (molecular biomarkers; 51–100 mm total length [TL]) and late (visual examination of the gonads; ≥100 mm TL) in the juvenile phase. Both techniques indicated similar sex determination trends at each temperature, with overall sex ratios assessed as 49.9% male at 15°C, 74.5% male at 19°C, and 98.2% male at 23°C. Growth rates were highest at 23°C and lowest at 15°C, with intrinsically fast- and slow-growing individuals at all temperatures. At 15°C and 19°C, females comprised a higher proportion among the fast growers than they did among the slow growers. These data show that California halibut exhibit TSD, with temperatures of 19°C and 23°C masculinizing fish while 15°C appears to produce a 1:1 sex ratio. This study will help optimize sex ratios and growth in hatcheries through thermal manipulation. Furthermore, the developed biomolecular tools and identified temperature thresholds will be important in future work to understand the influence of global warming on wild population demographics.