Search

This study assessed the effect of different dietary ingredients on the quantity and characteristics of faecal waste produced by rainbow trout (Oncorhynchus mykiss). Seven ingredients were tested: fish meal (FM), mussel meal (MM), poultry meal wet rendered (PM-W) or dry rendered (PM-D), insect meal (IM), single cell protein (SCP) and brewers grain protein (BGP). Eight experimental diets were formulated: a control diet (CON) being predominantly plant-based and seven test diets, which contained 70% of the CON diet and 30% of one of the test ingredients. Rainbow trout juveniles (65 g, 30 fish/tank) were fed the experimental diets at satiation for six weeks, in triplicate groups. Dry matter (DM) and nutrient digestibility of diets and the test ingredients were measured. Furthermore, to estimate the faecal characteristics, particle size distribution (PSD) and removal efficiency of the faecal waste were determined. Nutrient digestibility of diets and ingredients differed significantly. Growth did not differ between the experimental diets, but DM digestibility was affected by the diet. Diets affected the amount of faecal waste produced, its removal efficiency (%) and the amount of non-removed faeces (g DM/kg DM feed). The highest and lowest removal efficiency were observed at FM and BGP diets, respectively. Accordingly, FM diet resulted in the lowest (37 g DM/kg DM feed), while BGP diet resulted in the highest (125 g DM/kg DM feed) amount of non-removed faeces. Further, it was also observed that differences in faecal removal efficiency can compensate for the variation in the quantity of faecal waste produced. Consistent with the faecal removal efficiency data, faeces PSD was also influenced by diets. FM and MM diets resulted in faeces with lowest proportion of particles of

Aquaculture feed formulation is shifting from fish meal and fish oil toward other ingredients, such as plant-based ingredients, which lead to different levels and forms of minerals in diets. Dietary minerals are essential not only for growth, but also for acid-base balance and the homeostasis of fish body fluids. The gastrointestinal tract (GIT) is involved in the process of osmoregulation of salt and water during digestion, but this process can be hampered when the dietary electrolyte balance (dEB = Na - K + Cl, mEq/kg) is altered. To better understand the interaction between feeding and osmoregulation along the GIT, two diets with low (−100 mEq/kg) and high (+600 mEq/kg) dEB were designed. Freshwater rainbow trout (average initial weight, 306 g) were fed for 6 weeks. Faeces were collected during the last week for digestibility analyses. The final sampling took place over three days, with fish being sampled at 2 time points after feeding (3 and 7 h) and dissected to collect chyme from 4 GIT segments: stomach, proximal, middle and distal intestine. Chyme was analysed for dry matter, pH, osmolality, crude protein (CP) and mineral content. Yttrium oxide (Y2O3) was used as an inert marker to measure water fluxes, mineral fluxes, and nutrient digestibility from different gut segments. Both dEB and time after feeding altered (p < 0.05) chyme characteristics and water fluxes in the stomach and proximal intestine, but there was no interaction effect (p > 0.05). dEB also affected (p < 0.01) chyme pH and sodium (Na) fluxes in the stomach. Faecal digestibility of dry matter (DM) and CP was higher (p < 0.001) when fish were fed a high dEB diet compared to a low dEB diet. The opposite was observed for Ca availability, which was lower at the high dEB diet than at the low dEB diet (p < 0.001). In contrast to faecal digestibility, CP and mineral digestibility measured in the different gut segments were different predominantly in the middle and distal intestine. Our findings suggest that the GIT plays a role in regulating the alterations caused by the contrasting dEB, and that this regulation is stronger in the middle and distal intestine. Furthermore, this regulation affects nutrient and mineral digestibility in the middle and distal segments of the GIT.

Atlantic salmon were fed diets containing graded levels of EPA + DHA (1·0, 1·3, 1·6 and 3·5 % in the diet) and one diet with 1·3 % of EPA + DHA with reduced total fat content. Fish were reared in sea cages from about 275 g until harvest size (about 5 kg) and were subjected to delousing procedure (about 2·5 kg), with sampling pre-, 1 h and 24 h post-stress. Delousing stress affected plasma cortisol and hepatic mRNA expression of genes involved in oxidative stress and immune response, but with no dietary effects. Increasing EPA + DHA levels in the diet increased the trace mineral levels in plasma and liver during mechanical delousing stress period and whole body at harvest size. The liver Se, Zn, Fe, Cu, and Mn and plasma Se levels were increased in fish fed a diet high in EPA + DHA (3·5 %) upon delousing stress. Furthermore, increased dietary EPA + DHA caused a significant increase in mRNA expression of hepcidin antimicrobial peptide (HAMP), which is concurrent with downregulated transferrin receptor (TFR) expression levels. High dietary EPA + DHA also significantly increased the whole-body Zn, Se, and Mn levels at harvest size fish. Additionally, the plasma and whole-body Zn status increased, respectively, during stress and at harvest size in fish fed reduced-fat diet with less EPA + DHA. As the dietary upper limits of Zn and Se are legally added to the feeds and play important roles in maintaining fish health, knowledge on how the dietary fatty acid composition and lipid level affect body stores of these minerals is crucial for the aquaculture industry.

Dietary electrolyte balance is the equilibrium of monovalent cations and anions that influence the acid-base balance of the feed (dEB = Na + K − Cl, mEq kg−1). Dietary electrolytes/minerals can influence the physiological changes during smoltification in Atlantic salmon. In this context, we aimed to study if the dEB of the freshwater feeds can be used to pre-adapt the hypoosmotic functionality and the associated effects on mineral metabolism. The dEB of commercial freshwater Atlantic salmon feeds in Norway varied from −9 to 400 mEq kg−1 feed. Three experimental feeds were formulated to study incremental levels of dEB reflecting the low (L-dEB, −50 to 0), median (M-dEB, 200–250) and high (H-dEB, 350–400). Triplicate groups of Atlantic salmon parr (36 g) were fed one of the three feeds for 8 weeks in freshwater at 12 °C. The fish were transferred to full strength seawater in indoor tanks and fed a commercial diet for 6 weeks. Growth was not differentially affected by dEB levels, neither in the freshwater phase nor in the seawater. Plasma electrolytes (Na+ and Cl−) and gill mRNA expression of sodium potassium ATPase (NKA a1b, seawater isoform) were significantly lower in L-dEB fed fish. In the intestine, carbonate precipitates 24 h after seawater transfer was higher in fish fed both L-dEB and H-dEB feeds compared to the M-dEB fed fish. Whole body and plasma mineral levels were significantly affected by dEB levels in freshwater feeds. Interestingly, the carryover effect of dEB in freshwater feeds was significant after 6 weeks in seawater for plasma and whole-body Zn status, with the H-dEB fed fish showing significantly increased body Zn status compared to L-dEB and M-dEB fed fish. The study revealed that mineral metabolism and intestinal response to seawater transfer can be pre-adapted by modulating the electrolyte and/or mineral balance in freshwater feeds in Atlantic salmon. Further, dEB did not affect long term development of cataract or vertebral deformities.

The hypothesis of the present study was that increased growth in spring, stimulated by increasing temperature and daylength, leads to oxidative stress in Atlantic salmon with accumulation of oxidation products in the tissues and increased utilization of antioxidants. The drop in fillet pigmentation and astaxanthin, often observed in spring by the industry, could be explained by oxidative stress. Furthermore, oxidative stress may cause production related diseases such as development of cataracts and melanin spots in the fillet. We sampled Atlantic salmon from two cages in a commercial scale experiment in Northern Norway (67°N), every month from April until August and then every second month until December (510 ± 160–3060 ± 510 g, mean weight ± std). The specific growth rate (SGR) increased with increasing temperature until midsummer and decreased thereafter. We found that vitamin E in the fillet and vitamin C in the liver were depleted in the spring and were restored in the autumn, even though the dietary concentrations were stable. Astaxanthin concentration in the muscle was constant during the spring and summer and increased in the autumn, concomitant with an increase in astaxanthin supplementation. Cataract increased from zero in May until July, when 90% of the fish were affected. The glutathione based redox-potential in the lenses became more reduced from June, indicating a protective mechanism against oxidative stress and cataract. The number of fish with melanin spots was high in June and decreased in August and October, but the size and intensity of the remaining spots increased in the same period. The change in vitamin C and E concentrations, cataract and glutathione metabolism during spring and early summer, indicate that the fish became oxidized in this period, while malon-di-aldehyde (MDA) and astaxanthin concentrations did not support the hypothesis. There are too few data to draw conclusions on possible effects of oxidative stress on melanin spots.