Quantification of Flavobacterium psychrophilum in rainbow trout, Oncorhynchus mykiss (Walbaum), tissues by qPCR

A qPCR assay was developed for rapid and sensitive detection of Flavobacterium psychrophilum, the aetiological agent of bacterial cold-water disease and rainbow trout fry syndrome in salmonid fish worldwide. A set of F. psychrophilum-specific primers based on 16S rRNA gene sequences was designed and validated for specific detection and quantification of DNA isolated from representative strains of F. psychrophilum. The qPCR assay exhibited a high specificity for the 16S rRNA gene of F. psychrophilum (from 4 × 10(8) down to 11 copies per reaction) but not for other Flavobacterium species or other bacteria including fish pathogens. This qPCR-based method proved to be useful in the quantification of the F. psychrophilum titre present within organs dissected out from diseased fish. As the F. psychrophilum genome contains six copies of the 16S rRNA gene, we could infer a limit of detection corresponding to two bacteria per reaction, corresponding to 800 bacteria per fish tissue sample, and therefore 20 F. psychrophilum cells mg(-1) of tissue (for sample weighing 40 mg). The qPCR assay reported here could be a useful tool for veterinary diagnostic laboratories to monitor the F. psychrophilum infection level in fish farms.     

Development of a protein binding assay for teleost insulin-like growth factor (IGF)-like: relationships between growth hormone (GH) and IGF-like in the blood of rainbow trout (Oncorhynchus mykiss)

Using rainbow trout plasma protein (IGF-BP) which specifically binds human insulin-like growth factor (IGF) (Niu and Le Bail 1993), we have developed an assay to measure plasma IGF-like levels in different teleost species. Before the assay and to prevent interference by IGF-BP, IGF-like was extracted from all samples, using SP Sephadex C-25 in acidic conditions. After this treatment, contamination of the IGF fraction by IGF-BP which was estimated by binding assay, was approximately 5%, and was not detectable by western ligand blot. Human IGF-I was used as standard and labelled hormone. Sensitivity of the assay was 0.15–0.40 ng/ml (ED90) and ED50 was 1–3 ng/ml. hIGF-II crossreaction was partial and no significant displacement was observed with Insulin from different species or with other hormones. Inhibition curves were obtained with plasma IGF fractions (but not with tissue extracts) from teleost and mammals and are parallel to the standard curve. These results suggest that the protein binding assay can quantify an IGF-like factor in the plasma of teleost and that the binding sites of IGF are well conserved during vertebrate evolution. Using this IGF binding assay, IGF-like was measured in parallel with growth hormone (GH) in plasma from young rainbow trout killed every 1.5h throughout one day. The daily profiles for both hormones, which appear pulsatile, are similar. A significant correlation was observed between GH levels and IGF-like levels with a 1.5h delay. Analogous observations were obtained in individual catheterized adult rainbow trout. Although plasma GH levels differ greatly between fish, less variability is found with IGF-like. In a third experiment, rainbow trout were starved or submitted to bovine GH treatment for four weeks. Starved fish, in which plasma GH levels increased, had plasma IGF-like level significantly lower than in fed fish. In bGH injected fish, plasma IGF-like level was significantly higher than in non-injected fish. These results suggest that, as in mammals, IGF-like secretion depends on plasma GH level and could be modulated by the nutritional status of fish.     

Predation on native fish eggs by invasive round goby revealed by species‐specific gut content DNA analyses

1. Conservation of riverine fish often aims to improve access to spawning grounds and restore longitudinal connectivity by removing migration barriers, and involves substantial investments. However, these investments also enable non‐native predators to invade upstream into spawning areas and potentially adversely affect the recruitment of threatened freshwater fish through egg or fry predation.
2. Detecting egg predation is often challenging. Visual inspections of fish gut contents may underestimate predation of soft materials such as eggs and fry, which limits the discovery of predators preying upon these life‐stages. DNA‐based detection assays may offer a more sensitive tool to assess predation of soft materials.
3. A conservation issue was confirmed by developing and applying a species‐specific DNA‐based detection assay: invasive round goby (Neogobius melanostomus) prey on the eggs or fry of the threatened common nase (Chondrostoma nasus) in Switzerland.
4. DNA‐based detection assays were also developed for five other valuable native fish species, including endangered salmonid and cyprinid river spawners. The applicability of the assays was confirmed in a series of laboratory and field feeding experiments involving eggs and fish tissue. In addition, this work provides a guiding framework for conservation managers regarding the use and applicability of different DNA‐based detection approaches for gut content analysis.
5. The results of this study could inform local conservation measures – such as temporary reductions in the density of round goby at spawning sites prior to spawning – and demonstrate how targeted application of species‐specific molecular markers may advance freshwater fish management.

     

Seasonal variation of muscle metabolic organization in rainbow trout (Oncorhynchus mykiss)

This study examined how muscle metabolic organization varied during an annual cycle in which rainbow trout (Oncorhynchus mykiss) were held in outdoor holding ponds in which they were exposed to natural changes in temperature (range 0.2 to 15.6°C) and photoperiod. We examined the activities of glycolytic and mitochondrial enzymes in red and white muscle to evaluate whether trout enhance their capacity for lipid and carbohydrate oxidation during cold-acclimization. When assayed at habitat temperature, the enzyme activities generally increased in spring to reach a maximum in summer followed by a decrease in the fall. This led to significantly higher activities at warm than cold periods for all enzymes measured in red muscle and all but one in white muscle. The activities at 10°C provided little evidence for compensatory adjustments of aerobic capacity. Particularly in red muscle, enzyme levels at 10°C were generally lower during cold than warm periods. The variation of enzyme activities throughout the cycle was not due to changes in protein concentration, as the same responses were observed when activities were expressed per g wet mass or per mg protein. Although the aerobic capacity did not increase with cold-acclimatization, the relative capacity for lipid oxidation was higher in winter than in summer trout. In contrast, the relative capacity for aerobic glycolysis was higher in summer than in winter trout. Thus, the metabolic capacities of trout muscle undergo seasonal reorganization.     

Using fishers’ ecological knowledge to assess the status and cultural importance of sawfish in Guinea‐Bissau

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Analysis of fine‐scale daily movement patterns of juvenile Pristis pectinata within a nursery habitat

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Effect of an intensive mechanical removal effort on a population of non‐native rainbow trout Oncorhynchus mykiss in a South African headwater stream

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Aqualase®, a yeast‐based in‐feed probiotic,modulates intestinal microbiota,immunity and growth of rainbow trout Oncorhynchus mykiss

Yeast probiotics have great promise, yet they received little attention in fish. This study investigated the influence of Aqualase^®, a yeast‐based commercial probiotic composed of Saccharomyces cerevisiae and Saccharomyces elipsoedas, on health and performance of rainbow trout (Oncorhynchus mykiss). Probiotics were incorporated in the diets at three different inclusion levels (1%, 1.5% and 2%) and administered to the fish for a period of 8 weeks. After the feeding trial, intestinal total viable aerobic bacterial count was significantly higher in fish group that received 2% in‐feed probiotics. In addition, a significant increase in at least 11% in intestinal lactic acid bacteria population was observed in all probiotic‐fed groups. Total protein level and lysozyme activity in skin mucus were significantly elevated following probiotic feeding. Inhibitory potential of skin mucus against fish pathogens was significantly enhanced by at least 50% in probiotic‐fed groups. Humoral and cellular immune parameters were influenced by probiotic feeding and the effects were dependent on inclusion level. Digestive physiology was affected by in‐feed probiotics through improvement of intestinal enzyme activities. All growth performance parameters were significantly improved following probiotic administration specifically at inclusion rate 1.5% and above. Taken together, the results revealed that Aqualase^® is a promising yeast‐based probiotic for rainbow trout with the capability of modulating the intestinal microbiota, immunity and growth.     

Population prioritization for conservation of imperilled warmwater fishes in an arid‐region drainage

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Seasonal use of a macrotidal estuary by the endangered dwarf sawfish,Pristis clavata

1. Sawfishes (Family: Pristidae) are one of the most imperilled fish families worldwide. There is an increasingly urgent need to better understand the biology, ecology, and population status of the five sawfish species to develop more effective conservation measures. The dwarf sawfish, Pristis clavata, is one of the least researched members of the pristids, with literature limited to analysing disparate datasets or collations of rare encounters in northern Australia.
2. This study examined the spatial ecology of dwarf sawfish using targeted surveys and acoustic telemetry to determine its habitat use in a macrotidal estuary in northern Australia. Seventeen dwarf sawfish were tagged with acoustic transmitters and monitored in the Fitzroy River estuary and adjacent King Sound (Kimberley, Western Australia) between August 2015 and November 2017.
3. Dwarf sawfish observed within the Fitzroy River estuary and King Sound were juveniles, ranging between 740 and 2,540 mm in total length. Catch per unit effort of dwarf sawfish in the late dry season was relatively high in the estuary, with the catch rate in 2015 being one of the highest reported for any sawfish species.
4. Acoustic detections revealed a distinct seasonal pattern in the use of different parts of the estuary and King Sound, which was found to be driven by salinity. Dwarf sawfish predominately occupied a single large pool near the terminus of the tidal limit in the late dry season (August–November), before transitioning to regions in closer proximity to the river mouth or in King Sound in the wet and early dry seasons (December–July).
5. Given the high abundance and residency of dwarf sawfish in the Fitzroy River estuary, this area is an important nursery for the species during the late dry season and should be formally recognized as a habitat protection area for the species.

     

Underwater video analysis as a non‐destructive alternative to electrofishing for sampling imperilled headwater stream fishes

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Characterization of the trade in manta and devil ray gill plates in China and South‐east Asia through trader surveys

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Seasonal and spatial variation in a prairie stream-fish assemblage

Abstract – Stream-fish assemblage and environmental data for 13 sites in the upper Brazos River, Texas, USA during 1997 and 1998 were used to assess the relationship between environmental conditions, and seasonal and spatial variation in fish species abundance and distribution patterns. There was considerable spatial variation in environmental conditions among sites. Spatial variation in species diversity and species composition was related to variation in conductance (salinity), depth and current velocity among sites and streams. Species diversity increased downstream and species composition shifted from primarily cyprinodontids upstream to cyprinids downstream. Among all dominant species, spatial components of variation in fish abundance were greater than seasonal components, suggesting that assemblage structure is determined more by average or persistent differences in environmental conditions among sites than by seasonal variation in environmental conditions.     

Relationship between vitamin C and plasma concentrations of testosterone in female rainbow trout,Oncorhynchus mykiss

Two-year old rainbow trout females were fed diets containing 0, 30, 110, 220, 440 and 870 mg kg^-1 ascorbyl-2-monophosphate Mg⁺ salt (groups 1, 2, 3, 4, 5, and 6, respectively) from August until March. At the time of spawning (February–March) blood was sampled and the ovulating females were hand stripped. Estradiol (E₂) and testosterone (T) concentrations in plasma, and ascorbic acid (AA) concentrations in plasma and eggs were determined. The mean plasma concentrations of T were higher in group 4, 5, or 6 than in group 2 or 3 (p < 0.05). Moreover, the average plasma concentration of T in fish fed the diets with AA level below National Research Council (NRC) recommendations (groups 1, 2 and 3) was significantly lower (p<0.01) than the average plasma concentration in fish fed diets with AA level above NRC recommendations (groups 4, 5, and 6). These data are consistent with the hypothesis that AA can influence production of steroids in female rainbow trout.Corresponding author     

Monitoring a loss: Detection of the semi‐aquatic crocodile lizard (Shinisaurus crocodilurus) in inaccessible habitats via environmental DNA

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Introduction of exotic fish species and decline of native species in the lower Po basin,north‐eastern Italy

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