The freshwater eel Anguilla japonica is rapidly decreasing in number and has not yet been successfully mass produced. This may be at least partially attributable to the unique and long early life history of the eel. Therefore, we investigated its ontogeny of morphometry and growth pattern in larval stages to provide baseline information for understanding the early life history and improving seed rearing technology. This study was conducted for 200 days after hatching (DAH) and analyzed morphometry and allometry for eel larvae. The following cultured eel larval stages were identified: the yolk sac larvae stage (0–6 DAH, 3.23–6.85 mm total length (TL)), the pre-leptocephalus stage (7–30 DAH, 6.85–15.31 mm TL), and the leptocephalus stage (50–200 DAH, 15.31–60.06 mm TL). Cultured and wild eel larvae could be divided into characteristic larval stages at similar sizes. However, compared to wild eels, cultured eels had a slower growth rate and fewer preanal myomeres. Meanwhile, cultured eel larvae rarely had a mixed feeding period as the absorption of endogenous reserves was completed by 7 DAH. The lower jaw of eel larvae was significantly longer than the upper jaw from 50 DAH. In the pre-leptocephalus and leptocephalus stages, eel larvae showed continuous positive allometric growth at trunk height and tail muscle height with change to the willow leaf-like form. These growth characteristics may be the result of adaptation to the migration over long distances and to a diel vertical migration. The inflection point in the body parts growth patterns showed only before 30 DAH, and mass mortality appeared at this period. Therefore, to improve the growth and survival rates of cultured eel seed, it is necessary to focus on improving the feeding and rearing protocol until 30 DAH.

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.

(1) Background: The small yellow croaker, an economically important fish in East Asia, has been subjected to population declines due to overfishing and environmental pressures. The development of effective breeding programs is considered crucial for the species, and accurate parentage assignment is deemed essential for such programs. (2) Methods: The assembled reference genome of the small yellow croaker was utilized to select highly polymorphic microsatellite markers. A multiplex PCR system was optimized for the simultaneous amplification of these markers. The system’s accuracy was validated using controlled mating pairs and subsequently applied to a group mating scenario. (3) Results: The developed multiplex PCR system demonstrated high accuracy in assigning offspring to their parents in both the controlled and group mating scenarios. (4) Conclusions: The system is presented as a valuable tool for pedigree management, selective breeding, and conservation efforts for the small yellow croaker, facilitating sustainable aquaculture practices and genetic improvement.

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.

Pre-harvest starvation is extensively used in aquaculture; however, its physiological effects on chub mackerel (Scomber japonicus) remain poorly characterized. In this study, we evaluated a 21-day starvation period on growth, tissue histology, lipid/protein mobilization, antioxidant capacity, immune function, and stress responses in reared chub mackerel. The hepatosomatic index declined significantly under starvation conditions from day 5. Whole-body crude lipid decreased significantly from day 3 and plateaued by day 7; muscle crude lipid decreased from day 7; and crude protein declined only after day 14, demonstrating a metabolic transition from lipid catabolism to protein mobilization. Histological analysis revealed hepatocyte shrinkage from day 7 and progressive thinning of the perimysium from day 14 without alterations in the muscle fiber area or diameter. Extended starvation (>14 days) increased plasma antioxidant enzyme activity (GPX and SOD) and lipid peroxidation (MDA), upregulated hepatic gpx4a, sod1, nrf2, il1β, and tnfα, suppressed adaptive immunity (plasma IgM), and elevated plasma HSP70 and cortisol on day 21. Overall, controlled starvation up to 7 days reduces excess fat without depleting structural protein or inducing detectable oxidative or endocrine stress, whereas starvation beyond 14 days results in nutrient depletion and immune suppression.

Although mass-spawning pooling systems are widely used for small yellow croaker (Larimichthys polyactis) aquaculture, they often induce severe genetic bottlenecks driven by reproductive skew. This study evaluated cross-generational genetic diversity and spawning patterns to propose an optimal genetic management strategy. We analyzed 1049 adult broodstock and 950 juvenile offspring using nine microsatellite markers. To mitigate reproductive skew, fertilized eggs were collected via multi-time sampling (19 times) over a two-month spawning season and reared to the juvenile stage. Genetic diversity was highly conserved across generations, with expected heterozygosity maintained at 0.860 in the offspring. Parentage assignment succeeded for 96.2% of the offspring (914 individuals), revealing 802 unique families, of which 89.9% (721 families) were singletons. Also, 60.9% of the broodstock contributed to reproduction, exhibiting a right-skewed participation distribution. Importantly, comparisons with a short-term single-event collection control group demonstrated that our multi-time strategy effectively prevented drastic reductions in effective population size (N_e). These patterns highlight the species asynchronous spawning physiology and confirm that the strategy approximates random mating with minimal genetic drift. We suggest this long-term, multi-time egg collection method as an effective protocol for the sustainable genetic management of multiple-spawning marine fish.