Fishes

Freshwater fish community surveys are an important component of aquatic ecosystem management. However, the standard method for taxonomic identification currently used for these surveys, wherein fishes are manually identified in the field by a taxonomic expert, has several shortcomings. These include handling-related fish injury and mortality, the need for a fish-identification expert to be present during field sampling, and additional fish mortality due to physical voucher collection. These shortcomings may be overcome using new methods such as environmental DNA (eDNA) or image analyses. While eDNA can provide fish community data through metabarcoding, it is costly and provides little ecological information. A novel, image-based method for taxonomic identification (“batch-image identification”), which addresses the shortcomings of standard and eDNA methods, was tested in this study. Fishes were captured in the field and photographed in small groups (“batches”) within fish viewers for subsequent identification by taxonomic experts. Comparing taxonomist-based identifications from batch images to specimen-based identification, batch-image identification yielded an overall species-level correct-identification rate (CIR) of 49.7%, and an overall genus-level CIR of 61.2%. CIR increased with taxonomist expertise, reaching 83% when identification was performed by expert taxonomists. Batch-image identification data also produced rarefaction curves and fish-length measurements comparable to those obtained through standard methods. Potential methodological improvements to batch-image identification, including procedural adjustments and alternative identification methods, provide direction for the continued testing and improvement of this method. Full article

Walleye pollock (Gadus chalcogrammus) is a commercially important species widely distributed in cold-water regions. We have been culturing this species artificially since 2015. The average embryo diameter was 1.43 ± 0.056 mm, and hatching occurred at 5 °C approximately 339 h post-fertilization. Gonadal development became visibly apparent at a body size of 10–15 cm after ~180 days, initially distinguishing ovaries with the development of germ cells, whereas testes were observable after further maturation. We discovered two intersexes from F1 and F2 generations, and one abnormal mature individual from F2. Morphologically, intersex walleye pollock exhibited distinct characteristics of ovary and testes, with male gonads visibly connected to the end of ovaries. In intersex walleye pollock aged 3–6 years, the gonads developed normally, and oocyte resorption was restricted to the area near the connections between testicular and ovarian tissues, with numerous atretic oocytes observed in the resorption zone. Primordial germ cells were found together in individuals who had not undergone spawning, indicating an abnormal maturation pattern. Although no significant differences in the gonadosomatic index were observed between intersex and normal individuals, further research on intersexuality is necessary to understand the reproductive development of this species and the health of offspring spawned by intersex individuals, which are rare. Full article

The present study evaluates garlic powder (GP) effects on growth performance, feed utilization, gill parasitic treatment, and monogenean diversity. Thus, a trial was performed under controlled conditions with 84 juvenile Nile tilapia, Oreochromis niloticus (39.8 ± 8.8 g initial weight), from culture ponds with monogenean parasite presence for 30 days. Four balanced diets in protein (32.5%) and lipids (6.4%) with GP inclusion levels of 0%, 1%, 2%, and 3% were formulated, manufactured, and supplied daily at approximately 6.5% body weight/tank. The GP diets, compared to the Control (without GP), indicated that the three inclusion levels did not affect the water quality, survival, growth performance, and feed utilization parameters (p > 0.05). No differences were observed in the parasitological index of prevalence (20–25%), mean intensity (9.6–28), and mean abundance (2.7–5.3) among the experimental diets (p > 0.05), evidencing no effect by inclusion level. Efficacy among GP diets indicated a potential decrease in parasite number (13.4–45.6%) but not all monogenean gill parasites. In conclusion, GP diets did not affect the Nile tilapia survival, growth performance, and feed utilization parameters; therefore, its use is suggested as a preventive alternative for monogenean gill parasites. Full article

Chinese herbal medicines have become a new green feed additive in the aquaculture industry. The aim of this study is to investigate the effects of traditional Chinese herbal medicines (Isatidis radix, Forsythia suspensa, and Schisandra chinensis) on the growth performance, antioxidant capacity, and intestinal microbiota of hybrid snakehead (Channa maculata ♀ × Channa argus ♂). A total of 600 fish (mean weight: 15.85 ± 0.15 g) were randomly assigned to five groups, including the control group (CG), I. radix extract group (IRE), F. suspensa extract group (FSE), S. chinensis extract group (SCE), and the Chinese herbal medicine mixture group (CHMM; a mixture of extracts of I. radix, F. suspensa, and S. chinensis at the ratio of 1:1:1) for 6 weeks. The results show that the IRE-supplemented diet improved the survival rate (SR), feed efficiency ratio (FE), and condition factor (CF) compared to others. Compared to the control group, the activity of superoxide dismutase (SOD) in plasma and intestine was significantly increased in the FSE and CHMM groups, whereas the content of malondialdehyde (MDA) in plasma and liver was significantly reduced in the SCE group. A 16s rRNA analysis indicates that dietary supplementation with FSE significantly promoted the proliferation of Fusobacteriota, while IRE supplementation increased the alpha diversity of intestinal bacteria. In conclusion, the addition of I. radix to the diet of hybrid snakehead improves growth, antioxidant capacity, and liver and intestine health, and modulates the intestinal microbiota of snakehead positively. Full article

The Mediterranean aquaculture industry has recently been confronted with the appearance of a bile imprint on fish filets, which to-date remains of unknown etiology. This study investigates the involvement of common procedures applied before (fasting), during (confinement), and after (ice storage) fish harvesting. Two experiments were designed, one for gilthead seabream (Sparus aurata) and one for red seabream (Pagrus major). The fish were grouped according to fasting duration (1, 2, 3 days), harvesting method (stressed, unstressed), and ice storage (0 h, 48 h). In both species, the imprint appeared in all ice-stored fish for 48 h but not in fresh fish (0 h), the color of the imprint became darker as Days of Fasting increased, stressed fish had darker imprints than unstressed fish, and plasma and bile osmolality and cholesterol were significantly affected by treatments. The histological examination of the gallbladder in red seabream showed great variability in the muscularis thickness and appearance, regardless of treatment. These results are not conclusive as to the cause of the bile imprint appearance. However, they offer a first insight into an issue that bears significant impact in the marketing of aquaculture products and may foster further investigation in the search of the underlying causes of this reoccurring issue. Full article

Estuary perch (Percalates colonorum) are a long-lived and slow-growing species which are distributed throughout southeastern Australian estuaries. In Tasmania, only one known population remains which is located in the Arthur River, on the remote northwest coast. Tasmania is the only Australian state to list estuary perch as a wholly protected species (recreational and commercial), while other populations across their mainland distribution appear to be robust. A survey undertaken on estuary perch in the Arthur River in 2014–2015 indicated that the population was small (825–2375 fish) and dominated by three age classes between 12 and 14 years old. This population also has extremely low genetic diversity and is genetically distinct to mainland populations due to its geographic isolation. During a re-survey in December 2023, 378 estuary perch were captured, with 191 tagged and released, and 31 fish retained for age and gonad analysis. Although three strong age classes were detected (five, eight and twenty-three years), numerous weak and missing cohorts were apparent. This study also confirms this population has a delayed spawning season in comparison to mainland populations, with growth models and tag recaptures (32 fish) demonstrating a very slow but sexually dimorphic growth rate. By combining the datasets of the 2014/15 and 2023 survey, we present an updated understanding of the biology and population dynamics of this unique Tasmanian population, which has previously never been published. The results confirm the population is still reproductively active and stable; however, due to their restricted distribution, variable recruitment, slow growth, and small population, further conservation efforts are still required. Further legislative listings at both the state and federal levels are planned, which if successful will provide additional protection and assist in applications for external funding and resources. This will support the development of a recovery plan which will consider re-stocking efforts and re-establishment of previously inhabited waters and investigate potential actions to improve the genetics of the Tasmanian population. Additionally, educational campaigns to inform the public and recreational fishers on the status of estuary perch in Tasmania will be undertaken. Full article

Fishing vessels are known to have a fatality rate from accidents nearly 100 times higher than that of merchant ships. However, since most cases are resolved internally without maritime tribunals, obtaining accurate statistics on accidents is challenging, making quantitative analysis and evaluation of accident risks difficult. Studies using inconsistent analytical methods often fail to converge on unified results or preventive measures, which contributes to the persistently high occurrence of fishing vessel accidents. Therefore, this study aimed to establish a standardized method for analyzing marine accidents on fishing vessels by applying the Formal Safety Assessment (FSA) technique, originally developed for merchant ships. The research focuses on the globally prevalent trap boat fishery, specifically examining common injuries occurring on fishing vessels. Quantitative data were collected from insurance approval records, while qualitative data were obtained through surveys. The research followed the five stages of the FSA framework: hazard identification, risk estimation, identification of risk control options, cost-benefit assessment, and recommendations for decision-making. The findings suggest that the FSA framework can be effectively applied to analyze fishing vessel accidents based on insurance data, leading to selective and effective preventive measures. Full article

Aeromonas species are among the main pathogens causing rainbow trout infections. Silver nanoparticles (AgNPs) have a broad spectrum of antimicrobial properties and are usually produced by various green-synthesis methods. However, the application of commercialized AgNPs has not fully been clarified. Thus, the objective of this study was to evaluate the antibacterial activities of commercialized AgNPs (range of sizes 10–12 nm) on two contrasting A. salmonicida strains (I-1 and I-4), isolated from rainbow trout; the antibacterial mechanism, histopathological alterations and the expression of immune-related genes were investigated. In vitro, the minimal inhibitory concentration (MIC) was 10 µg/mL for I-1, and lowered to 9.5 µg/mL for I-4, respectively. AgNPs were shown to disrupt both the cell wall and membrane of I-1 and I-4, resulting in cell lysis and degradation. In vivo, rainbow trout challenged by immersed or intraperitoneally injected infection, the 10 µg/mL AgNP-treated groups, both showed delayed deaths and lower mortalities compared to the control groups, without any clinical signs and pathological changes. Especially for the virulent I-4, the enhanced expressions of immune-related genes TNF-α, IL-1β, IL-10 and IL-11 were significantly reduced in the AgNP-treated group, indicating a lesser inflammation due to the application of AgNPs. This study would lay theoretical foundation for the wide application of silver nanoparticles in fish diseases. Full article

Melatonin is a critical regulator of biological rhythms across organisms, transducing light signals into neuroendocrine signals that facilitate reproductive regulation in response to environmental cues. However, the precise mechanisms through which melatonin regulates reproduction in fish require further investigation. In this study, we employed molecular and organizational biological techniques to examine the expression patterns of melatonin and its five receptor subtypes (LcMTNR1A1, LcMTNR1A2, LcMTNR1B1, LcMTNR1B2, and LcMTNR1C) in various tissues of the large yellow croaker (Larimichthys crocea). Our results revealed significant expression of all receptors in the pituitary and testes, with distinct gender differences, including a lack of expression in the ovary. Moreover, our data indicate that melatonin and its receptors are primarily expressed during stage III, highlighting their role in sexual maturity. Enzyme- linked immunosorbent assay (ELISA) results further demonstrated that in vitro melatonin incubation in the brain of L. crocea influenced gonadotropin-releasing hormone (GnRH) and testosterone secretion in a dose-dependent manner, suggesting actions beyond the classical hypothalamic–pituitary–gonadal (HPG) axis. Overall, our findings provide new evidence supporting the role of the melatonin system in reproductive regulation in marine teleosts. Full article

The costly production of live microalgal feed prevents the inclusion of an extended nursery phase in bivalve aquaculture. One method of feeding juvenile bivalves that has received minimal attention is the use of dissolved nutrients to reduce the reliance on live microalgae as a sole feed input. This study aimed to determine whether dissolved sucrose could work as a supplement to live microalgae. Two different concentrations of dissolved sucrose (i.e., 100 μg mL⁻¹ and 1 mg mL⁻¹) were each provided daily for 2 h and 4 h to juvenile Greenshell^™ mussels as a supplement to a diet of live microalgae. The growth and survival of the mussels were measured over three weeks. All combinations of sucrose concentrations and exposures improved the growth of spat compared with the control without sucrose. However, the best-performing spat were provided with a concentration of 1 mg mL⁻¹ of dissolved sucrose for an exposure time of 4 h, which induced 57% greater spat growth daily compared with the control diet. The mussel spat supplemented with dissolved sucrose also accumulated greater carbohydrate content compared with those in the control treatment, indicating they were in greater nutritional condition. This demonstration that dissolved sucrose can significantly improve the growth and nutritional composition of mussel spat over periods as short as 2 h shows promise for the commercial application of sucrose as low-cost supplementary feed in bivalve nurseries. Full article

The aim of this study was to investigate the effects of dietary yeast polysaccharide (YPS) supplementation on the growth performance, whole-body composition, antioxidant capacity, and immunity of juvenile largemouth bass (Micropterus salmoides). In this study, five diets with YPS levels of 0.00% (control), 0.05% (0.05Y), 0.10% (0.10Y), 0.15% (0.15Y), and 0.20% (0.20Y) were designed and prepared. A total of 300 healthy fish (3.20 ± 0.03 g) were randomly divided into 15 floating cages (1 × 1 × 1 m) in five different groups, with three replicates per group, for an 8-week culture experiment. The 0.10Y and 0.20Y groups had significantly higher feed conversion ratios compared with the control group. There was no significant effect on any of the other growth indicators. Plasma biochemical indices showed that the 0.10Y group exhibited the highest plasma alkaline phosphatase content and the 0.20Y group exhibited the highest plasma glucose content. Plasma antioxidant indices (total antioxidant capacity, superoxide dismutase, and glutathione) and antioxidant genes (catalase and superoxide dismutase) were elevated in the 0.05Y or 0.10Y groups, and the malondialdehyde content decreased with increasing YPS concentration. Moreover, the 0.05Y group showed significantly higher immune-related gene (nuclear factor-kappa B, interleukin-8, and interleukin-10) mRNA expression. Altogether, our results indicate that dietary YPS supplementation enhances the antioxidant and immune capacity of M. salmoides, but with no positive effect on their growth. Full article

The presence of the white shark (Carcharodon carcharias) in the Mediterranean Sea is well documented, but mainly through historical and opportunistic records. Historically, the Adriatic Sea, particularly its eastern coastline, was considered a hotspot for white sharks, with relatively frequent reports of juvenile specimens suggesting a potential nursery area. However, since the second half of the 20th century, the abundance of white sharks in the Adriatic has experienced a dramatic decline, with the last confirmed sighting recorded in 2011. In this study, we report the recent capture of a young-of-the-year (YOY) white shark off the Croatian coast, previously misidentified as a porbeagle shark (Lamna nasus). In addition, we revisit historical records of white sharks in the Adriatic Sea to assess whether this region can be considered a nursery area. Our findings highlight significant gaps in the understanding of white shark spatial ecology and reproductive biology in the Mediterranean Sea. Furthermore, this study emphasizes the critical role citizen science and public engagement can play in documenting occurrences of these elusive and endangered predators, offering valuable insights for future conservation efforts. Full article

The coastal waters of Zhejiang feature a complex aquatic environment and abundant biological resources, creating an ideal habitat for various fish species. However, the systematic monitoring of fish larvae in these offshore waters is limited. This study collected 24,232 fish larvae using large plankton nets during April and November 2022, as well as February and July 2023, and identified 93 species, primarily warm-temperate and warm-water species, with a peak occurrence in summer. The dominant species include Larimichthys croceus, Sebastiscus marmoratus, Lateolabrax japonicus, and Odontamblyopus lacepedii, among others, and these species exhibit frequent seasonal changes. Fish larvae are typically found to be aggregated along estuaries and bays in spring, autumn, and summer, while in winter, they tend to shift towards areas near the boundaries of motor trawler fisheries areas. Our cluster analysis revealed spatial heterogeneity in the community structure, driven by an abundance of dominant and important species. Our Mantel tests and canonical correspondence analysis (CCA) identified seawater temperature and salinity as core drivers of the aggregation and distribution of fish larvae, interacting with factors such as the chlorophyll-a concentration, water turbidity, water depth, and dissolved oxygen. This research provides a scientific basis for the dynamic monitoring of spawning grounds and effective management of fishery resources in Zhejiang’s coastal waters. Full article

Monitoring stress in captive fish is crucial for their welfare, but continuous physiological measures in unrestrained animals are challenging. Rainbow trout (Oncorhynchus mykiss) exhibit divergent personalities, ranging from bold to shy, which correlate with cortisol-mediated stress responses. To determine whether personality affects the sympathetic nervous system, heart rate was measured during three potentially stressful events as a proxy for sympathetic nervous system responses. Firstly, trout were classified as bold or shy, using a novel object test. Subsequently, trout were implanted with biologgers to record heart rate in vivo at rest during and after the behavioral tests. Following recovery, the fish underwent a second novel object test, a confinement test, a pair-wise contest, and a final novel object test to explore the degree of boldness over the experimental period, which remained consistent. Heart rate was relatively higher in both bold and shy animals during the confinement test and the pair-wise contest compared with the novel object test, which indicated that heart rate monitoring was a valid gauge of the valence of the experience. Heart rate responses did not differ between bold and shy trout, indicating that behavioral phenotype did not influence the autonomic stress response. Thus, heart rate is a reliable indicator of stress without the need to account for intra-specific behavioral variations. Full article

The Australian lungfish, Neoceratodus forsteri, is one of the world’s oldest vertebrate lineages, with a slow life-history and threatened status, requiring immediate conservation efforts. The main threats to lungfish populations are degradation and availability of key macrophyte habitats, water regulation and flow modification. As this long-lived species (at least 77 years) has delayed maturity (mature at 10 years), field monitoring alone will not be enough to inform the challenge of ensuring sustainable populations. A stochastic metapopulation model was developed for the Burnett River (Southeast Queensland, Australia), an important habitat for the lungfish that is a highly regulated system with extensive water infrastructure. The model consists of three interacting populations, where the ecology and biology of the species were translated into an 80-year-class population projection matrix for each population, each with post-development streamflow, habitat and movement rules. The model highlights the longer-term interaction between dams and stream flows on habitat availability and subsequent recruitment. Through a pre-development streamflow, we quantify the impact of high regulation and development on the lungfish population in the Burnett River: a minor decline in the upstream population (e.g., 9.8% decline), a large decline in the middle population (64.2% decline), virtually no change in the downstream population (e.g., 1.2% decline) and a moderate decline in the overall metapopulation (e.g., 22.3% decline). The loss of spawning and feeding habitat remains the main reason for population decline, with implications that the loss will lead to greater pressure on remaining downstream habitat due to combined flow and dam effects and, in turn, to extended periods of recovery of spawning habitat. Our modeling approach substantially advances conservation management of this species, as it can be adapted to suit other populations in other river systems and used to test sensitivity to recovery actions. Full article

Aromatase inhibitor letrozole (LZ) and 17α-methyltestosterone (MT) are commonly used for sex control breeding to induce female-to-male sex reversal in aquaculture. However, studies on the molecular regulatory mechanisms of LZ and MT are still limited. Here, a comparative transcriptome analysis was conducted on the hypothalamus, the gonad, and the liver of golden pompano (Trachinotus ovatus) after 30 days of LZ and MT treatment, which demonstrated significant sexual size dimorphism. Following LZ treatment, the transcriptome data identified 156, 890, and 1286 different expression genes (DEGs) in the hypothalamus, the liver, and the gonad, respectively. After MT treatment, the transcriptome data identified 114, 876, and 950 DEGs in the hypothalamus, the liver, and the gonad, respectively. Functional enrichment analysis revealed that DEGs were involved in oocyte meiosis and steroid-hormone synthesis following LZ treatment. Similarly, the analysis indicated that DEGs were associated with cysteine and methionine metabolism and steroid-hormone synthesis after MT treatment. Further analysis showed that LZ treatment can promote the first meiotic division but inhibit the second meiotic division of the oocyte. MT treatment promotes polyamine synthesis to induce oocyte apoptosis, both of which affect the trajectory of ovarian differentiation in golden pompano. However, LZ and MT have converse effects in sex hormone synthesis. LZ inhibits estrogen metabolism but promotes cholesterol synthesis by regulating sqle, cyp3a40, ugt1a1, and ugt2a1, while MT promotes estrogen metabolism but inhibits the conversion of cholesterol to pregnenolone by regulating cyp19a1, cyp1a, and hsd17b7. This study reveals that both LZ and MT may inhibit the ovarian differentiation and induce masculinization through different regulatory mechanisms, which provides a fundamental insight into the sex control breeding in aquaculture. Full article

Spawning grounds may shift due to climate change and subsequent variations in the marine environment, but few studies have aimed to project shifts in the spawning grounds of chub mackerel driven by climate change. We projected the effects of climate change on the spawning grounds of chub mackerel (Scomber japonicus) by developing and applying a suitable spawning ground index based on a regional ocean circulation model for the western North Pacific. Our model indicated that the potential spawning grounds of chub mackerel extended from southern waters of the East China Sea to the Korea Strait, Yellow Sea, and Japan/East Sea. Despite some uncertainty, our model based on climate change scenarios projected that, by the 2050s, spawning grounds will shift northward due to warming of the ocean surface, resulting in a subsequent westward shift of nursery grounds from the Japan/East Sea to the Korea Strait and Yellow Sea. Our projections will contribute to clarifying the impacts of climate change on the distribution of exploitable chub mackerel, the adaptation of fisheries to climate change, and the reliability of stock assessments used for fisheries management in the region. Full article

Spotted Seatrout Cynoscion nebulosus (hereafter, trout) are one of the most heavily targeted sportfish along the Gulf coast. With recent regulation changes intended to reduce the harvest mortality of trout and promote a healthy trophy-class trout fishery in Texas, understanding differences between trophy-class (>28 in, 712 mm) and adult-class (12–28 in, 305–710) trout has become especially relevant for fisheries managers. Here, we leveraged a long-term (1983–2023) gill net dataset collected by the Texas Parks and Wildlife Department to (1) compare the distribution of size classes both spatially and temporally, (2) determine environmental drivers of the abundance of each size class, and (3) compare the species associated with each size class in the Upper Laguna Madre. Kernel density distribution estimates, hot spot analyses, and space–time pattern mining methods were used to compare the distribution of the two size classes of trout spatially and temporally. Boosted regression trees (BRTs) were used to determine the environmental drivers of abundance, and multivariate analyses were utilized to compare the species associated with each size class. Trophy-class trout exhibited a greater degree of spatial clustering than adult-class trout, with abundance peaks concentrated along specific shorelines in Baffin Bay. Significant changes in trophy-class trout distribution over time were also found, indicating that the spatial clustering of trophy-class trout has intensified over the past four decades. Salinity and bay were strong predictors for the abundance of both size classes, with peak abundances of both trophy-class and adult-class trout in Baffin Bay and its tertiary bays and at salinities of 10–30 and above 60 psu. Dissolved oxygen (DO) concentration and temperature were good predictors of the abundance of trophy-class trout, which were most abundant at temperatures of 25–30 °C and DO levels of 5–8 mg/L. Depth, turbidity, and season were good predictors of the abundance of adult-class trout, which were most abundant at increased depths (>2 m), low-to-moderate turbidity (~125 NTU), and in the spring season (April–June). Multivariate analyses revealed differences in the communities associated with trophy-class and adult-class trout occurrence, with species such as Black Drum Pogonias cromis, Striped Mullet Mugil cephalus, and Ladyfish Elops saurus more abundant in samples associated with trophy-class trout. These results indicate that trophy-class trout display unique spatial patterns and species associations and that their abundance may be driven by a distinct set of hydrological and biotic factors, which may have important implications for the management of trophy fisheries. Full article

A primary cell culture derived from the gill tissues of largemouth bass (Micropterus salmoides) was successfully established and characterized, providing a physiologically relevant model for virological research. Gill tissues were enzymatically dissociated, and their cells were cultured in M199 supplemented with 20% fetal bovine serum at 25 °C, yielding optimal growth. Viral replication within these primary cells was confirmed by transmission electron microscopy, and further qRT-PCR demonstrated the upregulation of antiviral genes (IFN1, Mx1, ISG15, and Viperin). These primary gill cells of spindle-like morphology exhibited significantly higher susceptibility to Micropterus salmoides rhabdovirus (MSRV) compared to established cell lines, as evidenced by higher viral titers, thus establishing their suitability for studying host–virus interactions. Furthermore, these cells were amenable to genetic manipulation, with the successful transfection of an mCherry reporter gene using commercially available reagents. These findings highlight the utility of the largemouth bass gill-derived primary cell culture as an alternative in vitro system for investigating MSRV pathogenesis and host immune responses, which serves as a stepping stone for improved antiviral strategies in largemouth bass aquaculture. Full article