Practical diets for California yellowtail, Seriola dorsalis: Use of advanced soybean meal products on growth performance, body composition, intestinal morphology, and immune gene expression.

California yellowtail (CYT), Seriola dorsalis, is a promising candidate for aquaculture due to its rapid growth and high-quality flesh, particularly in markets like Japan, Australia, China, and the United States. Soy protein has shown success as a replacement for marine protein sources in CYT diets, reducing fishmeal levels, though concerns about potential intestinal inflammation persist with the inclusion of solvent-extracted soybean meal. To address this, processing strategies like fractionation, enzymatic treatment, heat treatment, and microbial fermentation have been employed to mitigate the negative impacts of soybean meal on fish nutrition and immune systems. This study focuses on optimizing soybean meal inclusion levels by incorporating advanced soy variants into CYT diets. The eight-week feeding trial, conducted in a recirculation system, featured six diets with sequential inclusion levels (0, 50, 100%) of high protein low oligosaccharide soybean meal (Bright Day, Benson Hill, St Louis, MO) and enzyme-treated soybean meal (HP 300, Hamlet Protein Inc., Findlay, OH), replacing solvent-extracted soybean. The study compares these formulations against a soy-free animal protein-based diet. At the end of the trial, fish were sampled for growth performance, body proximate composition, intestinal morphology, and immune response from gut samples. Results showed consistent FCR (P = 0.775), weight gain (P = 0.242), and high survival rate (99.4 ± 0.5%) among dietary treatments (P>0.05). Histological evaluations revealed no gut inflammation and gene expression analysis demonstrated no significant variations in immune, physiological, and digestive markers apn (P = 0.687), mga (P = 0.397), gpx1 (P = 0.279), atpase (P = 0.590), il1ß (P = 0.659). The study concludes that incorporating advanced soybean meal products, replacing up to 20% of fishmeal does not negatively affect CYT's growth and intestinal health. This suggests that all three soy sources, contributing 35% of total protein (15.4 g 100 g-1 diet), can be included in practical diets without compromising CYT's intestinal integrity or growth. These findings have positive implications for the commercial production of CYT and future research on the incorporation of plant-based proteins in aquaculture diets.

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.

Ion-transporting capacity and aerobic respiration of larval white seabass (Atractoscion nobilis) may be resilient to ocean acidification conditions.

Ocean acidification (OA) has been proposed to increase the energetic demand for acid-base regulation at the expense of larval fish growth. Here, white seabass (Atractoscion nobilis) eggs and larvae were reared at control (542 ± 28 µatm) and elevated pCO2 (1831 ± 105 µatm) until five days post-fertilization (dpf). Skin ionocytes were identified by immunodetection of the Na+/K+-ATPase (NKA) enzyme. Larvae exposed to elevated pCO2 possessed significantly higher skin ionocyte number and density compared to control larvae. However, when ionocyte size was accounted for, the relative ionocyte area (a proxy for total ionoregulatory capacity) was unchanged. Similarly, there were no differences in relative NKA abundance, resting O2 consumption rate, and total length between control and treatment larvae at 5 dpf, nor in the rate at which relative ionocyte area and total length changed between 2 and 5 dpf. Altogether, our results suggest that OA conditions projected for the next century do not significantly affect the ionoregulatory capacity or energy consumption of larval white seabass. Finally, a retroactive analysis of the water in the recirculating aquarium system that housed the broodstock revealed the parents had been exposed to average pCO2 of ~1200 µatm for at least 3.5 years prior to this experiment. Future studies should investigate whether larval white seabass are naturally resilient to OA, or if this resilience is the result of parental chronic acclimation to OA, and/or from natural selection during spawning and fertilization in elevated pCO2.

Pathology of ocular lesions associated with gas supersaturation in white seabass.

Cultured juvenile white seabass Atractoscion nobilis (WSB) can suffer from intraocular emphysemas and exophthalmia in the hatchery environment. To identify the cause, two size-groups of WSB were exposed to five gas saturation levels, ranging from 98% to 122% total gas pressure (TGP), over a 96-h exposure period in 18 degrees C and 23 degrees C seawater. Histological examination revealed that the gross and subgross lesions associated with gas supersaturation included corneal and orbital emphysema, along with subretinal, optic nerve, and iridial hemorrhage. Corneal emphysema was the most prominent gross lesion, with the severity and prevalence increasing between size-groups and water temperatures as TGP increased. Following the same pattern was orbital emphysema, which affected more than 93% of the fish examined and caused hemorrhage in the subretinal space, around the optic nerve, in the iris, or a combination thereof. Iridial hemorrhage occurred in 91% of the fish examined and decreased significantly with fish size. The prevalence and severity of hemorrhage in the subretinal space increased significantly with TGP and fish size but not with temperature. Optic nerve hemorrhage was absent in small fish exposed at 18 degrees C but increased significantly with temperature and fish size. The reverse was true for the large fish.

Toward responsible stock enhancement: broadcast spawning dynamics and adaptive genetic management in white seabass aquaculture.

The evolutionary effects captive-bred individuals that can have on wild conspecifics are necessary considerations for stock enhancement programs, but breeding protocols are often developed without the knowledge of realized reproductive behavior. To help fill that gap, parentage was assigned to offspring produced by a freely mating group of 50 white seabass (Atractoscion nobilis), a representative broadcast spawning marine finfish cultured for conservation. Similar to the well-known and closely related red drum (Sciaenops ocellatus), A. nobilis exhibited large variation in reproductive success. More males contributed and contributed more equally than females within and among spawns in a mating system best described as lottery polygyny. Two females produced 27% of the seasonal offspring pool and female breeding effective size averaged 1.85 per spawn and 12.38 seasonally, whereas male breeding effective size was higher (6.42 and 20.87, respectively), with every male contributing 1-7% of offspring. Further, females batch spawned every 1-5 weeks, while males displayed continuous reproductive readiness. Sex-specific mating strategies resulted in multiple successful mate pairings and a breeding effective to census size ratio of ≥0.62. Understanding a depleted species' mating system allowed management to more effectively utilize parental genetic variability for culture, but the fitness consequences of long-term stocking can be difficult to address.