Mass mortality events associated with cyprinid herpesvirus 2 (CyHV-2) infection in wild Prussian carp Carassius gibelio in the Netherlands, and molecular biology of virus strains.

In 2011 and 2015, four mass mortalities of Prussian carp (Carassius gibelio) were observed in a recreational freshwater lake and open freshwater in the western part of the Netherlands. Cyprinid herpesvirus 2 (CyHV-2) infection was suspected in these cases, based on presumptive gross diagnosis. To elucidate the cause of the mass mortalities diagnostic PCR assays were performed for CyHV-2, based on the helicase gene. Furthermore, the viral isolates were genotyped by sequencing the enlarged marker A and marker B sequences. Diagnostic PCR revealed that three of four samples were positive for CyHV-2, indicating these three mass mortalities were associated with CyHV-2 infection. The marker A sequence from one of the isolates found in this study was identical to those from different locations such as Asia and Middle East, suggesting a link among the isolates. This is the first detailed report on mass mortalities of Prussian carp associated with CyHV-2 infection in natural aquatic environments in the Netherlands. Since 2015, additionally, in total three CyHV-2 associated outbreaks of Dutch Prussian carp were seen in 2016 and 2020. These outbreaks in Prussian carp from lakes and open water suggest that the virus has been spreading in natural freshwaters in the Netherlands.

Assessing the transmission risk of red sea bream iridovirus (RSIV) in environmental water: insights from fish farms and experimental settings.

Aquatic animal viruses are considered to be transmitted via environmental water between fish farms. This study aimed to understand the actual transmission risk of red sea bream iridovirus (RSIV) through environmental water among fish farms. An environmental DNA (eDNA) method using iron-based flocculation coupled with large-pore filtration was used to monitor RSIV DNA copies in seawater from fish farms and from an experimental infection model. RSIV dispersion in seawater from a net pen where the disease outbreak occurred was visualized by the inverse distance weighting method using multiple-sampling data sets from a fish farm. The analysis demonstrated that the center of the net pen had a high viral load, and RSIV seemed to be quickly diluted by the tidal current. To evaluate the transmission risk of RSIV in environmental water, the red sea bream Pagrus major (approximately 10 g) was exposed to RSIV-contained seawater (103, 104, 105, 106, and 107 copies/L) for 3 days, which mimicked field exposure. A probit analysis of the challenge test indicated that the inferred infection rates of seawater containing 105.9 copies/L and 103.1 copies/L of RSIV were 50% and 0.0001%, respectively. In the surveillance for 3 years at 10 fixed points (n = 306), there were only seven samples in which the viral load exceeded 104 copies/L in seawater. These results suggest that the transmission of RSIV among fish farms via seawater is highly associated with the distance between the net pens, and the environmental water is not always an infection source for the transmission of RSIV between fish farms. IMPORTANCE Our surveillance of viral loads for red sea bream iridovirus (RSIV) by monitoring environmental DNA in fish farms suggested that the viral loads in the seawater were low, except for the net pens where RSIV outbreaks occurred. Furthermore, our experimental infection model indicated that the infection risk of RSIV-contained seawater with less than 103 copies/L was extremely low. The limited risk of environmental water for transmission of RSIV gives an insight that RSIV could be partly transmitted between fish farms due to the movement of equipment and/or humans from the fish farm where the disease outbreaks. Since our data suggest that seawater can function as a potential wall to reduce the transmission of RSIV, biosecurity management, such as disinfection of equipment associated with fish farms could be effective, even in the semi-open system aquaculture that the environmental water can be freely transferred, to reduce the risk of RSIV outbreaks.

Reproductive strategy of Delta Smelt Hypomesus transpacificus and impacts of drought on reproductive performance.

Understanding reproductive biology and performance of fish is essential to formulate effective conservation and management programs. Here, we studied reproductive strategies of female Delta Smelt Hypomesus transpacificus, an endangered fish species in the State of California, the United States, focusing on (1) better understanding their distribution pattern during the winter and spring spawning season at very fine scale to predict their possible spawning grounds and (2) assessing impacts of a recent, severe drought on their reproductive performance. We formulated our hypotheses as follows; (1) female Delta Smelt migrate to particular locations for spawning so that mature females can be frequently found in those locations throughout the spawning season and (2) reproductive performance of individual female fish declined during the drought. To test the first hypotheses, we analyzed relationships between water quality parameters and maturity/distribution pattern of Delta Smelt. Salinity better explained the distribution pattern of Delta Smelt at subadult and adult stages compared with water temperature or turbidity. Although there are some freshwater locations where mature Delta Smelt can frequently be found during the spawning season, Delta Smelt at the final maturation stage (Stage 5: hydration) and post spawners appeared to be widespread in the area where salinity was below 1.0 during the spawning season. Therefore, Delta Smelt could theoretically spawn in any freshwater locations, with more specific spawning requirements in the wild (e.g., substrate type and depth) still unknown. Delta Smelt, which experienced dry and critically dry conditions (the 2013 and 2014 year-classes), showed smaller oocytes, and lower clutch size and gonadosomatic index compared with the fish caught in a wet year (2011 year-class) at the late vitellogenic stage (Stage 4 Late), suggesting reproductive performance was negatively affected by environmental conditions during the drought.

Microcystin-Induced Immunotoxicity in Fishes: A Scoping Review.

Cyanobacteria (blue-green algae) have been present on Earth for over 2 billion years, and can produce a variety of bioactive molecules, such as cyanotoxins. Microcystins (MCs), the most frequently detected cyanotoxins, pose a threat to the aquatic environment and to human health. The classic toxic mechanism of MCs is the inhibition of the protein phosphatases 1 and 2A (PP1 and PP2A). Immunity is known as one of the most important physiological functions in the neuroendocrine-immune network to prevent infections and maintain internal homoeostasis in fish. The present review aimed to summarize existing papers, elaborate on the MC-induced immunotoxicity in fish, and put forward some suggestions for future research. The immunomodulatory effects of MCs in fish depend on the exposure concentrations, doses, time, and routes of exposure. Previous field and laboratory studies provided strong evidence of the associations between MC-induced immunotoxicity and fish death. In our review, we summarized that the immunotoxicity of MCs is primarily characterized by the inhibition of PP1 and PP2A, oxidative stress, immune cell damage, and inflammation, as well as apoptosis. The advances in fish immunoreaction upon encountering MCs will benefit the monitoring and prediction of fish health, helping to achieve an ecotoxicological goal and to ensure the sustainability of species. Future studies concerning MC-induced immunotoxicity should focus on adaptive immunity, the hormesis phenomenon and the synergistic effects of aquatic microbial pathogens.

Covariance of Phytoplankton, Bacteria, and Zooplankton Communities Within Microcystis Blooms in San Francisco Estuary.

Microcystis blooms have occurred in upper San Francisco Estuary (USFE) since 1999, but their potential impacts on plankton communities have not been fully quantified. Five years of field data collected from stations across the freshwater reaches of the estuary were used to identify the plankton communities that covaried with Microcystis blooms, including non-photosynthetic bacteria, cyanobacteria, phytoplankton, zooplankton, and benthic genera using a suite of analyses, including microscopy, quantitative PCR (qPCR), and shotgun metagenomic analysis. Coherence between the abundance of Microcystis and members of the plankton community was determined by hierarchal cluster analysis (CLUSTER) and type 3 similarity profile analysis (SIMPROF), as well as correlation analysis. Microcystis abundance varied with many cyanobacteria and phytoplankton genera and was most closely correlated with the non-toxic cyanobacterium Merismopoedia, the green algae Monoraphidium and Chlamydomonas, and the potentially toxic cyanobacteria Pseudoanabaena, Dolichospermum, Planktothrix, Sphaerospermopsis, and Aphanizomenon. Among non-photosynthetic bacteria, the xenobiotic bacterium Phenylobacterium was the most closely correlated with Microcystis abundance. The coherence of DNA sequences for phyla across trophic levels in the plankton community also demonstrated the decrease in large zooplankton and increase in small zooplankton during blooms. The breadth of correlations between Microcystis and plankton across trophic levels suggests Microcystis influences ecosystem production through bottom-up control during blooms. Importantly, the abundance of Microcystis and other members of the plankton community varied with wet and dry conditions, indicating climate was a significant driver of trophic structure during blooms.

Potential Impacts on Treated Water Quality of Recycling Dewatered Sludge Supernatant during Harmful Cyanobacterial Blooms.

Cyanobacterial blooms and the associated release of cyanotoxins pose problems for many conventional water treatment plants due to their limited removal by typical unit operations. In this study, a conventional water treatment process consisting of coagulation, flocculation, sedimentation, filtration, and sludge dewatering was assessed in lab-scale experiments to measure the removal of microcystin-LR and Microcystis aeruginosa cells using liquid chromatography with mass spectrometer (LC-MS) and a hemacytometer, respectively. The overall goal was to determine the effect of recycling cyanotoxin-laden dewatered sludge supernatant on treated water quality. The lab-scale experimental system was able to maintain the effluent water quality below relevant the United States Environmental Protection Agency (US EPA) and World Health Organisation (WHO) standards for every parameter analyzed at influent concentrations of M. aeruginosa above 106 cells/mL. However, substantial increases of 0.171 NTU (Nephelometric Turbidity Unit), 7 × 104 cells/L, and 0.26 µg/L in turbidity, cyanobacteria cell counts, and microcystin-LR concentration were observed at the time of dewatered supernatant injection. Microcystin-LR concentrations of 1.55 µg/L and 0.25 µg/L were still observed in the dewatering process over 24 and 48 h, respectively, after the initial addition of M.aeruginosa cells, suggesting the possibility that a single cyanobacterial bloom may affect the filtered water quality long after the bloom has dissipated when sludge supernatant recycling is practiced.

Hamp Type-1 Promotes Antimicrobial Defense via Direct Microbial Killing and Regulating Iron Metabolism in Grass Carp (Ctenopharyngodon idella).

Hepcidin is an antimicrobial peptide and regulator of iron homeostasis which has two isoforms in most fishes and some mammals. Previous studies have reported that the two hepcidin isoforms have different roles. Hamp type-1 plays a regulatory role in iron metabolism and hamp type-2 mostly performs an antimicrobial role. In this study, we found that Ctenopharyngodon idella (C. idella) have only one hepcidin isoform (hamp type-1), which showed both broad-spectrum antibacterial and iron regulatory functions. C. idella hepcidin mature peptide (hepcidin-25) and truncated peptide (hepcidin-20) exhibited bactericidal activities against both Gram-positive and Gram-negative bacteria in a dose-dependent manner in part through membrane rupture and binding to bacterial genomic DNA. The data from challenge tests demonstrated that the administration of hepcidin-25 significantly reduced mortality rates of C. idella by A. hydrophila infection, probably due to direct bactericidal activities of the peptide and a reduction of iron content in the fish serum. In addition, a comparison between hepcidin-20 and -25 suggests that the N terminal 5 amino acids play a critical role in reducing iron content in fish serum. Our findings revealed an important role of hamp type-1 in maintaining iron homeostasis and fighting against bacterial infections, suggesting the hepcidin has implications for the prevention and control of bacterial infection in aquaculture.

First Isolation of a Novel Aquatic Flavivirus from Chinook Salmon (Oncorhynchus tshawytscha) and Its In Vivo Replication in a Piscine Animal Model.

The first isolation of a flavivirus from fish was made from moribund Chinook salmon (Oncorhynchus tshawytscha) from the Eel River, California, USA. Following the observation of cytopathic effect in a striped-snakehead fish cell line, 35-nm virions with flaviviral morphology were visualized using electron microcopy. Next-generation sequencing and rapid amplification of cDNA ends obtained the complete genome. Reverse transcriptase quantitative PCR (RT-qPCR) confirmed the presence of viral RNA in formalin-fixed tissues from the wild salmon. For the first time, in vivo replication of an aquatic flavivirus was demonstrated following intracoelomic injection in a Chinook salmon model of infection. RT-qPCR demonstrated viral replication in salmon brains up to 15 days postinjection. Infectious virus was then reisolated in culture, fulfilling Rivers' postulates. Only limited replication occurred in the kidneys of Chinook salmon or in tissues of rainbow trout (Oncorhynchus mykiss). The proposed salmon flavivirus (SFV) has a 10.3-kb genome that encodes a rare dual open reading frame, a feature uncharacteristic of classical flaviviruses. Phylogenetic analysis places SFV in a basal position among a new subgroup of recently recognized aquatic and bat flaviviruses distinct from the established mosquito-borne, tick-borne, insect-only, and unknown-vector flavivirus groups. While the pathogenic potential of the virus remains to be fully elucidated, its basal phylogeny and the in vivo infection model will allow SFV to serve as a prototype for aquatic flaviviruses. Ongoing field and laboratory studies will facilitate better understanding of the potential impacts of SFV infection on ecologically and economically important salmonid species.IMPORTANCE Chinook salmon are a keystone fish species of great ecological and commercial significance in their native northern Pacific range and in regions to which they have been introduced. Threats to salmon populations include habitat degradation, climate change, and infectious agents, including viruses. While the first isolation of a flavivirus from wild migrating salmon may indicate an emerging disease threat, characterization of the genome provides insights into the ecology and long evolutionary history of this important group of viruses affecting humans and other animals and into an expanding group of recently discovered aquatic flaviviruses.

Analysis of Covalently Bound Microcystins in Sediments and Clam Tissue in the Sacramento-San Joaquin River Delta, California, USA.

Harmful cyanobacterial blooms compromise human and environmental health, mainly due to the cyanotoxins they often produce. Microcystins (MCs) are the most commonly measured group of cyanotoxins and are hepatotoxic, neurotoxic, and cytotoxic. Due to MCs ability to covalently bind to proteins, quantification in complex matrices is difficult. To analyze bound and unbound MCs, analytical methods were optimized for analysis in sediment and clam tissues. A clean up step was incorporated to remove lipids, improving percent yield. This method was then applied to sediment and clam samples collected from the Sacramento-San Joaquin River Delta (Delta) in the spring and fall of 2017. Water samples were also tested for intracellular and extracellular MCs. These analyses were used to quantify the partitioning of MCs among sediment, clams, and water, and to examine whether MCs persist during non-summer months. Toxin analysis revealed that multiple sediment samples collected in the Delta were positive for MCs, with a majority of the positive samples from sites in the San Joaquin River, even while water samples from the same location were below detection limit. These data highlight the importance of analyzing MCs in complex matrices to accurately evaluate environmental risk.

Toxic effects of fluridone on early developmental stages of Japanese Medaka (Oryzias latipes).

The herbicide fluridone is intensively applied to control invasive aquatic plants globally, including in the Sacramento and San Joaquin Delta (the Delta), California, USA. Our previous study revealed that the adult stage of Delta Smelt showed acute and sub-lethal adverse effects following 6 h of exposure to environmentally relevant concentrations of fluridone. To further investigate mechanisms of toxicity of fluridone and to assess its toxicity to early life stages of fish, we performed additional exposures using the fish model Japanese Medaka (Oryzias latipes). Male and female Medaka embryos were exposed to concentrations of fluridone for 14 d and showed reduced hatching success in a dose dependent manner. The half maximal effective concentration for the hatching success was 2.3 mg L-1. In addition, male and female Medaka larvae were acute exposed to fluridone for 6 h to assess their swimming behavior and gene expression patterns. Fish exposed to fluridone at 4.2 mg L-1 or higher became lethargic and showed abnormal swimming behavior. The response to the stimuli was significantly impaired by fluridone at 21 mg L-1 and above in males, and at 104 mg L-1 in females. Transcriptome analysis identified a total of 799 genes that were significantly differentially expressed, comprising 555 up-regulated and 244 down-regulated genes in males exposed to 21 mg L-1 of fluridone. The gene set enrichment analysis indicated a number of biological processes altered by fluridone. Among the genes involved in those biological processes, the expression of the genes, acetylcholinesterase, retinoic acid receptor, insulin receptor substrate, glutathione reductase, and glutathione S transferase, exhibited dose- and sex-dependent responses to fluridone. The study indicated that fluridone exposure led to detrimental toxic effects at early developmental stages of fish, by disturbing the biological processes of growth and development, and the nervous system, inducing oxidative stress and endocrine disruption.

Toxicity of herbicides to cyanobacteria and phytoplankton species of the San Francisco Estuary and Sacramento-San Joaquin River Delta, California, USA.

The herbicides glyphosate, imazamox and fluridone are herbicides, with low toxicity towards fish and invertebrates, which are applied to waterways to control invasive aquatic weeds. However, the effects of these herbicides on natural isolates of phytoplankton and cyanobacteria are unknown. Three species of microalgae found in the San Francisco Estuary (SFE)/Sacramento-San Joaquin River Delta (Delta) (Microcystis aeruginosa, Chlamydomonas debaryana, and Thalassiosira pseudonana) were exposed to the three herbicides at a range of concentrations in 96-well plates for 5-8 days. All three algal species were the most sensitive to fluridone, with IC50 of 46.9, 21, and 109 µg L-1 for M. aeruginosa, T. pseudonana and C. debaryana, respectively. Imazamox inhibited M. aeruginosa and T. pseudonana growth at 3.6 × 104 µg L-1 or higher, and inhibited C. debaryana growth at 1.0 × 105 µg L-1 or higher. Glyphosate inhibited growth in all species at ca. 7.0 × 104 µg L-1 or higher. Fluridone was the only herbicide that inhibited the microalgae at environmentally relevant concentrations in this study and susceptibility to the herbicide depended on the species. Thus, the application of fluridone may affect cyanobacteria and phytoplankton community composition in water bodies where it is applied.

A pilot study of the performance of captive-reared delta smelt Hypomesus transpacificus in a semi-natural environment.

A captive breeding programme was developed in 2008 for delta smelt Hypomesus transpacificus in reaction to dramatic population decline over several decades. We took 526 sub-adult captive-reared delta smelt and cultured them for 200 days without providing artificial food or water quality management to assess their performance once released in the wild. The results indicated captive-reared sub-adult delta smelt could survive in a semi-natural environment with uncontrolled water quality and naturally produced wild prey through spawning and into their post spawning phase.

Plant detritus is selectively consumed by estuarine copepods and can augment their survival.

Particulate material comprising the detrital remains of terrestrial plants and macrophytes is a substantial source of organic matter to estuaries and therefore has the potential to support the energy demands of the pelagic aquatic food web. Despite the prevalence of macrophytic or terrestrial particulate organic carbon (tPOC), phytoplankton are nutritionally superior and are thought to be the primary food resource for zooplankton. However, estuarine phytoplankton primary productivity abundances can wax and wane, and often production cannot meet heterotrophic energy needs. In this study, we examined how tPOC (detritus of macrophytes and grasses) may affect survival of a calanoid copepod (Eurytemora affinis) common in the San Francisco Estuary (SFE), an estuary with relatively low phytoplankton primary productivity. Using chemical biomarkers and a targeted DNA metagenomic methodology, we show that E. affinis consumed tPOC (dominated by Schoenoplectus sp., or tule) even when phytoplankton were abundant and tPOC was scarce. Furthermore, we found that a mixed diet of phytoplankton and terrestrial material (1:3 carbon ratio) enhanced the survival of E. affinis over a diet of phytoplankton alone. These data show that tPOC can be a vital supplementary food source for zooplankton, perhaps extending survival during low phytoplankton periods, and may help explain elevated zooplankton abundances in tidal wetlands and other detrital-dominated regions.

Biodiversity of cyanobacteria and other aquatic microorganisms across a freshwater to brackish water gradient determined by shotgun metagenomic sequencing analysis in the San Francisco Estuary, USA.

Blooms of Microcystis and other harmful cyanobacteria can degrade water quality by producing cyanotoxins or other toxic compounds. The goals of this study were (1) to facilitate understanding of community structure for various aquatic microorganisms in brackish water and freshwater regions with emphasis on cyanobacteria, and (2) to test a hypothesis that Microcystis genotypes that tolerate higher salinity were blooming in brackish water environments during the severe drought, 2014. Shotgun metagenomic analysis revealed that cyanobacteria dominated the brackish water region while bacteria dominated the freshwater region. A group of cyanobacteria (e.g., Aphanizomenon, Microcystis, Planktothrix, Pseudanabaena), bacteria (e.g., Bacillus, Porphyrobacter), and diatoms (Phaeodactylum and Thalassiosira) were abundant in the brackish water region. In contrast, Hassallia (cyanobacteria) and green algae (Nannochloropsis, Chlamydomonas, and Volvox) were abundant in the landward freshwater region. Station variation was also apparent. One landward sampling station located downstream of an urbanized area differed substantially from the other stations in terms of both water chemistry and community structure, with a higher percentage of arthropods, green algae, and eukaryotes. Screening of the Microcystis internal transcribed spacer region revealed six representative genotypes, and two of which were successfully quantified using qPCR (Genotypes I and VI). Both genotypes occurred predominantly in the freshwater region, so the data from this study did not support the hypothesis that salinity tolerant Microcystis genotypes bloomed in the brackish water region in 2014.

Sub-lethal effects of herbicides penoxsulam, imazamox, fluridone and glyphosate on Delta Smelt (Hypomesus transpacificus).

Concerns regarding non-target toxicity of new herbicides used to control invasive aquatic weeds in the San Francisco Estuary led us to compare sub-lethal toxicity of four herbicides (penoxsulam, imazamox, fluridone, and glyphosate) on an endangered fish species Delta Smelt (Hypomesus transpacificus). We measured 17ß-estradiol (E2) and glutathione (GSH) concentrations in liver, and acetylcholinesterase (AChE) activity in brain of female and male fish after 6 h of exposure to each of the four herbicides. Our results indicate that fluridone and glyphosate disrupted the E2 concentration and decreased glutathione concentration in liver, whereas penoxsulam, imazamox, and fluridone inhibited brain AChE activity. E2 concentrations were significantly increased in female and male fish exposed to 0.21 µM of fluridone and in male fish exposed to 0.46, 4.2, and 5300 µM of glyphosate. GSH concentrations decreased in males exposed to fluridone at 2.8 µM and higher, and glyphosate at 4.2 µM. AChE activity was significantly inhibited in both sexes exposed to penoxsulam, imazamox, and fluridone, and more pronounced inhibition was observed in females. The present study demonstrates the potential detrimental effects of these commonly used herbicides on Delta Smelt.

Evaluation of water quality during successive severe drought years within Microcystis blooms using fish embryo toxicity tests for the San Francisco Estuary, California.

In the San Francisco Estuary, California, the largest estuary on the Pacific Coast of North America, the frequency and intensity of drought and associated cyanobacteria blooms are predicted to increase with climate change. To assess the impact of water quality conditions on estuarine fish health during successive severe drought years with Microcystis blooms, we performed fish embryo toxicity testing with Delta Smelt and Medaka. Fish embryos were exposed to filtered ambient water collected from the San Francisco Estuary during the Microcystis bloom season in 2014 and 2015, the third and fourth most severe recorded drought years in California. Medaka embryos incubated in filtered ambient waters exhibited high mortality rates (>77%), which was mainly due to bacterial growth. Medaka mortality data was negatively correlated with chloride, and positively correlated with water temperature, total and dissolved organic carbon, and ambient and net chlorophyll a concentration. Delta Smelt embryo mortality rates were lower (<42%) and no prominent seasonal or geographic trend was observed. There was no significant correlation between the Delta Smelt mortality data and water quality parameters. Aeromonas was the dominant bacteria that adversely affected Medaka. The growth of Aeromonas was suppressed when salinity was greater than or equal to 1psu and resulted in a significant reduction in mortality rate. Bacterial growth test demonstrated that the lysate of Microcystis cells enhanced the growth of Aeromonas. Toxin production by Microcystis is a major environmental concern, however, we conclude that dissolved substances released from Microcystis blooms could result in water quality deterioration by promoting growth of bacteria. Furthermore, a distinctive developmental deformity was observed in Medaka during the toxicity tests; somite formation was inhibited at the same time that cardiogenesis occurred and the functional heart was observed to be beating. The exact cause of the embryonic developmental deformity is still unknown.

Impairment of antioxidant mechanisms in Japanese Medaka (Oryzias latipes) by acute exposure to aluminum.

Increasing aluminum (Al) concentrations in aquatic habitats as a result of anthropogenic acidification and industrialization is a global issue. Moreover, in extensive areas of the humid tropics and subtropics, high Al concentrations in freshwater are observed because of both naturally low pH and high Al concentrations in soil. Al increases production of reactive oxygen species and enhances oxidative damage in mammals. However, no studies have examined the effect of environmentally relevant concentrations of Al at low pH on oxidative stress in fish. This study assessed Al-induced effects on enzymatic and non-enzymatic antioxidants, lipid peroxidation, and on expression of oxidative stress-related genes at low pH using Japanese Medaka (Oryzias latipes). Larval fish were exposed to dissolved Al concentrations of 0, 1.7, 6.2 and 16.7µgL-1 for 4days at pH5.3 in soft water. Al caused a significant reduction in activity of glutathione peroxidase at 6.2 and 16.7µgL-1, and of glutathione reductase at 16.7µgL-1 in whole body homogenates. No changes were observed in the expression of the glutathione peroxidase gene, and expression levels of the glutathione reductase gene were too low to be quantitated. Even though there was an overall decrease in the activity of catalase and in the concentration of glutathione, differences were not significant compared to the control. Changes in lipid peroxidation were not found. This study showed that exposure to environmentally relevant concentrations of Al at low pH impairs antioxidant defense mechanisms of Medaka.

Effects of low levels of ultraviolet radiation on antioxidant mechanisms of Japanese Medaka (Oryzias latipes).

Extreme weather events like drought are expected to increase with climate change, which will increase exposure of freshwater fish to ultraviolet (UV) radiation. Compared to fully grown adult fish, fish in early life stages are more susceptible to UV radiation due to the lack of well-developed pigmentation. Even though several studies have described affectation of fish health after exposure to UV radiation, most of the studies have used intensities that are only found on the surface of the earth crust or at shallow depths in water bodies, and little is known about impacts of weaker UV radiation, which can be found in deep water. This study showed effects on the antioxidant system of Japanese Medaka after 7 days of exposure to very low intensities of UV radiation, levels that can be found at deep locations in lakes and rivers. Exposure to UV radiation (UVA: 360.1 ± 18.4 µW cm-2 for a dose of 21.6 ± 1.2 mJ cm-2 min-1, and UVB: 6.3 ± 0.5 µW cm-2 for a dose of 0.38 ± 0.03 mJ cm-2 min-1) caused a reduction in the catalase activity (over 50%) and enhanced oxidative damage to lipids. Results of this study showed that environmentally relevant, low levels of UV radiation affect mechanisms by which fish deal with enhanced production of reactive oxygen species and oxidative damage. The results raise concerns about early life stages of fish under scenarios of increased exposure to solar light, such as in tropic regions or during summer in temperate regions.

Foraging and metabolic consequences of semi-anadromy for an endangered estuarine fish.

Diadromy affords fish access to productive ecosystems, increasing growth and ultimately fitness, but it is unclear whether these advantages persist for species migrating within highly altered habitat. Here, we compared the foraging success of wild Delta Smelt-an endangered, zooplanktivorous, annual, semi-anadromous fish that is endemic to the highly altered San Francisco Estuary (SFE)-collected from freshwater (<0.55 psu) and brackish habitat (≥0.55 psu). Stomach fullness, averaged across three generations of wild Delta Smelt sampled from juvenile through adult life stages (n = 1,318), was 1.5-fold higher in brackish than in freshwater habitat. However, salinity and season interacted, with higher fullness (1.7-fold) in freshwater than in brackish habitat in summer, but far higher fullness in brackish than freshwater habitat during fall/winter and winter/spring (1.8 and 2.0-fold, respectively). To examine potential causes of this interaction we compared mesozooplankton abundance, collected concurrently with the Delta Smelt, in freshwater and brackish habitat during summer and fall/winter, and the metabolic rate of sub-adult Delta Smelt acclimated to salinities of 0.4, 2.0, and 12.0 psu in a laboratory experiment. A seasonal peak in mesozooplankton density coincided with the summer peak in Delta Smelt foraging success in freshwater, and a pronounced decline in freshwater mesozooplankton abundance in the fall coincided with declining stomach fullness, which persisted for the remainder of the year (fall, winter and spring). In brackish habitat, greater foraging 'efficiency' (prey items in stomachs/mesozooplankton abundance) led to more prey items per fish and generally higher stomach fullness (i.e., a higher proportion of mesozooplankton detected in concurrent trawls were eaten by fish in brackish habitat). Delta Smelt exhibited no difference in metabolic rate across the three salinities, indicating that metabolic responses to salinity are unlikely to have caused the stomach fullness results. Adult migration and freshwater spawning therefore places young fish in a position to exploit higher densities of prey in freshwater in the late spring/summer, and subsequent movement downstream provides older fish more accessible prey in brackish habitat. Thus, despite endemism to a highly-altered estuary, semi-anadromy provided substantial foraging benefits to Delta Smelt, consistent with other temperate migratory fish.

Co-infection of Acipenserid herpesvirus 2 (AciHV-2) and Streptococcus iniae in cultured white sturgeon Acipenser transmontanus.

A mortality event in cultured white sturgeon Acipenser transmontanus (Richardson, 1836) sub-adults was investigated. After transfer between farms, high mortality was observed in fish, associated with back arching, abnormal swimming, and ulcerative skin lesions. Necropsy of moribund individuals revealed hemorrhagic ascites and petechial hemorrhages in the coelomic peritoneum and serosa of internal organs. Acipenserid herpesvirus 2 (AciHV-2) was isolated from external tissue samples, then identified and genotyped by sequencing of the terminase and polymerase genes. In addition, Streptococcus iniae was recovered from internal organs of affected fish. Histologic changes were limited to interstitial hematopoietic areas of the kidney and consisted of small foci of necrosis accompanied by fibrin deposition, minimal inflammatory response, and small numbers of bacterial cocci compatible with streptococci. Identity was confirmed by partial sequencing of the 16S rRNA, rpoB, and gyrB genes. Genetic fingerprinting demonstrated a genetic profile distinct from S. iniae isolates recovered from previous outbreaks in wild and cultured fish in North America, South America, and the Caribbean. Although the isolates were resistant to white sturgeon complement in serum killing assays, in vivo challenges failed to fulfill Koch's postulates. However, the clinical presentation, coupled with consistent recovery of S. iniae and AciHV-2 from moribund fish, suggests viral and bacterial co-infection were the proximate cause of death. To our knowledge, this represents the first report of AciHV-2 and S. iniae co-infection in cultured white sturgeon.