Bacteria associated with early life stages of the great scallop, Pecten maximus: impact on larval survival

International audienceA bacteriological study was carried out at a scallop (Pecten maximus) hatchery near Bergen, western Norway following a severe increase in mortality rates during the larval stages of the scallops. No larvae survived to settling, except for those in groups treated prophylactically with chloramphenicol. In order to identify pathogenic strains of bacteria, we performed a challenge test on 10- to 16-day-old larvae using isolated bacterial strains from the hatchery. Infection with six of these strains produced mortalities that were not statistically different from that resulting from infection with the known pathogen Vibrio pectenicida. However, about 5% of the strains tested in the challenge experiment produced higher motility rates than found in the unchallenged control group, indicating a possible probiotic effect. On the basis of 16S rDNA analysis on these strains, the phylogenetic tree indicated two groups of apparent pathogens: (1) one strain, LT13, grouped together with Alteromonas/Pseudoalteromonas; (2) a cluster of strains grouped together with Vibrio splendidus (LT06, LT21, LT73, PMV18 and PMV19). Strain LT13 was isolated from cultures of the microalga Chaetoceros calcitrans used for feed, while the other strains were isolated from larval cultures. Transmission electron microscopy showed intracellular bacteria that resembled bacteria in the groups Chlamydiaceae and Rickettsiaceae

Susceptibility of Atlantic cod Gadus morhua, halibut Hippoglossus hippoglossus and wrasse (Labridae) to Aeromonas salmonicida subsp. salmonicida and the possibility of transmission of furunculosis from farmed salmon Salmo salar to marine fish

The possibility of transmission of Aeromonas salmonicida subsp. salmonicida from Atlantic salmon Salmo salar to cod Gadus morhua, halibut Hippoglossus hippoglossus, and wrasse (Labridae) was studied in both laboratory cohabitation and injection challenge trials, and in a field situation during an outbreak of furunculosis in Atlantic salmon in 2 sea farms containing both cod and salmon. A, salmonicida subsp, salmonicida was isolated from cod, halibut and wrasse, but only occasionally. The injection challenge studies with A, salmonicida subsp. salmonicida were carried out on cod and halibut. Despite challenge with 4.6 x 10(3), 4.6 x 10(4) and 4.6 x 10(5) CFU (colony-forming units) of the bacterium, only 1 halibut died, whereas no mortality occurred in the cod groups. In cohabitation challenge trials in which cod, halibut and wrasse were kept together with infected salmon, 1 cod and 1 halibut died, with subsequent isolation of A, salmonicida subsp. salmonicida from their kidneys. In a group of salmon challenged in the same way with infected salmon cohabitants, all the salmon died. In the field study, there were high mortalities of salmon due to furunculosis. However, A, salmonicida subsp. salmonicida was isolated from only 1 of the wrasse used as 'cleaner fish' for the salmon. A. salmonicida was not isolated from cod during the field study. The results suggest that A, salmonicida subsp. salmonicida can be transmitted from salmon and cause disease in cod, halibut and wrasse, but that this is a relatively rare event

Green water in larviculture - An experiment with natural phytoplankton in tanks for first feeding of halibut larvae (Hippoglossus hippoglossus L.)

At 232 day degrees, halibut larvae were transferred from indoor tanks to 1.7 m^3 outdoor tanks for first feeding. The number in each tank was approximately 750. Three tanks were continuously given algal suspension ("green water") and supplied nonenriched Artemia instar II. Six tanks were given filtered deep water ("clear water"). Three of the six were supplied nonenriched Artemia, and three were supplied Artemia prefed in green water. Feeding incidence at day 3 was 47 % in green water and 0 % in clear water. Larval growth was significantly higher in green water compared to clear water, while no significant difference was found between the clear water groups given prefed and nonenriched Artemia. The mean myotome heights for all groups were 0.75 - 0.78 mm at day 7. At day 14 and 21, the mean heights were 1.49 and 1.86 mm in the green water group and 0.84 and 1.05 mm in the clear water groups. The survival rates were also much higher in green water. Out of a total of approximately 2250 halibut larvae in the green water tanks, 684 larvae were found alive at the end of the experiment. Corresponding numbers for the clear water tanks were 57 out of 4500. Preliminary results indicate no nutritional effect of the algae

Susceptibility and pathology in juvenile atlantic cod gadus morhua to a marine viral haemorrhagic septicaemia virus isolated from diseased rainbow trout oncorhynchus mykiss

The first known outbreak caused by a viral haemorrhagic septicaemia virus (VHSV) strain of genotype III in rainbow trout occurred in 2007 at a marine farm in Storfjorden, Norway. The source of the virus is unknown, and cod and other marine fish around the farms are suspected as a possible reservoir. The main objective of this study was to test the susceptibility of juvenile Atlantic cod to the VHSV isolate from Storfjorden. As the pathology of VHS in cod is sparsely described, an additional aim of the study was to give a histopathological description of the disease. Two separate challenge experiments were carried out, using both intra peritoneal (ip) injection and cohabitation as challenge methods. Mortality in the ip injection experiment leveled at approximately 50% three weeks post challenge. Both immunohistochemical and rRT-PCR analysis of organs sampled from diseased and surviving fish confirmed VHSV infection. No VHSV was detected in the cohabitants. The results indicate that Atlantic cod has a low natural susceptibility to this VHSV genotype III strain. One of the most extensive pathological changes was degeneration of cardiac myocytes. Immunohistochemistry confirmed that the lesions were related to VHSV. In some fish, the hematopoietic tissue of spleen and kidney showed degeneration and immunostaining, classical signs of VHS, as described in rainbow trout. Positive immunostaining of the capillaries of the gills, suggests this organ as a useful alternative when screening for VHSV.publishedVersio