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Amoebic gill disease (AGD) is an increasing problem in Northern Europe and selective breeding has been shown as a good strategy to manage this disease in Tasmania. To explore the possibilities to perform selective breeding for AGD-resistance in Norwegian Atlantic salmon, two controlled challenge tests and one field (on-farm) test were performed with fish from two different breeding companies. For the field test, full-sibs of the same families from one of the controlled challenge tests were exposed to AGD naturally in a net-cage in the sea in Ireland. In all three experiments, two rounds of AGD infection were run, and all fish were assessed when the average gill score had progressed to an advanced threshold. Following assessment, all fish were bathed in freshwater. Heritability for resistance against both first and second AGD infection in the challenge tests and the field test ranged from 0.09 to 0.20. Estimated genetic correlation between gill score in the challenge test and field test resistance ranged from 0.02 to 0.34 and was not significantly different from 0. The genetic correlation between gill score in the first and second infection of the field test was 0.69 and ranged from 0.09 to 0.18 in the two challenge tests. These results indicate that AGD resistance in Norwegian Atlantic salmon can be improved by selective breeding. However, further work to optimize the controlled challenge test to better predict field test resistance is required.

Estimates of genetic parameters of susceptibility of Atlantic salmon to amoebic gill disease (AGD) were obtained from a bath challenge test with two repeated infections (1612 and 1582 fish, the offspring of 50 sires and 100 dams) and from a field test of their sibs (1156 fish) that were naturally infected and scored once for AGD. A third sibling group were reared in an AGD-free environment and their body weights recorded at harvest. In both challenge tests, susceptibility to AGD were measured using an adapted Taylor gill-score (0–5) where gill-score 3 was divided into three sub-classes 3A, 3B and 3C. In the field test, one gill arch of each animal was swabbed to quantify the amount of Paramoeba perurans by RT-qPCR, and a random sample of 126 of the fish were also analysed by RT-qPCR for Paranucleospora theridion and Branchiomonas cysticola. In the field test, body weights of the fish were recorded at time of gill-scoring and seven months later. In both tests, the distributions of gill-score was quite narrow (large proportion with gill-score 2 and 3A, and none with gill-score 4 and 5). In the field test, average body weight of fish with gill-score 1, 2, 3A and 3B was 17.6, 9.4, 17.9 and 22.2% lower, respectively than fish with gill-score 0. The genetic correlation between gill-score in the bath and the field test was close to zero. Therefore, the present bath challenge test for susceptibility to AGD cannot replace a field test in a selective breeding program. In the AGD-affected environment, the genetic correlation of gill-score with CT was −0.81 ± 0.16 and with body weight − 0.88 ± 0.09. These high genetic correlations indicate that CT and growth may be used as indirect trait measures of susceptibility to AGD. The high genetic correlation between body weights in the AGD-affected and the AGD-free environment (0.86 ± 0.05) indicate a true favourable genetic correlation between susceptibility to AGD and growth in Atlantic salmon. Consequently, selection for increased growth rate will result in a favourable genetic correlated response in susceptibility to AGD. The magnitude of these correlations need to be verified, in particular as the negative effect of decreasing CT-values of P. theridion on body weight was found to be larger than that of P. perurans and that growth of the fish in the AGD-free environment may be affected by other gill pathogens with negative effect of growth.

The objective of this study was to estimate the genetic correlations between vaccine-induced side effects, disease resistance and growth in Atlantic salmon. This was obtained using data from four independent sib groups from each of 144 to 279 families of the same year-class from SalmoBreed AS. Resistance to furunculosis was recorded as survival of unvaccinated (FUR, 279 families) and vaccinated (FUR-V, 150 families) presmolts in two separate challenge tests, while resistance to infectious salmon anemia (ISA) was recorded as survival in a third challenge test with unvaccinated presmolt (ISA, 279 families). Vaccine-induced side effects and growth (body weight at harvest) of vaccinated fish was evaluated on the fourth sample 12 months after sea transfer (SW12, 144 families). The genetic correlations of vaccine-induced side effects with resistance to furunculosis (of both vaccinated and unvaccinated fish) and ISA (unvaccinated fish) and with growth were not significantly different from zero. Therefore, selection for one of these traits is not expected to result in large unfavorable correlated response in the other traits. However, a low, albeit significant, unfavorable genetic correlation was found for growth with resistance to furunculosis in vaccinated fish (FUR-V), while that with unvaccinated fish (FUR) was not significantly different from zero. This may indicate that the best responders to vaccination may direct more resources into immunological reactions which, in accordance to resource reallocation theory, may leave fewer resources for growth.

In order to determine which salinity is needed to prevent desmoltification in Atlantic salmon, Salmo salar, changes in the hypoosmoregulatory capacity of postsmolts reared at different salinities were monitored. Smolting occurred under natural light and temperature conditions, and was completed by mid-June. Thereafter the smolts were divided into six groups reared at different salinities (0, 10, 15, 21, 26 and 31‰). The hypoosmoregulatory capacity was measured by 24 h seawater challenge tests. A decrease in hypoosmoregulatory capacity was evident in the groups kept at 0 and 10‰, and 3 weeks after smolting there was a significant difference between the 0‰ group and the groups kept on 15‰ or higher. Two weeks later both the 0‰ and the 10‰ groups had regained a high hypoosmoregulatory capacity, indicating that desmoltification did not result in a lasting state of low hypoosmoregulatory capacity, but rather appeared as a transient process lasting no more than 5–6 weeks.

Salmonid life histories are characterised by the ages at which the two primary developmental conversions, smolting and sexual maturation, are made. For Atlantic salmon, the critical seasons at which the physiological decisions are taken are thought to be soon after midsummer for the smolting conversion, and in mid-autumn for the maturity conversion. Maturity is inhibited if genetically determined performance thresholds are not met. Experiments were designed to test whether smolting was controlled in a similar sense. Sibling upper modal growth group fish (mean wet weight 10.34 ± 0.02 g) were divided into two replicated treatment groups on 11 February. One treatment group was fed a standard ration of 3% body weight/day until I June; the other group was fed this same ration but only during I week in four. All fishes had silvered by early May. On 11 May, when 96-h challenge tests in 35‰ artificial seawater were started, the mean weights of the unrestricted and restricted groups were 16.3 ± 1.7 and 9.4 ± 1.2 g, respectively. By 18 May all the unrestricted and 47.5% of the restricted groups survived the challenge. By 25 May. survival of the restricted group had increased to 65%. On 1 June only one fish from the unrestricted and none from the restricted group survived the transfer. Survival after transfer was related significantly to weight at transfer: larger smolts survived better than did smaller smolts (p < 0.001). Food-deprived fish survived as well as control fish of the same size. Food restriction limited weight growth to only 12.5% at maximum (compared to 85% among the controls) over the 3-month interval, and so had an indirect effect on survival of deprived fish, but did not switch off smolting for the majority of individuals. It is concluded that smolting is not controlled in a manner analogous to maturation, but is a negative developmental decision. This is consistent with the hypothesis proposed earlier that smolting represents a developmental pathway taken by fish that have failed to mature in that developmental cycle.

The results described in this article are based on historical data. A total of 57 945 Atlantic salmon (Salmo salar L.) smolts were tested in 12 cohabitation challenge tests with Aeromonas salmonicida, over a period of 5 years. Analyses of the 12 tests demonstrated that acute furunculosis with high mortality numbers was induced in all tests. However, the time from the first mortality among cohabitant fish to the first mortality among test fish varied by 5 days between different challenge tests, and the time from introduction of cohabitant fish to a level of 50% cumulative mortality among test fish varied by 19 days between the same tests. The reason for these variations in mortality pattern could be caused by variation in infectious load between test tanks but also due to the fact that the test groups compared were heterogenous both with respect to genetic origin and rearing conditions prior to challenge. As all families from one breeding farm were challenged in one large test tank, the applied challenge test was regarded as suitable to rank families of fish within one generation. The test system was regarded as less suitable to measure improvements in genetic resistance to furunculosis from one generation to the next due to a possible variation in challenge pressure between challenge tanks from different years.

The oyster pathogen Bonamia ostreae was introduced with importations in 1980 into beds of the European flat oyster, Ostrea edulis , in The Netherlands. Control measures were taken and yearly experiments were carried out to study their effects on the presence and activities of the oyster pathogen. The period of small-scale experiments (1981–1984) showed a sharp decline of bonamiasis, but the further results of large-scale experiments in 1985 and 1986 showed an asymptotic trend of decline. These results indicate the need for a longer period of control than was expected.

The development of hypoosmoregulatory ability assessed by monthly seawater challenge tests, changes in serum thyroxine levels, and the timing of downstream migratory behavior was studied during smoltification of yearling hatchery-reared (Hokkaido Fish Hatchery, Mori, Hokkaido) and wild (Oippe River, Aomori Prefecture) masu salmon, Oncorhynchus masou . The best hypoosmoregulatory performance was found in hatchery-reared smolts collected in May, and coincided with the peak of smoltification assessed by external appearance. Analysis of serum samples of wild (1982) masu salmon revealed that the levels of thyroxine were consistently high (7–12 ng/ml) during smoltification with a distinct surge (about 25 ng/ml) on the day of the new moon in late April. These results suggest that thyroid hormones may play an important role in the process of the parr-smolt transformation in the masu salmon. The onset of downstream migratory behavior in hatchery-reared masu salmon kept in an artificial stream was observed around the time of the new moon in May, 1982 and 1983. Moreover, the largest numbers migrated immediately after rainfall following the new moon. Similar observations were obtained from wild masu salmon in the Oippe River (1982). These results implicate the new moon and rainfall as important environmental factors which may initiate downstream migration in the masu salmon.