Molecular and physiological responses to long-term sublethal ammonia exposure in Atlantic salmon (Salmo salar).

The objective of this study was to determine the underlying physiological and molecular responses to long-term sublethal ammonia exposure in Atlantic salmon (Salmo salar) parr. Previous studies have predominately focused on mechanisms during acute, short-term exposure. For that purpose Atlantic salmon parr were exposed to four ammonia concentrations between 4 and 1800 µmol l(-1) total ammonia nitrogen (TAN), and subjected to two feeding regimes for 15 weeks. Elevated environmental ammonia and full feeding strength caused an initial increase in plasma ammonia levels ([T(amm)]) after 22 days of exposure, which thereafter declined and remained similar to the control animals towards the end of the study. On the other hand, a progressive decrease in plasma urea levels was evident throughout the entire exposure period and depended on the concentration of environmental ammonia, with the largest decrease in urea levels observed at the highest ammonia concentrations (1700 and 1800 µmol l(-1) TAN). We hypothesized that the successful adaptation to long-term elevated ammonia levels would involve an increased capacity for carrier-facilitated branchial excretion. This hypothesis was strengthened by the first evidence of an up-regulation of branchial transcription of the genes encoding the Rhesus (Rh) glycoproteins, Rhcg1 and Rhcg2, urea transporter (UT) and aquaporin 3a (Aqp3a), during long-term exposure. Of the Rhesus glycoprotein (Rh) mRNAs, Rhcg1 was up-regulated at all tested ammonia levels, while Rhcg2 showed a concentration-sensitive increase. Increased transcription levels of V-type H(+)-ATPase (H(+)-ATPase) were observed at the highest ammonia concentrations (1700 and 1800 µmol l(-1) TAN) and coincided with an up-regulation of Rhcg2 at these concentrations. Transcription of UT and Aqp3a was increased after 15 weeks of exposure to low ammonia levels (470 and 480 µmol l(-1) TAN). A significant increase in brain glutamine (Gln) concentration was observed for full fed Atlantic salmon after 22 days and in fish with restricted feeding after 105 days of exposure to 1800 and 1700 µmol l(-1) TAN, respectively, without any concomitant decrease in brain glutamate (Glu) concentrations. These results suggest that Gln synthesis is an ammonia detoxifying strategy employed in the brain of Atlantic salmon parr during long-term sublethal ammonia exposure. Full feed strength had an additive effect on plasma [T(amm)], while the restricted feeding regime postponed the majority of the observed physiological and molecular responses. In conclusion, Atlantic salmon parr adapts to the long-term sublethal ammonia concentrations with increased branchial transcription levels of ammonia and urea transporting proteins and ammonia detoxification in the brain.

A prospective matched nested case-control study of bacterial gill disease outbreaks in Ontario, Canada government salmonid hatcheries.

Early-rearing salmonids in Ontario, Canada government fish hatcheries have been persistently affected by bacterial gill disease (BGD), and outbreaks at these locations have often been associated with high morbidity and mortality. The causative agent of BGD, Flavobacterium branchiophilum, is ubiquitous in fresh water, and outbreaks of BGD are considered to be associated with deleterious environmental conditions. This paper summarizes a 14-month rearing unit-level prospective nested matched case-control investigation at six Ontario government hatcheries (raising a total of six different salmonid species) to identify, and quantify the effects of, important predictors of BGD outbreaks. Ongoing husbandry data were collected on all early-rearing (<9 months of age) fish tank-lots ("tank-lot"=a group of fish from a specific lot existing in a single hatchery tank for a given period during the study time frame) at participating hatcheries, and all outbreaks of BGD were confirmed by light microscopy during the study period. Control tank-lots were selected at the end of the study and matched to individual cases based on time, hatchery, and species. Data were analyzed using logistic regression modeling, controlling for fish age. The final multivariable model indicated that affected tank-lots were significantly more likely to have had lower fish numbers, lower individual fish weights, higher mortality levels and higher feeding rates during the week preceding observed BGD outbreaks than were asymptomatic control tank-lots. Refinements in the observation and manipulation of these factors could therefore aid in the prevention of fish losses associated with observable BGD outbreaks. The predictive (as opposed to causal) nature of the identified factors needs to be considered, and further research is required to understand the relationships between these factors and BGD.

Rearing unit-level factors associated with bacterial gill disease treatment in two Ontario, Canada government salmonid hatcheries.

Early-rearing salmonids in Ontario Ministry of Natural Resources (OMNR) fish hatcheries have been consistently affected by bacterial gill disease (BGD) (causative agent: Flavobacterium branchiophilum) for many years. Separate retrospective epidemiological investigations of BGD treatments at two OMNR fish hatcheries (Hatcheries A and B) for the 1999 production year were conducted using on-site hatchery records. Both investigations were carried out at the rearing unit-level, with early-rearing (<9 months of age) "tank-lot" as the unit of analysis to identify unique fish populations over time. Multivariable repeated measures logistic regression models were created for both hatchery datasets, controlling for lot-level and species effects. For Hatchery A, the species brook trout (Salvelinus fontinalis) and brown trout (Salmo trutta) were significantly associated with BGD treatment, as well as lower water exchange rate, and higher feeding and mortality percentages during the 2 weeks previous to BGD treatment. At Hatchery B, the species brook trout (S. fontinalis) and splake (Salvelinus namaycush x S. fontinalis) were significantly associated with BGD treatment, as well as lower individual fish weights and treatment for BGD during the previous week. These results emphasize the importance of water quality, feeding rate, fish size and prior mortality on the development of BGD. Significant hatchery and species effects were evident, and future observational research on BGD must account for these factors in their design and analysis.

Factors associated with the incidence of bacterial gill disease in salmonid lots reared in Ontario, Canada government hatcheries.

Bacterial gill disease (BGD) (causative agent: Flavobacterium branchiophilum) has been a persistent problem in early-rearing salmonids in the Ontario Ministry of Natural Resources (OMNR) fish hatchery system. Retrospective epidemiological investigations of BGD diagnoses and treatments in OMNR fish hatcheries during the period 1991-2001 were conducted using University of Guelph Fish Health Laboratory and OMNR central office data. All investigations were conducted at the lot-level, which is the major within-hatchery-level of population aggregation. Survivorship of BGD diagnosis in early-rearing lots within seven individual hatcheries ranged from 84.2 to 100%; within individual species groups, survivorship was lowest (84.6%) in brook trout (Salvelinus fontinalis) lots. Annual risk percentages (cumulative incidence) for BGD diagnosis within hatchery and species groups varied considerably among years. Multivariable proportional hazards survival analysis indicated that the species brook trout, and the Spring (March-May) season, were significantly associated with treatment for BGD. Combined, these results emphasize the importance of hatchery, species, and time on the development of BGD. Future observational research on this disease must consider these factors in their design and analysis.