such Norwegian studies on Salmo salar provided a dataset for the time period 1996 to 2011 on 25 118 release groups comprising 657 624 fish released and 3 989 recaptured. The overall risk ratio 26 (RR), calculated as the probability of being recaptured in the treated group divided by the 27 probability of being recaptured in the control group, was estimated to be 1.18 (95 % CI: 1.07-281.30). The effect varied strongly between groups, quantified by Higgins measure of heterogeneity 29 (I 2 = 40.1%). Over 70% of this heterogeneity could be explained by the release location, time 30 period and baseline survival. The most important predictor variable was baseline survival. In 31 groups with low recapture in the control group (low baseline survival), the effect of treatment 32 was high (RR = 1.7), while in groups with high recapture in the control group (high baseline 33 survival), there was no effect of treatment (RR ~ 1.00). The most prevalent parasite in the region 34 affected by the drugs administered was Lepeophtheirus salmonis. Hence, the meta-analysis 35 supports the hypothesis that anti-parasitic treatment protects S. salar smolts from L. salmonis 36 during outward migration. However, the effect of treatment was not consistent, but was evidently 37 strongly modulated by other risk factors. The results suggest that the population level effects of 38 parasites cannot be estimated independently of other factors affecting the marine survival of 39Salmo salar. 40

Spinal deformities in Atlantic salmon, Salmo salar L., have been described as a disease of multifactorial origin for which vaccines and time of vaccination have been suggested as risk factors. A vaccine efficacy trial where spinal deformity became evident was continued by the observational study reported here. In the preharvest part of the study 17 months post-sea transfer, there was a prevalence of 11.3% spinal deformity, with deformities present only in one vaccine group indicating a strong vaccine involvement. At slaughter, the prevalence of spinal deformities was 11.7%, and deformed fish had only 62% of normal slaughter weight. Visual analogue scales (VAS) were used for continuous recordings of vaccine-induced abdominal lesions and deformity. A logistic regression model associating presence of spinal deformity with markers of abdominal lesions was developed. The odds ratio for spinal deformity was 5.7 (95% CI: 3.4-9.4) for each unit increase in adhesion score (0-6) and 4.9 (2.9-3.4) for each unit increase in melanin on abdominal organs (0-3). Lesions in the dorsal caudal part of the abdomen gave an odds ratio for spinal deformity of 2.2.

The development of ectothermic embryos is strongly affected by incubation temperature, and thermal imprinting of body growth and muscle phenotype has been reported in various teleost fishes. The complex epigenetic regulation of muscle development in vertebrates involves DNA methylation of the myogenin promoter. Body growth is a heritable and highly variable trait among fish populations that allows for local adaptations, but also for selective breeding. Here we studied the epigenetic effects of embryonic temperature and genetic background on body growth, muscle cellularity and myogenin expression in farmed Atlantic salmon (Salmo salar). Eggs from salmon families with either high or low estimated breeding values for body growth, referred to as Fast and Slow genotypes, were incubated at 8°C or 4°C until the embryonic ‘eyed-stage’ followed by rearing at the production temperature of 8°C. Rearing temperature strongly affected the growth rates, and the 8°C fish were about twice as heavy as the 4°C fish in the order Fast8>Slow8>Fast4>Slow4 prior to seawater transfer. Fast8 was the largest fish also at harvest despite strong growth compensation in the low temperature groups. Larval myogenin expression was approximately 4–6 fold higher in the Fast8 group than in the other groups and was associated with relative low DNA methylation levels, but was positively correlated with the expression levels of the DNA methyltransferase genes dnmt1, dnmt3a and dnmt3b. Juvenile Fast8 fish displayed thicker white muscle fibres than Fast4 fish, while Slow 8 and Slow 4 showed no difference in muscle cellularity. The impact of genetic background on the thermal imprinting of body growth and muscle development in Atlantic salmon suggests that epigenetic variation might play a significant role in the local adaptation to fluctuating temperatures over short evolutionary time.

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Methods for investigating patterns of mortality and quantifying cause-specific mortality in Atlantic salmon Salmo salar farming were developed. The methods were further used to investigate mortality and patterns of mortality for the first 3 mo after sea transfer in the 2006 year-class autumn smolts (S0) of Norwegian farmed Atlantic salmon. In the study population, which consisted of 20 pens at 10 sites, cause-specific mortality was examined by 11 fish health professionals during 8 visits to each pen. Cause-specific mortality proportions were used to convert crude mortality into cause-specific mortality. Cumulative mortality in the study period was 2.1% in the study population compared with 3.7% for the 2006 year-class S0s in the national database. Of this cumulative mortality, 73 and 59% took place in 20% of the pens in the study and the reference population, respectively. Daily mortality rates in the study population showed a variation from 0 to 2376 per 100 000 fish where the majority of mortality was observed during disease outbreaks. All study pens had periods of low baseline mortality and some pens had no increased mortality during the study period. Of 2088 dead fish examined, 92% (1929 fish) were assigned a specific cause of death, and in 97% of these 1929 fish the investigators reported the given cause of death to be likely or very likely. Ulcers were the main cause of death, accounting for 43% of the assigned mortality, and infectious agents were involved in 64% of the total mortality. The study shows that probable causes of death can be established in Atlantic salmon farming and their contribution to total mortality measured.KEY WORDS: Salmo salar · Atlantic salmon · Mortality · Cause-specific mortality · Patterns of mortality Resale or republication not permitted without written consent of the publisherDis Aquat Org 81: [99][100][101][102][103][104][105][106][107] 2008 tions and inconsistencies in measures, making comparisons among companies, years and countries difficult. However, despite the inherent limitations, the loss at sea of farmed Atlantic salmon Salmo salar is substantial and represents a major economic challenge for the industry. A major part of the observed loss is caused by various diseases, representing a substantial portion of the cost of diseases in the salmon industry (Menzies et al. 2002, Brun et al. 2003, Olsen et al. 2007.While some data are available for general mortality, statistics on cause-specific mortality are limited, and the methodology for estimating cause-specific mortality and costs of specific diseases is not standardised. In Norway, cause-specific statistics are restricted to the official statistics of number of outbreaks of the major infectious diseases, mainly being notifiable diseases (Olsen et al. 2007) or national statistics on losses categorised in crude blocks: mortalities, fish condemned at slaughter, escapees, counting errors and other causes (Anonymous 2007a). Studies of specific diseases often report disease-associated mortality (Jarp et al. 199...