The threeline grunt (Parapristipoma trilineatum) is a recently introduced aquaculture species that has attracted considerable interest in Korea. However, research on its aquaculture potential and reproductive biology remains limited. This study aimed to investigate the natural reproductive cycle and the feasibility of manipulating the spawning period of the threeline grunt through environmental control. We designed an environmental changes protocol involving a gradual increase in water temperature (16 °C → 21 °C) and extension of photoperiod (10 L → 14 L) from January to April and compared the group exposed to these conditions to those maintained under natural conditions. The experimental group exhibited significantly higher plasma sex steroid hormone levels at 60 and 90 days and significantly higher gonadosomatic index at 100 days. In addition, the experimental group spawned between April and June, approximately 2 months earlier than that of the control group. Despite earlier spawning, no significant differences were found in egg diameter, hatching rate, and larval notochord length. These findings suggest that spawning can be accelerated through photothermal changes without negatively impacting egg and larval quality. This study provides valuable insights into the reproductive biology of the threeline grunt and highlights the potential of utilizing photothermal control to enhance its aquaculture production.

To evaluate the overwintering performance of threeline grunt (Parapristipoma trilineatum), field experiments were conducted on 3000 subadults and 5000 juveniles reared in sea cages along the southern coast of Korea (SC group), and 600 subadults and 700 juveniles reared in inland tanks on Jeju Island (IT group), under thermally suitable overwintering conditions. Fish were sampled from January to April. The SC group maintained acceptable survival rates of 77.2% in subadults and 75.8% in juveniles. Body weight and standard length in the SC group did not significantly change during overwintering; condition factor and hepatosomatic index varied at specific sampling points. The SC group juveniles exhibited lower plasma glucose levels than the IT group ones. Overwintering in sea cages influenced body composition, characterized by reduced crude protein and increased crude lipid content. Plasma cortisol levels in the SC group juveniles increased significantly in January, whereas hepatic heat shock protein-70 levels showed no distinct changes. Principal component analysis revealed no clustering among groups. Physiological variations in the SC group reflected seasonal adaptation, evidenced by stable survival and no strong coordinated changes among physiological responses. These findings highlight the potential of threeline grunt to overwinter in sea cages along the southern coast of Korea.