Background
Vaccination enhances survival in farmed rainbow trout (Oncorhynchus mykiss) exposed to certain pathogens, but vaccinated fish may develop mild-to-severe side-effects1. Indeed, industry experience testifies that growth may be impaired, resulting in up to 20% lower harvest mass. The growth impairments stem from an initial reduction in appetite, leaving less energy available for growth2. This might be mediated through cross talk between the endocrine systems regulating growth and immunity3. In addition, the reduced growth may result from increased metabolic demands from immune activation or adjustment of homeostatic imbalances. Vaccine-induced effects have primarily been studied in fresh water, while less is known about the effects in sea water, even though salmonids are often transferred to and reared in sea water after vaccination. This knowledge-gap warrants attention, especially considering the observed negative salinity-dependent effect on growth in trout that likely relates to effects on metabolism and appetite4. Thus, as both vaccination and seawater-acclimation may interfere with processes underlying growth, we hypothesized that their combination may lead to additive effects. We therefore analysed the effects of vaccination on growth in rainbow trout acclimated to fresh water or sea water, and compared the effects on metabolism, osmoregulation, and endocrine growth-regulation.
Materials and methods
Rainbow trout were either immunized with Alpha Ject 3000, or sham-injected with phosphate-buffered saline. The fish were fed daily until satiation with ≤2% of body weight. Starting 12 days post injection (dpi), salinity was gradually increased to 31 ppt over 10 days for half of the vaccinated and unvaccinated fish, while the other half remained in fresh water. During acclimation, feeding was limited to ≤1% of body weight, and then returned to initial levels until the experiment ended at 52 dpi. Length and weight were recorded at 0, 12 and 22 dpi. At 44-50 dpi, the metabolism and aerobic scope of the fish was examined by manually chasing the fish for 5 minutes to elicit a maximal metabolic response, before measuring standard metabolic rate after 48 hours of recovery. Fish were then euthanized, length and weight measured, and blood plasma acquired for measuring concentrations of growth hormone (GH) and insulin-like growth factor I (IGF-I). In addition, the kidney, gill, and intestine were sampled for analyses of Na+/K+-ATPase (NKA) activity.
Results and discussion
Size was initially uniform across treatment groups (weight and length for all fish [mean ± s.e.m.]: 32.8±0.8 g; 14.3±0.1 cm). Vaccinated fish grew less than sham-injected fish for two weeks after injection (Fig. 1A), which is in line with research showing that initial appetite inhibition can limit growth for two weeks after vaccination2. However, during the seawater-acclimation phase, the vaccinated fish grew faster than non-vaccinated fish (Fig. 1A), compensating for the initially reduced growth. When the fish were fully acclimated and their feeding rates increased, freshwater trout grew faster than seawater trout (Fig. 1A). This is in line with research showing a negative relationship between growth and salinity in rainbow trout4. During this phase, growth rates were numerically lowest in the vaccinated trout acclimated to sea water, although this was not significant.
Yet, there was no interaction effect of salinity and vaccination on growth rates when analysed across the entire experimental period (Fig. 1B). However, there was a strong trend for lower growth in sea water (p = 0.07), where vaccinated trout again had the numerically lowest growth rates (Fig. 1B). While this pattern was similar for both weight and length growth, the effect on length appeared to be more pronounced (data not shown). Over 50 days, this did not cause any differences in condition factor, but may indicate ongoing spinal deformation, which is a known risk of vaccination in Atlantic salmon5.
We did not find conclusive support for our hypothesis that metabolic differences could explain differences in growth. For example, aerobic scope was similar across treatment groups (Fig. 1C). However, fish in seawater had significantly lower standard metabolic rate than fish in freshwater, and it was numerically lowest in vaccinated seawater-acclimated trout (Fig. 1C). This could be associated with reduced growth rates in fish fed ad libitum6. The analyses of endocrine and osmoregulatory effects are ongoing and will be complemented later. In conclusion, we show that the growth of rainbow trout is compromised following vaccination with Alpha Ject 3000, and the effect was more pronounced in seawater where it appeared to persist for at least 50 days. This may partly relate to metabolic changes, and it remains to be seen if the differences in growth can be explained by endocrine and/or osmoregulatory changes.
References
1 Villumsen, K R, Koppang, E O and Raida, M K. 2015. Adverse and long-term protective effects following oil-adjuvanted vaccination against Aeromonas salmonicida in rainbow trout. Fish Shellfish Immunol. 42: 193-203.
2 Sørum, U and Damsgård, B. 2004. Effects of anaesthetization and vaccination on feed intake and growth in Atlantic salmon (Salmo salar L.). Aquac. 232: 333-341.
3 Alzaid, A, Castro, R, Wang, T, Secombes, C J, Boudinot, P, Macqueen, D J and Martin S A M. 2016. Cross talk between growth and immunity: Coupling the IGF axis to conserved cytokine pathways in rainbow trout. Endocrinol. 157(5): 1942-1955.
4 Morgan, J D and Iwama, G. K. 1991. Effects of salinity on growth, metabolism, and ion regulation in juvenile rainbow and steelhead trout (Oncorhynchus mykiss) and fall chinook salmon (Oncorhynchus tshawytscha). Can. J. Fish Aquat. Sci. 48: 2083-2094.
5 Berg, A, Rødseth, O M, Tangerås, A and Hansen, T. 2006. Time of vaccination influences development of adhesions, growth and spinal deformities in Atlantic salmon Salmo salar. Dis. Aquat. Org. 69: 239-248.
6 Auer, S K, Salin, K, Rudolf, A M, Anderson, G J, Metcalfe, N B. 2014. The optimal combination of standard metabolic rate and aerobic scope for somatic growth depends on food availability. Funct. Ecol. 29(4): 479-486.