Population Dynamics of Vibrio and Pseudomonas Species Isolated from Farmed Tasmanian Atlantic Salmon (Salmo salar L.): A Seasonal Study

Vibrio and Pseudomonas species have been shown to be part of the normal microbiota of Atlantic salmon (Salmo salar L.), with some strains causing disease in fish. The factors affecting their prevalence and persistence in the salmon gut, however, have not been well studied. In this study, we collected 340 Vibrio and 150 Pseudomonas isolates from the hindgut of farmed Tasmanian Atlantic salmon, fed with two commercially available diets. Samples were collected every 6–8 weeks between July 2011 and May 2012. Isolates from selective agar were initially identified using biochemical tests and confirmed using genus-specific primers and 16S ribosomal RNA (16S rRNA) sequencing. Random amplified polymorphic DNA (RAPD) PCR was used to type both Pseudomonas and Vibrio; the latter was further typed using a biochemical fingerprinting method (PhP-RV plates). We observed low species diversity with strains comprising Vibrio ichthyoenteri/Vibrio scophthalmi, Vibrio crassostreae/Vibrio splendidus, Aliivibrio finisterrensis, Photobacterium phosphoreum and Pseudomonas fragi. Out of 340 Vibrio isolates, 238 (70 %) belonged to 21 clonal types and were found predominantly during summer when water temperatures reached 15 to 21 °C. Of these, the four major clonal types were found in multiple samples (70 %). P. fragi, on the other hand, was only found during the colder water temperatures and belonged to 18 clonal types. The presence of both groups of bacteria and their clonal types were independent of the fish diets used, suggesting that the water temperature was the main factor of the prevalence and persistence of these bacteria in the gut of Atlantic salmon.     

The Bacterial Flora of the Atlantic Salmon (Salmo salar L.) in Relation to its Environment

S ummary . The aerobic flora of the skin of 56 Atlantic salmon from coastal, estuarine and river waters was analysed quantitatively; 50 skin and 33 gill samples were analysed qualitatively. The water at each sampling station was also analysed. The principal genera on the skin and gills were Moraxella, Flavobacterium, Cytophaga and Pseudomonas ; members of Acinetobacter, Bacillus, Aeromonas, Vibrio , the Entrobacteriaceae, Micro-coccaceae and some coryneforms were also present. The gill flora was similar to that of the skin, which reflected that of the environment.     

Cataract Development in Atlantic Salmon (Salmo salar L) in Fresh Water

Irreversible bilateral cataracts were diagnosed by slit-lamp biomicroscopy in 178 of 200 farm-raised Atlantic salmon (Salmo salar L) fed a standard diet over a five-month period. Initial changes were anterior polar opacities, progressing to involve both the anterior and posterior cortex before changes in the lens nucleus were seen. The lens changes were recorded and given scores according to the severity of the cataracts. At each of 3 samplings, after 2, 4 and 5 months, 200 fish were measured, weighed and examined by slit-lamp biomicroscopy. At all 3 samplings, there was a significant correlation between body length and both cataract incidence and cataract severity. There was also a significant correlation between body weight and cataract incidence and severity for the 2 last samplings. There was a significant correlation between K-factor as a measure of the shape of the fish, and both cataract incidence and severity, at all 3 samplings. Evaluation of specific growth rate in the periods between the examinations showed that the rapidly-growing fish were most susceptible to cataract formation. After cataract developed, however, the growth rate slowed. Follow-up examination of severely affected fish 3 months after transfer to sea water showed a normal cortical zone in the periphery of the lens in 24 out of 28 fish.     

The hearing of the Atlantic Salmon, Salmo salar

The hearing of the salmon, Salmo salar L., was studied by means of a cardiac conditioning technique. Fish were trained to show a slowing of the heart, on hearing a sound, in anticipation of a mild electric shock applied later. The minimum sound level to which the fish would respond was determined for a range of pure tones, both in the sea, and in the laboratory. The fish responded only to low frequency tones (below 380 Hz), and particle motion, rather than sound pressure, proved to be the relevant stimulus. The sensitivity of the fish to sound was not affected by the level of sea noise under natural conditions but hearing is likely to be masked by ambient noise in a turbulent river. Sound measurements made in the River Dee, near Aberdeen, lead to the conclusion that salmon are unlikely to detect sounds originating in air, but that they are sensitive to substrate borne sounds. Compared with the carp and cod the hearing of the salmon is poor, and more like that of the perch and plaice.     

Functional Feeds Reduce Heart Inflammation and Pathology in Atlantic Salmon (Salmo salar L.) following Experimental Challenge with Atlantic Salmon Reovirus (ASRV)

Heart and Skeletal Muscle Inflammation (HSMI), recently associated with a novel Atlantic salmon reovirus (ASRV), is currently one of the most prevalent inflammatory diseases in commercial Atlantic salmon farms in Norway. Mortality varies from low to 20%, but morbidity can be very high, reducing growth performance and causing considerable financial impact. Clinical symptoms, including myocarditis, myocardial and red skeletal muscle necrosis, correlate with the intensity of the inflammatory response. In the present study, the effects of two functional feeds (FF1 and FF2) were compared to a standard commercial reference feed (ST) in Atlantic salmon subjected to an ASRV challenge. The functional feeds had reduced levels of total lipid and digestible energy, and different levels and proportions of long-chain polyunsaturated fatty acids (LC-PUFA). The objective was to determine whether these feeds could provide effective protection by decreasing the inflammatory response associated with HSMI. Histopathology, viral load, fatty acid composition and gene expression of heart tissue were assessed over a period of 16 weeks post-infection with ASRV. The viral load and histopathology scores in heart tissue in response to ASRV infection were reduced in fish fed both functional feeds, with FF1 showing the greatest effect. Microarray hierarchical cluster analysis showed that the functional feeds greatly affected expression of inflammation/immune related genes over the course of the ASRV infection. Viral load correlated with up-regulation of pro-inflammatory genes at the early-mid stages of infection in fish fed the ST diet. Expression of inflammatory genes 16-weeks after ASRV challenge reflected the difference in efficacy between the functional feeds, with fish fed FF1 showing lower expression. Thus, severity of the lesions in heart tissue correlated with the intensity of the innate immune response and was associated with tissue fatty acid compositions. The present study demonstrated that dietary modulation through clinical nutrition had major influences on the development and severity of the response to ASRV infection in salmon. Thus, HSMI was reduced in fish fed the functional feeds, particularly FF1. The modulation of gene expression between fish fed the different feeds provided further insight into the molecular mechanisms and progression of the inflammatory and immune responses to ASRV infection in salmon.     

The vgll3 Locus Controls Age at Maturity in Wild and Domesticated Atlantic Salmon (Salmo salar L.) Males

Wild and domesticated Atlantic salmon males display large variation for sea age at sexual maturation, which varies between 1–5 years. Previous studies have uncovered a genetic predisposition for variation of age at maturity with moderate heritability, thus suggesting a polygenic or complex nature of this trait. The aim of this study was to identify associated genetic loci, genes and ultimately specific sequence variants conferring sea age at maturity in salmon. We performed a genome wide association study (GWAS) using a pool sequencing approach (20 individuals per river and phenotype) of male salmon returning to rivers as sexually mature either after one sea winter (2009) or three sea winters (2011) in six rivers in Norway. The study revealed one major selective sweep, which covered 76 significant SNPs in which 74 were found in a 370 kb region of chromosome 25. Genotyping other smolt year classes of wild and domesticated salmon confirmed this finding. Genotyping domesticated fish narrowed the haplotype region to four SNPs covering 2386 bp, containing the vgll3 gene, including two missense mutations explaining 33–36% phenotypic variation. A single locus was found to have a highly significant role in governing sea age at maturation in this species. The SNPs identified may be both used as markers to guide breeding for late maturity in salmon aquaculture and in monitoring programs of wild salmon. Interestingly, a SNP in proximity of the VGLL3 gene in humans (Homo sapiens), has previously been linked to age at puberty suggesting a conserved mechanism for timing of puberty in vertebrates.     

The Thyrotropic Cell in the Atlantic Salmon,Salmo salar

The thyrotropin (TSH) producing cells are distributed in the rostral and proximal pars distalis. This cell type is the smallest and most infrequent cell of the adenohypophysis. Its cytology is similar to the smallest gonadotropic (GTH) cells although the two cell types can be separated by the size of the small secretory granules (diameter less than 200 nm) in the TSH cells. In presmolts and smolts the cells are more numerous than in parr and adult salmon and have cytological features indicating an increased activity. This was also the case after intraperitoneal injections of synthetic TRH. Antisera to carp GTH and salmon GTH cross-reacted with both the GTH and the TSH cells. Anti-human TSH cross-reacted only with the TSH cells which confirms the assumption of antigenic similarity between human and fish TSH.     

Candida utilis and Chlorella vulgaris Counteract Intestinal Inflammation in Atlantic Salmon (Salmo salar L.)

Intestinal inflammation, caused by impaired intestinal homeostasis, is a serious condition in both animals and humans. The use of conventional extracted soybean meal (SBM) in diets for Atlantic salmon and several other fish species is known to induce enteropathy in the distal intestine, a condition often referred to as SBM induced enteropathy (SBMIE). In the present study, we investigated the potential of different microbial ingredients to alleviate SBMIE in Atlantic salmon, as a model of feed-induced inflammation. The dietary treatments consisted of a negative control based on fish meal (FM), a positive control based on 20% SBM, and four experimental diets combining 20% SBM with either one of the three yeasts Candida utilis (CU), Kluyveromyces marxianus (KM), Saccharomyces cerevisiae (SC) or the microalgae Chlorella vulgaris (CV). Histopathological examination of the distal intestine showed that all fish fed the SC or SBM diets developed characteristic signs of SBMIE, while those fed the FM, CV or CU diets showed a healthy intestine. Fish fed the KM diet showed intermediate signs of SBMIE. Corroborating results were obtained when measuring the relative length of PCNA positive cells in the crypts of the distal intestine. Gene set enrichment analysis revealed decreased expression of amino acid, fat and drug metabolism pathways as well as increased expression of the pathways for NOD-like receptor signalling and chemokine signalling in both the SC and SBM groups while CV and CU were similar to FM and KM was intermediate. Gene expression of antimicrobial peptides was reduced in the groups showing SBMIE. The characterisation of microbial communities using PCR-DGGE showed a relative increased abundance of Firmicutes bacteria in fish fed the SC or SBM diets. Overall, our results show that both CU and CV were highly effective to counteract SBMIE, while KM had less effect and SC had no functional effects.     

Cytogenetic studies of Atlantic salmon, Salmo salar L., in Scotland

Chromosome numbers for Atlantic salmon, Salmo salar L., from ten populations in Scotland were ascertained. The majority of fish had 2n = 58, NF = 74 karyotypes, and no polymorphisms between populations were found. The findings suggest that Atlantic salmon in Scotland are cytogenetically homogeneous.     

The effects of handling on flounder (Platichthys flesus L.) and Atlantic salmon (Salmo salar L.)

The effect of 9 min of net confinement on two marine teleost species, the flounder and the Atlantic salmon, was investigated in order to gauge how different species respond to the same stress stimulus. Net confinement in both species induced significant elevations in plasma cortisol, glucose, lactate, osmolality, and monovalent ion levels; the responses to net confinement in salmon were generally of a greater magnitude. In both species, handling induced significant alterations in PFFA levels though there were marked species differences in the qualitative and quantitative aspects of this response. Plasma protein levels were significantly elevated only in the flounders. In general, the two species responded similarly to the net confinement with differences only in the absolute levels and durations of the responses.     

Sample displacement chromatography of Atlantic Salmon (Salmo salar) thrombin

A modified method of sample displacement chromatography (SDC) was used to purify active salmon thrombin on a heparin-coupled matrix to near homogeneity in milligram amounts from 117 ml plasma. This was achieved by combining a low-pressure multi-column affinity chromatography system with non-homogenous sample application in the order of increasing affinity to Heparin Sepharose. The results suggest that this modified method could be useful in protein purification. Some characteristics of salmon thrombin are presented.