Large-scale Mortalities Research Articles

The thermal dependent biology of cultured lumpfish (Cyclopterus lumpus Linnaeus 1758): Differences due to temperatures during early life and how tolerance is measured

The use of lumpfish as a “cleaner fish” in Atlantic salmon aquaculture is increasing. However, during the summer months, there have been large-scale mortalities of lumpfish at some North Atlantic salmon sea-cage sites. There is evidence that manipulating temperatures during early life can improve the thermal tolerance of fishes. Thus, we reared lumpfish embryos (eggs), larvae and juveniles under different temperature regimes, and measured their metabolic physiology and upper thermal limits, with the goal of better understanding their physiology and mitigating the effects of high summer water temperatures on their welfare. The specific incubation (egg) / rearing temperature combinations were 6 °C / 9 °C (normal production temperatures), 8.5 °C / 9 °C, 6–11 °C / 9 °C, 8.5 °C / 9–11 °C and 6–11 °C / 9–11 °C; with a range of temperatures indicating that fish were exposed to stochastic temperature changes.Hatching success was 61 % in the control (6 °C) group, as compared to ~47 % and 26.5 % in the groups exposed to 8.5 °C and the stochastic temperature regime (6–11 °C), respectively. Groups exposed to stochastic incubation and rearing temperatures also had the lowest survival rate (33.7 %) as opposed to those incubated and reared at constant temperatures (66.1 and 50.7 %) when unexpectedly exposed to high, and variable, ambient water temperatures that were experienced for roughly 7 weeks during rearing (due to abnormal water temperatures in the bay supplying our aquaculture facility). Interestingly, the critical thermal maximum (CTMax), incremental temperature maximum (ITMax) and metabolic capacity (i.e., aerobic scope) of 30-40 g lumpfish did not differ between the groups, and the first two parameters on average were 22.85 ± 0.12 °C and 20.63 ± 0.02 °C, respectively.

An apicomplexan parasite drives the collapse of the bay scallop population in New York

The bay scallop, Argopecten irradians, represents a commercially, culturally and ecologically important species found along the United States’ Atlantic and Gulf coasts. Since 2019, scallop populations in New York have been suffering large-scale summer mortalities resulting in 90–99% reduction in biomass of adult scallops. Preliminary investigations of these mortality events showed 100% prevalence of an apicomplexan parasite infecting kidney tissues. This study was designed to provide histological, ultrastructural and molecular characteristics of a non-described parasite, member of the newly established Marosporida clade (Apicomplexa) and provisionally named BSM (Bay Scallop Marosporida). Molecular diagnostics tools (quantitative PCR, in situ hybridization) were developed and used to monitor disease development. Results showed that BSM disrupts multiple scallop tissues including kidney, adductor muscle, gill, and gonad. Microscopy observations allowed the identification of both intracellular and extracellular stages of the parasite. Field surveys demonstrated a strong seasonal signature in disease prevalence and intensity, as severe cases and mortality increase as summer progresses. These results strongly suggest that BSM infection plays a major role in the collapse of bay scallop populations in New York. In this framework, BSM may synergistically interact with stressful environmental conditions to impair the host and lead to mortality.

Lowering temperature to 1 °C results in physiological changes, stress and mortality in cultured Atlantic Salmon (Salmo salar)

Atlantic salmon (Salmo salar) reared in sea-cages in Atlantic Canada and Iceland can be exposed to temperatures as low as 0–1 °C during the winter months, and this can result in large-scale mortalities. Despite this threat to salmon aquaculture, most of our knowledge of this species' thermal biology is at temperatures above 10 °C. In this laboratory experiment, Atlantic salmon post-smolts (175 ± 24 g) were exposed to a temperature decline of 1 °C/week (from 8 to 1 °C) with an additional week at 1 °C (to simulate temperature changes in sea-cages in Atlantic Canada during the winter months), or a constant temperature of 8 °C (control), and sampled weekly. Growth was reduced by 58% in the cold-exposed group and was associated with a decrease in appetite beginning at 6 °C. Starting at 4–5 °C, an osmoregulatory disturbance ensued characterized by higher plasma [Cl−], lower plasma [K+] and increased osmolality. Hsp70 and hsp90 transcript expression also increased as temperature was reduced from 5 to 1 °C. However, plasma cortisol levels did not increase (to ∼18 ng/ml) until the fish reached 1 °C, and serpinh1 and cirbp expression were not good biomarkers of decreasing temperature. Symptoms suggestive of liver dysfunction (changes in plasma [aspartate aminotransferase] and an increase in liver size) were also evident at 1 °C. Finally, at 1 °C, 5% of the fish died, and these mortalities were typically associated with snout ulceration/erosion and fin rot. In summary, Atlantic salmon held at temperatures <5 °C suffer from decreased growth performance, osmoregulatory disturbance, liver dysfunction, stress, opportunistic infections, and increased mortality.

Isolation and identification of Streptococcus agalactiae in cage-cultured green chromide Etroplus suratensis in Kerala, India.

Streptococcus agalactiae is one of the main aetiological agents in large-scale mortalities of tilapia, having caused major economic losses to the aquaculture industry in recent years. This study describes the isolation and identification of the bacteria from cage-cultured Etroplus suratensis that experienced moderate to severe mortalities in Kerala, India. Gram-positive, catalase-negative S. agalactiae was identified from brain, eye and liver of the fish by antigen grouping and 16S rDNA sequencing. Multiplex PCR confirmed that the isolate belonged to capsular serotype Ia. Antibiotic susceptibility tests showed that the isolate was resistant to methicillin, vancomycin, tetracycline, kanamycin, streptomycin, ampicillin, oxacillin and amikacin. Histological sections of the infected E. suratensis brain revealed infiltration of inflammatory cells, vacuolation and meningitis. This report is the first description of S. agalactiae as a primary pathogen causing mortalities in E. suratensis culture in Kerala.

Largescale mortality of Oreochromis mossambicus in lakes and reservoirs of Karnataka due to coinfection of Tilapia Lake virus (TiLV) and multidrug-resistant Aeromonas veronii: An emerging fish disease in India

In India, culture of tilapines has been gaining importance over last few decades and it is the second widely farmed fish species in the World. There have been increased large-scale mortalities observed in last few years due to DNA and RNA virus associated with coinfection of bacteria, fungi, parasites etc., were considered as a major global threat to the tilapia and aquaculture industry. In this study, large scale mortality of tilapia (Oreochromis mossambicus) was observed in wild populations located in Krishna Raja Sagara reservoir in Mysuru, Thonnur Lake and Erode Lake in Pandavapura, Mandya. A detailed investigation was performed from these regions which reported mass mortality in these water bodies. Tissues from the fishes (brain, gill, kidney, spleen and liver) were collected for examination of parasites, histology and molecular detection (virus and bacteria). The results of polymerase chain reaction showed that all the samples were found negative for DNA viruses and for one RNA virus namely VNN but all the samples from 3 regions (reservoir and lakes) were positive for TiLV. Syncytial hepatitis lesion along with glial cells were observed in the TiLV-infected tilapia. TiLV isolated from this study (OM1 and OM2) shared 94.30% and 95.52% nucleotide identity with the TiLV isolated from West Bengal, India (MF502419.1) and Israel (KJ605629.1) respectively. NCBI-BLAST search results using gyrB gene revealed that the isolates G 1 & H 2 were identified and confirmed as Aeromonas veronii with 99.52% and 99.42% nucleotide similarity to the A. veronii isolate XX-57 (JX025939.1) from diseased Chub and A. veronii isolate JH-12 (JX025895.1) from diseased gold fish from China respectively. To the best of our knowledge, this is the first report of concurrent infection TiLV and A. veronii associated with mass mortality in Oreochromis mossambicus in Karnataka, India. In vitro study showed the cytopathic effects on OniL cell lines with initially elongated cells followed by rounding and finally detachment leading to death of the cells.

Dietary teprenone enhances non-specific immunity, antioxidative response and resistance to hypoxia induced oxidative stress in Lateolabrax maculatus

Persistent hypoxic or low-oxygen conditions in aquatic systems are becoming more frequent worldwide, causing large-scale mortalities to the aquaculture animals. Hence, it would be of great benefit to search for natural compounds that are clinically safe, yet able to induce host health and antioxidant defence system. In the experiment, Lateolabrax maculatus were fed with ketone compound teprenone and subsequently exposed to hypoxia condition. Afterwards, the effect of teprenone on the antioxidative response, non-specific immunity and hypoxia induced oxidative stress were investigated. We found that dietary supplementation of teprenone at 200 mg/kg (HT200), 400 mg/kg (HT400) and 800 mg/kg (HT800) significantly increase the total antioxidant capacity (T-AOC) during hypoxia stress. While, lower values of malondialdehyde (MDA) content, an indicator of oxidative stress, were recorded in teprenone supplemented groups. In addition, upregulation of non-specific immune parameters including lysozyme (LZM), Hsp70 and Hsp90 was observed in L.maculatus. We further demonstrated that teprenone supplementation protects L.maculatus from hypoxia and oxidative stress generated apoptosis. The pro-apoptotic signalling molecules, i.e., cytochrome C (CtyC), caspase-9 and caspase-3 production were significantly decreased in response to teprenone supplementation during the hypoxia stress. In contrast, the anti-apoptotic response, BCL-2 gene expression, was significantly increased. We have shown that dietary teprenone induces innate immunity and antioxidative response and protects L.maculatus against hypoxia induced-oxidative stress and apoptosis; therefore, suggesting their possible role to manage the stressful condition, in particular, hypoxia stress in aquaculture.

Bonamia exitiosa in farmed native oysters Ostrea angasi in Australia: optimal epidemiological qPCR cut-point and clinical disease risk factors.

Bonamiosis has developed as a problem in Australian native oysters Ostrea angasi since the parasite Bonamia spp. was first detected in Port Phillip Bay, Victoria, in the early 1990s. At that time, large-scale mortalities in both farmed and wild oysters saw the demise of the pilot native oyster culture industry. More recent attempts to farm the species resulted in subclinical infections that progressed over time to clinical disease. The aim of this work was to establish what environmental factors result in the clinical manifestation of disease; determine the diagnostic sensitivity and diagnostic specificity of histopathological examination and a quantitative polymerase chain reaction (qPCR) test for the diagnosis of B. exitiosa infection in clinically diseased farmed native oysters; and calculate the optimal qPCR threshold cycle (CT) epidemiological cut-point for classification of positive and negative cases. After applying a range of stressors to tank-held oysters, results indicated a 58% increased risk (95% CI: 16%, 99%) of a Bonamia-infected oyster dying if the oyster was held at a higher temperature (p = 0.048). Starving and tumbling oysters, in isolation, was not significantly associated with clinical bonamiosis, but a Bonamia-infected oyster was at the greatest risk of death when increased water temperature was combined with both starvation and increased motion (p = 0.02; odds ratio = 3.47). The diagnostic sensitivity and specificity of the World Organisation for Animal Health qPCR protocol were calculated for increasing CT value cut-points from ≤25 to ≤40, with an optimal cut-point identified at ≤34.5 (specificity: 92.2; 95% posterior credible intervals [PCI]: 76.2, 99.8; Sensitivity: 93.5; 95% PCI: 84.7, 99.1).

Increased Bacterial Richness Associated With Lesions Within the Porites spp. of Vietnam

Coral reefs worldwide are rapidly declining due to increasing anthropogenic stressors and environmental changes, with large-scale mortalities of coral reefs observed in many locations across the globe. It has become clear that the microbiome of corals is important in understanding the causes of coral infections, although its exact role is yet to be fully understood. Here, we characterize the bacteria and fungi associated with the non-lesional and lesional (identified by discoloration and tissue loss) tissues of coral species from Vietnam. Metabarcoding of the bacterial 16S rRNA gene and the fungal ITS rRNA gene region were performed. We sampled across two Porites species with potentially multiple causes of stresses, yet the bacterial compositions of lesional regions were consistently different from non-lesional areas of the same coral. These differences were driven by a considerable and significant increase in OTU richness within the lesional region compared to the non-lesional region. While no single OTU was consistently associated with lesional tissue, indicator analysis revealed that nine OTUs were significantly more persistent in the lesional regions that could represent useful bioindicators of stress. Meanwhile, there were no indicator OTUs in the non-lesional region. Further investigations are needed to determine whether changing bacterial communities play a mechanistic role in inducing lesioning, or are opportunistically colonizing stressed corals.

Structural Characterization of Open Reading Frame-Encoded Functional Genes from Tilapia Lake Virus (TiLV).

In recent years, large-scale mortalities are observed in tilapia due to infection with a novel orthomyxo-like virus named, tilapia lake virus (TiLV) which is marked to be a severe threat to universal tilapia industry. Currently, there are knowledge gaps relating to the antiviral peptide as well as there are no affordable vaccines or drugs available against TiLV yet. To understand the spreading of infection of TiLV in different organs of Oreochromis niloticus, RT-PCR analysis has been carried out. The gene segments of TiLV were retrieved from the NCBI database for computational biologyanalysis. The 14 functional genes were predicted from the 10 gene segments of TiLV. Phylogenetic analysis was employed to find out a better understanding for the evolution of tilapia lake virus genes. Out of 14 proteins, only six proteins show transmembrane helix region. Moreover, molecular modeling and molecular dynamics simulations of the predicted proteins revealed structural stability of the protein stabilized after 10-ns simulation. Overall, our study provided a basic bioinformatics on functional proteome of TiLV. Further, this study could be useful for development of novel peptide-based therapeutics to control TiLV infection.

Susceptibility of Indian major carp Labeo rohita to tilapia lake virus

Infection with tilapia lake virus (TiLV) has been reported to cause large-scale mortalities in tilapines, the second most cultured species in the world. Interestingly, in polyculture system, although tilapia is reported to suffer high mortalities due to infection with TiLV, the disease has not been reported in co-cultured species. Nevertheless, natural host switch is a frequent phenomenon among viruses of lower vertebrates, especially fish viruses. To analyse the risk of spread of virus to new host species, it is essential to study the susceptibility of other fish species to the virus. In India, tilapia is generally cultured along with Indian major carps (IMC) in polyculture systems. Keeping this in consideration, in the present study, the susceptibility of rohu Labeo rohita, an important major carp, to TiLV was evaluated following experimental infection and it was compared with a susceptible host, i.e. Nile tilapia Oreochromis niloticus. The results revealed that the experimentally-infected rohu did not exhibit any clinical signs and mortality. On the other hand, in tilapia, a cumulative mortality of 80% was observed during the experimental period of 12 days with clinical signs typical of TiLV disease. In histopathological examination of liver, one of the main target organ of TiLV, typical syncytial cells were observed in tilapia whereas, no alterations were observed in rohu. Importantly, in rohu, the virus copy numbers in liver and intestine were almost negligible (<3 virus copies) and did not increase with time course, and the virus was not detected in brain at any time point by qRT-PCR. However, in tilapia, there was significant increase in virus copy numbers in liver, brain as well as intestine up to 6 days post-infection (dpi), and thereafter, a marginal decrease was observed up to 12 dpi. Therefore, based on lack of clinical signs and mortality pattern, histopathological findings and results of qRT-PCR, it can be concluded that TiLV did not replicate in rohu, and it is not susceptible to infection with TiLV.

Expression of immune genes in Indian major carp, Catla catla challenged with Flavobacterium columnare

Columnaris disease, caused by Flavobacterium columnare, is one of the important bacterial diseases responsible for large-scale mortalities in numerous freshwater fishes globally. This disease can cause up to 100% mortality within 24 h of infection and is considered to be a cause of concern for aquaculture industry. Despite being a serious disease, scarce information is available regarding host-pathogen interaction, particularly the modulation of different immune genes in response to F. columnare infection. Therefore, in the present study, an attempt has been made to study expression of important immune regulatory genes, namely IL-1β, iNOS, INF-γ, IL-10, TGF-β, C3, MHC-I and MHC-II in gills and kidney of Catla catla following experimental infection with F. columnare. The expression analysis of immune genes revealed that transcript levels of IL-1β, iNOS, IL-10, TGF-β, C3 and MHC-I were significantly up-regulated (p < 0.05) in both the organs of the infected catla. IFN-γ and MHC-II were up-regulated in gills of infected catla whereas, both the genes showed down-regulation in kidney. The results indicate that important immune genes of C. catla are modulated following infection with F. columnare. The knowledge thus generated will strengthen the understanding of molecular pathogenesis of F. columnare in Indian major carp C. catla.

Prolonged exposure to low oxygen improves hypoxia tolerance in a freshwater fish.

Persistent hypoxic or low-oxygen conditions in aquatic systems are becoming more frequent worldwide, causing large-scale mortalities to aquatic fauna. It is poorly understood, however, whether species can acclimate to long-term hypoxic conditions. In two experiments, we exposed juvenile freshwater fish (Murray cod, Maccullochella peelii) to low-oxygen conditions and investigated acclimation effects. Experiment 1 determined how responses could be modified by exposure to different temperatures (20, 24 and 28°C) and oxygen conditions (control 6-8 mgO2 L-1 and low-oxygen 3-4 mgO2 L-1) over 30days. Experiment 2 determined the acclimation ability of fish exposed to two temperatures (20 and 28°C) and low-oxygen conditions (3-4 mgO2 L-1) for three different acclimation periods (7, 14 and 30days). Responses were measured by determining critical oxygen tension (P crit), loss of equilibrium and aerobic capacity using resting respirometry. In experiment 1, resting oxygen requirements were negatively affected by long-term low-oxygen exposure except at the highest temperature (28°C). However, long-term acclimation in low-oxygen improved tolerance as measured by loss of equilibrium but not P crit. In experiment 2, fish could tolerate lower oxygen levels before reaching loss of equilibrium after 7days acclimation, but this declined overtime. Murray cod were most tolerant to low-oxygen at the lowest temperature (20°C) and shortest exposure time (7days). Extended low-oxygen exposure resulted in reduced aerobic capacity of fish particularly at the lowest temperature. While prior exposure to low-oxygen may allow fish to cope with hypoxic conditions better in the long-term, acclimation time was inversely related to tolerance, suggesting that resistance to hypoxia might decrease as a function of exposure time. Our study fills a much-needed gap in our understanding of how freshwater species acclimate to hypoxia, and in particular, how exposure to prolonged periods of low-oxygen and elevated temperatures affect organisms physiologically.

Identification of a novel nidovirus as a potential cause of large scale mortalities in the endangered Bellinger River snapping turtle (Myuchelys georgesi).

In mid-February 2015, a large number of deaths were observed in the sole extant population of an endangered species of freshwater snapping turtle, Myuchelys georgesi, in a coastal river in New South Wales, Australia. Mortalities continued for approximately 7 weeks and affected mostly adult animals. More than 400 dead or dying animals were observed and population surveys conducted after the outbreak had ceased indicated that only a very small proportion of the population had survived, severely threatening the viability of the wild population. At necropsy, animals were in poor body condition, had bilateral swollen eyelids and some animals had tan foci on the skin of the ventral thighs. Histological examination revealed peri-orbital, splenic and nephric inflammation and necrosis. A virus was isolated in cell culture from a range of tissues. Nucleic acid sequencing of the virus isolate has identified the entire genome and indicates that this is a novel nidovirus that has a low level of nucleotide similarity to recognised nidoviruses. Its closest relatives are nidoviruses that have recently been described in pythons and lizards, usually in association with respiratory disease. In contrast, in the affected turtles, the most significant pathological changes were in the kidneys. Real time PCR assays developed to detect this virus demonstrated very high virus loads in affected tissues. In situ hybridisation studies confirmed the presence of viral nucleic acid in tissues in association with pathological changes. Collectively these data suggest that this virus is the likely cause of the mortalities that now threaten the survival of this species. Bellinger River Virus is the name proposed for this new virus.

A comparison of infectious agents between hatchery-enhanced and wild out-migrating juvenile chinook salmon (Oncorhynchus tshawytscha) from Cowichan River, British Columbia

Infectious diseases are likely contributing to large-scale declines in chinook salmon stocks in the Pacific Northwest, but the specific agents and diseases involved, and the prevalences in migratory salmon, are mostly unknown. We applied a high-throughput microfluidics platform to screen for 45 infectious agents in 556 out-migrating juvenile chinook salmon, collected from freshwater (FW) and saltwater (SW) locations in the Cowichan River system on Vancouver Island, Canada, during 2014. Nineteen agents (5 bacterial, 2 viral, and 12 parasitic) were detected, with prevalences ranging from 0.2% to 57.6%. Co-infections between Candidatus Branchiomonas cysticola Toenshoff, Kvellestad, Mitchell, Steinum, Falk, Colquhoun &amp; Horn, 2012, Paranucleospora theridion Nylund, Nylund, Watanabe, Arnesen &amp; Kalrsbakk, 2010, and gill chlamydia, all associated with gill disease, were observed in SW samples. We detected agents known to cause large-scale mortalities in Pacific salmon ( Ceratonova shasta (Noble, 1950), Parvicapsula minibicornis Kent, Whitaker &amp; Dawe, 1977), and agents only recently reported in Pacific salmon in BC ( Ca. B. cysticola, P. theridion, Facilispora margolisi Jones, Prosperi-Porta &amp; Kim, 2012 and Parvicapsula pseudobranchicola Karlsbakk, Saether, Hostlund, Fjellsoy &amp; Nylund, 2002). Wild and hatchery fish were most divergent in agent profiles in FW, with higher agent diversity in wild fish. Differences in prevalence largely dissipated once they converged in the marine environment, although hatchery fish may be infected by a greater diversity of agents sooner after ocean entry by virtue of their more rapid migration from nearshore to offshore environments.

Emergence of Tilapia Lake Virus associated with mortalities of farmed Nile Tilapia Oreochromis niloticus (Linnaeus 1758) in India

Tilapiines are one of the most widely farmed fish species and currently being cultured in >100 countries in the World. Over the last few years, large-scale mortalities have been reported in tilapia due to infection with orthomyxo-like virus i.e. Tilapia Lake Virus (TiLV) which is considered as a threat to global tilapia industry. In the present study, we report outbreaks of TiLV disease in farmed tilapia in two states, West Bengal and Kerala from India. Diseased fish exhibited lethargy, inappetance and skin erosions with >85% mortality. TiLV infection was confirmed on the basis of PCR amplification and sequencing of segment 3 of TiLV, histopathology, infection of fish cell line and bioassay. CLUSTALW alignment of the partial sequences of segment 3 of the TiLV revealed that North 24 Parganas (MF502419) and South 24 Parganas (MF582636) of West Bengal, India showed 97.2% and Ernakulam, Kerala, India (MF574205) showed 96.4% similarity with TiLV from Israel (KJ605629.1). In histopathology, typical syncytial giant cells in liver and congestion of the blood vessels along with haemorrhages in sections of brain tissue were observed. The filtered tissue homogenate prepared from liver and brain of affected tilapia produced cytopathic effects in CFF cell line derived from Pristolepis fasciatus. The disease was successfully reproduced in naive tilapia following injection of culture supernatant from infected cell line and TiLV was successfully reisolated from experimentally infected tilapia. This is the first report of TiLV from India and adds to the reports of TiLV outbreaks in five countries across three continents.

Testing hypoxia: physiological effects of long-term exposure in two freshwater fishes.

Hypoxic or oxygen-free zones are linked to large-scale mortalities of fauna in aquatic environments. Studies investigating the hypoxia tolerance of fish are limited and focused on marine species and short-term exposure. However, there has been minimal effort to understand the implications of long-term exposure on fish and their ability to acclimate. To test the effects of long-term exposure (months) of fish to hypoxia we devised a novel method to control the level of available oxygen. Juvenile golden perch (Macquaria ambigua ambigua), and silver perch (Bidyanus bidyanus), two key native species found within the Murray Darling Basin, Australia, were exposed to different temperatures (20, 24 and 28°C) combined with normoxic (6-8mgO2L-1 or 12-14kPa) and hypoxic (3-4mgO2L-1 or 7-9kPa) conditions. After 10months, fish were placed in individual respirometry chambers to measure standard and maximum metabolic rate (SMR and MMR), absolute aerobic scope (AAS) and hypoxia tolerance. Golden perch had a much higher tolerance to hypoxia exposure than silver perch, as most silver perch died after only 1month exposure. Golden perch acclimated to hypoxia had reduced MMR at 20 and 28°C, but there was no change to SMR. Long-term exposure to hypoxia improved the tolerance of golden perch to hypoxia, compared to individuals held under normoxic conditions suggesting that golden perch can acclimate to levels around 3mgO2 L-1 (kPa~7) and lower. The contrasting tolerance of two sympatric fish species to hypoxia highlights our lack of understanding of how hypoxia effects fish after long-term exposure.

Chryseobacterium sp. PLI2 and Aeromonas hydrophila co-infection in pacu, Piaractus brachypomus (Cuvier, 1817) fries cultured in West Bengal, India

The present study reports the first isolation of Chryseobacterium sp. and co-infection with Aeromonas hydrophila in pacu, Piaractus brachypomus fries cultured in West Bengal, India. The infected pacu fries showed white patches on gill, tail rot, skin discoloration, scale loss, skin peeling, emaciation, and kidney inflammation. The yellow pigmented colony from the kidney of pacu on selective cytophaga agar was identified as Chryseobacterium sp. based on colony pigmentation, cell morphology, phenotypic characterization, and 16S rDNA gene sequencing. Phylogenetically, the members of the genus Chryseobacterium clustered together as a separate clade and the strain PLI2 (NCBI accession number KP898212) branched with a non-type strain C. indologenes VIT-CMJ1 (accession number KJ437473) with high node value (100). Yet, the strain PLI2 was distinctly different from closely related type strain C. indologenes LMG 8337 (accession number LN681561) with 97% DNA homology and low (80) node value or other type strains of NCBI GenBank database. The challenge experiments indicated that Chryseobacterium sp. PLI2 can cause substantial mortalities (25%) in abraded and bath (2.2×107/mL) treated pacu fries. The kidney of challenged pacu became pale and inflamed. Most likely, this report is a new record on the isolation of Chryseobacterium sp. from the kidney of P. brachypomus. Statement of relevance•Co-infection of Chryseobacterium sp. and A. hydrophila in Piaractus brachypomus caused nephropathy and large scale mortalities.•The record of a new bacterial disease Chryseobacterium sp. infection in Indian aquaculture is a serious cause for concern.

Histopathological evaluation and molecular detection of natural Iridovirus infection in cultured grouper fish in Malaysia

Iridovirus is the causative agent of grouper iridovirus infection (GIV), which causes severe epizootics resulting in large-scale mortalities and huge economic losses in cultured and marine fishes worldwide. The current study evaluated the gross and histopathological lesions, and molecular detection of GIV in naturally infected grouper farms in Malaysia. A total of 150 moribund fish showing different clinical signs, presented to the Aquatic unit were used for this study. The fish were necropsied and visceral organs (spleen, kidney, heart, liver, intestine, brain, eyes, gills, and some parts of skin with ulcer) were collected and fixed in 10 % buffered formalin for histopathological processing and evaluation. Molecular detection of the virus was done by polymerase chain reaction (PCR). Grossly, the groupers had ulceration on the operculum and close to the caudal fin with hemorrhages on the margin of the caudal fin. In some fish, there was necrosis of the caudal fin, sloughing of the epidermis, dermal ulceration and popeyes Histological sections showed large basophilic cytoplasmic inclusions and vacuolations in the cytoplasm of hepatocytes. Basophilic or eosinophilic enlarged cells with the presence of mononuclear cellular infiltrations were seen in the kidney, liver, eye and gills, which is the distinctive feature of this disease. PCR detection showed positive amplification from 27 groupers. Based on the gross, histopathological and molecular detection GIV infection was established.

A hierarchy of conceptual models of red-tide generation: Nutrition, behavior, and biological interactions

Red tides – discolorations of the sea surface due to dense plankton blooms – occur regularly in coastal and offshore waters along much of the world's coastline. Red tides often cause large-scale mortalities of fish and shellfish and significant losses to the aquaculture and tourist industries of many countries. Therefore, understanding and predicting the mechanisms controlling the outbreak, persistence, spread, and decline of red tides are important concerns to scientists, officials, industry, and the public. With increasing knowledge of red-tide species and red-tide events, new mechanisms have been discovered. Based on the nutrition and behaviors of red-tide organisms and biological interactions among them, red-tide outbreaks can be categorized into a hierarchy of four generation mechanisms (GM1–GM4). In the simplest, GM1, all phototrophic red-tide species were treated as exclusively autotrophic organisms without the ability to swim. However, this GM cannot explain red-tide outbreaks in oligotrophic surface waters offshore. Vertical migration (considered in GM2) and mixotrophy (GM3) enable red-tide flagellates to acquire growth factors from nutrient-rich deep waters or co-occurring prey, respectively. In natural environments, all red tides occur by those species outgrowing co-occurring organisms; red-tide species dominate communities by eliminating other species or reducing their abundances. Thus, GM4 contains the direct biological interactions (i.e., inhibition by physical contact or chemical effects) and indirect biological interactions (i.e., acquiring resources faster than others) that can affect the dominance of red-tide species under given conditions. Correctly choosing one of these four GMs for red tides dominated by one causative species is important because the accuracy of predictions may be outweighed by the costs and time required to acquire the relevant information. In this study, mechanisms describing the outbreak, persistence, and decline of red tides were reviewed, the advantages and limitations of each mechanism were evaluated, and insights about the evolution of the mechanisms were developed.

Consequences of Anthropogenic Activities on Fish and the Aquatic Environment

Persistence of pollution into our natural environment has concurrently led to instability, disorder, harm or discomfort to both the physical systems and living organisms inhabiting the ecosystem. Potentially harmful substances such as pesticides, heavy metals, paper mill waste, polychlorinated biphenyl and crude oil are often released into the aquatic environment. When large quantities of these pollutants are released there may be an acute impact as measured by large-scale sudden mortalities of aquatic organisms such as fish kills. Lower levels of discharge may result in an accumulation of the pollutants in body composition of fish. The effects of water pollution on fish species can be classified into acute effects and long term chronic effects, which includes immune-suppression, reduced metabolism, and damage to gills and epithelia. Pollution-related diseases include fin/tail rot, gill disease, hepatic damage and ulceration. Specific examples of fish diseases that reflect the effects of pollution include, fin and tail rot caused by Amnonas hydrophila and Pseudomonas jluorescens. The polluted water may have undesirable colour, odour, taste, turbidity, harmful chemical contents, toxic and heavy metals, pesticides, industrial waste products, domestic sewage, virus, bacteria, protozoa and worms. Fish species are not only tolerant to severe contaminated waters but show a variety of specific morphological deformities and lesions that appear to reflect the level of water pollution. Assessing morphological deformities is one of the most straightforward methods to study the effects of contamination on fish because of the ease of recognition and examination when compared with other types of biomarkers. The indiscriminate disposal of pollutants without pre-treatment should be discouraged. In order to avoid the effects of water pollution on fish health, effluents discharged from industries and other sources should be properly treated.