Abstract:Trachinotus ovatus is one of the important species considered for deep-sea aquaculture due to its unique characteristics and significant contribution to marine fish aquaculture in China. Currently, deep-water net cage aquaculture of Trachinotus ovatus typically occurs in nearshore waters (＜20 m depth), often in bays or areas with island and reef cover, offering favorable aquaculture environmental conditions. However, transitioning to deep-sea aquaculture entails moving into deeper, more exposed waters where environmental factors such as flow velocity, wind, and waves can significantly impact fish farming and equipment. Fish are confined to specific aquaculture spaces and must constantly contend with tidal forces; excessive ocean currents can be detrimental, even leading to fish mortality. Therefore, understanding fish swimming ability and environmental adaptability is crucial when selecting species for deep-sea aquaculture. This study focused on Trachinotus ovatus, using a lane breathing apparatus to study its swimming ability under different flow velocities [at a temperature of (26.99±0.70) ℃]. Small [body length: (4.06±0.43) cm, weight: (2.61± 0.79) g] and large [body length: (11.06±0.70) cm, weight: (56.09±9.99) g] sizes of Trachinotus ovatus were selected for the experiment, and their swimming behavior, respiratory metabolism, and physiological and biochemical indicators were measured. The study aimed to understand Trachinotus ovatus' swimming ability, behavior, and exercise physiology, as well as to reveal its physiological and biochemical responses to changes in flow velocity, providing a technical reference for deep-sea net cage aquaculture of Trachinotus ovatus. The results showed that the induced velocities of small and large-sized Trachinotus ovatus were (3.90±0.64) and (0.96±0.13) BL/s, respectively. The critical swimming speeds were (91.83±10.97) and (99.78±12.66) cm/s, respectively, and the burst swimming speeds were about 112.20 and 114.25 cm/s. There was a significant linear positive correlation between tail wagging frequency and flow velocity. Under the same absolute flow velocity, the tail wagging frequency of small-sized Trachinotus ovatus was significantly higher than that of large-sized Trachinotus ovatus (P＜0.01). The static oxygen consumption rates were about (899±111) mg/(kg·h) and (525±95) mg/(kg·h) for small and large Trachinotus ovatus, respectively, higher than the exercise oxygen consumption rates of the 20%, 40%, and 60% U_crit flow rate groups. The exercise oxygen consumption rate and flow rate of both sizes of Trachinotus ovatus showed a power function increasing relationship. The minimum COT was observed in the 60% U_crit flow rate group. In summary, it is recommended to control the adaptive flow velocity for deep-water net cage aquaculture of Trachinotus ovatus to within 1.0 BL/s, with an aquaculture flow velocity not exceeding 100 cm/s, and the optimal flow velocity range of 54–60 cm/s. The sustained swimming abilities of several marine fish are as follows: Trachinotus ovatus ＞ Sciaenops ocellatus ＞ Acanthopagrus schlegeli ＞ Larimichthys crocea.

Abstract:Silver carp (Hypophthalmichthys molitrix) and bighead carp (Hypophthalmichthys nobilis) are endemic to China, with few natural hybrids found within their native distribution ranges. However, a significant number of natural hybrids emerged when these two species colonized the Mississippi River in the United States of America. To comprehend the morphological differences and biological characteristics of hybrid fish resulting from silver carp and bighead carp, comparative observations were conducted on external morphological traits, body coloration, pigment cells, and morphological changes in some organs and bones among silver carp, bighead carp, and their hybrid (silver carp ♀ × bighead carp ♂). The results indicated extremely significant differences (P＜0.05) in six traits out of 18 countable traits between the hybrids and their parents, with an average hybrid index of 72.2, indicating a slight bias towards paternal bighead carp. Utilizing 39 quantifiable and truss traits, principal component analysis, discriminant analysis, and cluster analysis revealed evident differences among the hybrids, silver carp, and bighead carp, although the hybrids exhibited closer proximity to maternal silver carp. Morphological disparities among the hybrids mainly centered on the head and trunk anterior to the dorsal fin. Body surface and peritoneum colors of the hybrids were intermediate between those of silver carp and bighead carp, leaning closer to silver carp. Nonetheless, the number of melanocytes in the scales was significantly higher in hybrids than in silver carp (P＜0.05). Moreover, gill rake characteristics of the hybrids resembled those of bighead carp but noticeably differed from those of silver carp. The shape and size of posterior swim bladders in hybrids differed from those of silver carp and bighead carp. While most skeletal characteristics of hybrids resembled those of silver carp, hybrid skeletal size ranged between that of silver carp and bighead carp, offering a scientific foundation for understanding the biological traits of hybrid offspring from silver carp and bighead carp, their ecological adaptability in migration areas, and the germplasm protection of both species in China.

Abstract:Early weaning stands as a critical phase in the breeding of juvenile largemouth bass (Micropterus salmoides), aiming to ascertain optimal size parameters and evaluate physiological and biochemical impacts. This study investigated the effects of early weaning on survival rates, antioxidant and digestive enzyme activities, and intestinal microbiota. Five length categories [(1.06±0.02) cm, (2.04±0.03) cm, (3.02±0.04) cm, (4.05±0.04) cm, (5.01±0.04) cm] of juvenile fish were subjected to early weaning experiments. Data on natural mortality, survival rates, and cannibalism rates were analyzed. Antioxidant enzyme activities in the liver and muscle, as well as digestive enzyme activities in the stomach, gut, and pyloric ceca, were assessed pre and post-early weaning. Intestinal microbiota analysis was conducted using high-throughput sequencing. Findings revealed: (1) Highest natural mortality (60.31%) and cannibalism rates (30.85%) occurred at 1 cm length level, with the lowest survival rate (8.84%). Conversely, the 4 cm and 5 cm length levels exhibited no natural mortality or cannibalism, with 100% survival. Increasing body length correlated with decreased natural mortality and cannibalism rates alongside heightened survival rates. (2) SOD activity in the liver and muscles increased by 86.97% and 16.62%, respectively (P＜0.05), post-early weaning, whereas CAT activity in the liver decreased by 58.60% (P＜0.05). T-AOC and MDA levels in the liver and muscles remained statistically unchanged. (3) Trypsin, lipase, and amylase activities were highest in the pyloric ceca, followed by the gut, and lowest in the stomach. Post-early weaning, trypsin, lipase, and amylase activities in the gut decreased significantly by 50.11%, 40.71%, and 42.79%, respectively. Trypsin activity in the stomach decreased by 18.33% (P＜0.05), with no significant changes observed in the pyloric ceca. (4) At the phylum level, Actinobacteria, Proteobacteria, Firmicutes, and Verrucomicrobiota were predominant in juvenile Micropterus salmoides guts. Actinobacteria decreased by 67.42% (P＜0.05), while Firmicutes increased by 360.44% (P＜0.05) post-early weaning. At the genus level, Mycobacterium, Microbacterium, Paracoccus, and Gammaproteobacteria were predominant pre-weaning, transitioning to Lactococcus, Mycoplasma, Bacillus, and Achromobacter post-weaning. LEfSe analysis identified 51 significantly altered bacterial groups in the gut post-early weaning. Findings suggest 3–4 cm length as the optimal early weaning size for juvenile Micropterus salmoides. Early weaning-induced stress leads to oxidative damage, affecting antioxidant enzyme activities. Digestive enzyme activities exhibit significant alterations during transfeeding, with a notable decline. Intestinal microbiota composition undergoes significant changes post-early weaning, with Firmicutes emerging as the predominant phylum. This study provides valuable insights for enhancing survival rates and digestive efficiency during the early weaning phase of juvenile Micropterus salmoides.

Abstract:Fish eggs play a crucial role as an initial source for replenishing fish populations, with their quantity and species composition being pivotal for determining the overall fish community structure. Variations in their abundance directly impact fishery resources. Identifying and analyzing the composition of fish eggs are indispensable for understanding fish ecology and conservation efforts, offering insights into spawning locations, timing, and reproductive dynamics across multiple species. Moreover, such analyses provide a foundational understanding for studying fish resource replenishment mechanisms, population dynamics, and sustainable fisheries management. Conventionally, research on fish eggs relies on morphological techniques, which often encounter challenges in accurately identifying egg species. To elucidate the spatial distribution, species composition, and their correlation with environmental factors of early fish resources in the Daya Bay, samples were collected from 14 sites in April 2022. Mitochondrial DNA cytochrome oxidase subunit I (CO I) fragments served as molecular markers for species identification. The findings unveiled 36 species of spring fish eggs in the Daya Bay, spanning 7 orders, 16 families, and 26 genera. Notably, 31 species, 3 genera, and 2 families were identified. The mean fish egg density stood at 10.12 ind/m³ , with the highest density recorded at site S7, reaching 37.3 ind/m³ . Dominant species (with dominance≥0.02) included Nuchequula nuchalis, Gerres decacanthus, Alepes kleinii, Stolephorus insularis, Thryssa mystax, Escualosa thoracata, and Photopectoralis bindus. Environmental factor analysis revealed no significant correlation between environmental variables and the distribution of eggs from dominant species, primarily due to their eurythermal euryhaline nature, exhibiting robust adaptability to marine environments. This study highlights a predominance of small, low-value species, such as Nuchequula nuchalis, Photopectoralis bindus, and Gerres decacanthus, with an absence of eggs from high-value economic fish. Such observations mirror the prevailing trend in Daya Bay’s fishery resources, characterized by a shift towards smaller fish groups with reduced individual size and economic worth. This trend reflects the adaptive responses of fish to marine environments. Daya Bay emerges as a crucial spawning ground for diverse fish species, underscoring the imperative to enhance the protection of its fishery resources and restore the functionality of its valuable germplasm resources.

Abstract:To study the biological characteristics of the breeding stocks of Scomber australasicus in the East China Sea, we analyzed the biological characteristics of the breeding stocks of Scomber australasicus, including fork length, body mass, feeding grade, fattening, gonad index, sex ratio, and muscle content, based on 227 Scomber australasicus samples collected in the south-central part of the East China Sea between January and April, 2016— 2023. The results showed that there was no significant difference in the relationship between fork length and body mass between the sexes of the breeding stocks of Scomber australasicus (P＞0.05), and the equation for the relationship between fork length and body mass fitted to both sexes was W=3×10^–6FL^3.2558. January is the overwintering period for Scomber australasicus, and the breeding period is from February to April. The peak spawning period of the Australian mackerel was concentrated in March, and the proportion of mackerel that had already spawned increased by late April. Female mackerel always have a higher gonadal index than male mackerel and less muscle mass than male mackerel, with equal gonadal development. Feeding intensity declined and then increased from the overwintering period to around the breeding period, whereas fattening increased and then declined before returning to pre-breeding levels. Males outnumbered females in early breeding in February, and females outnumbered males in late breeding from March to April. This variation in sex ratio during the breeding period is consistent with a breeding strategy that maximizes generation replenishment and reproductive gain. During the breeding season, the female Scomber australasicus organism allocates more energy to gonadal development than the male, while allocating less energy to muscle, to ensure population continuation. Female Scomber australasicus are more abundant than males during the breeding season, which may be related to the fact that Scomber australasicus is trying to increase the number of offspring to ensure the continuation of the population. The breeding season affects the feeding activity of Scomber australasicus, leading to a decrease in fat content. It was found that female Scomber australasicus outnumbered males in the large-bodied Scomber australasicus population, and that fork length and body weight were slightly higher than in males. However, further research is required to investigate this phenomenon. The biological characteristics of the reproductive stocks of Scomber australasicus in the south-central East China Sea were analyzed to enrich the biological data on the reproductive stocks of Scomber australasicus in the south-central East China Sea, which will provide a basis for the conservation and utilization of Scomber australasicus in the south-central East China Sea.

Abstract:Chub mackerel (Scomber japonicus) is an important fishery species in the Northwest Pacific Ocean, and its fishery involves multiple countries and various types of operations, resulting in its complexity. However, there are no appropriate conservation and management measures based on stock assessments of chub mackerel in this area. In this study, a management strategy for this key species in the Northwest Pacific Ocean was evaluated using traditional methods (resource projections) and data-limited approaches. Based on the stock assessment results of the JABBA and JABBA-Select models, the population dynamics were projected under different harvest control rules (HCRs). Four HCRs were set up for a chub mackerel fishery from 2023 to 2032, with total allowable catch (TAC)=(0.8–1.4)×Catch₂₀₂₀ (catch in 2020), maximum sustainable yield (MSY), B×F_MSY or SB×H_MSY (for JABBA and JABBA-Select respectively), and Catch_max (historical maxmum catch). The results showed that the stock was projected to be more negative in the JABBA-select model considering spawning stock biomass than in the JABBA model. Based on the MSY-related reference points, HCR_3 performed the best (TAC of 50–80 million tons and 50–66 million tons, respectively). The MSE then selected 12 strategies with better conservation management effects according to the settings, among which DAAC and SBT2 had the best conservation management effects. The TAC obtained from SBT2 was more appropriate for conservation, close to the TAC (520387 t) from HCR_3 and the recent catch. Robustness and sensitivity analyses showed that the catchability coefficient, natural mortality, and catch-related parameters (average catch, catch, catch reference) had a significant influence on management performance, requiring more attention to estimate accuracy and precision. In conclusion, the conservation and management of chub mackerel in the Northwest Pacific Ocean should consider catch limits based on B×F_MSY or SB×H_MSY by adjusting resource and catch levels.