Urea in feeds for sea water farmed Atlantic salmon: effect on growth, carcass quality and outbreaks of winter ulcer

The main objective of the present study was to evaluate whether dietary supplementation of urea might reduce the incidence of winter ulcer in sea water-farmed Atlantic salmon. Salmon destined to be S0 smolt were fed with a urea-supplemented diet (0 or 20 g kg⁻¹ urea) over an 8-week period prior to sea water transfer and were then fed supplemented diet (0, 5, 10 or 20 g kg⁻¹ urea) during the first and second winters in the sea. During the first winter positive relationships between dietary urea and plasma urea and between plasma urea and plasma osmolality were observed. Further, plasma osmolality displayed a negative relationship to mortality. Of the salmon that died during the first winter in the sea 90% had one or more skin ulcers. Both during the first and second winter there were fewer salmon with ulcer among fish fed with the diets supplemented with urea. Salmon fed with 20 g kg⁻¹ urea tended to have higher percentage water in the muscle. Mortality and incidence of salmon with ulcer seemed to relate to plasma osmolality amongst fish fed on diets that differed in levels of urea supplementation, suggesting that an osmotic imbalance may contribute to the development of winter ulcer in farmed salmon. Salmon fed with 20 g kg⁻¹ urea showed significantly greater body weight during the second winter in sea. Fish killed without prior starvation had significantly higher level of muscle urea in the 20 g kg⁻¹ urea group compared with fish fed with the unsupplemented diet. However, a 13-day starvation period reduced urea content in the muscle to the level of the control. No effects of dietary urea supplementation on the sensory quality of market size Atlantic salmon were observed.     

Effects of graded levels of medium chain triglycerides and cysteine on growth, digestive processes and nutrient utilization in sea water reared Atlantic salmon (Salmo salar, L.) under ad libitum feeding regime

Fifteen experimental diets with five levels of medium chain triglycerides (MCT) (0, 39, 78, 117 and 156 g kg^?1) and three levels of cysteine supplementations (0.0, 2.5 and 5.0 g kg^?1) were fed ad libitum to Atlantic salmon (mean weight at start 846 g) in sea water for 71 days. Nutrient digestibilities and protein retention improved significantly with increasing dietary MCT level following simple linear regression models. Positive effects were also observed on proteolytic enzyme activities in chyme of the pyloric region with peak value around 117 g kg^?1 MCT. However, strong inverse relationships were observed between MCT level and feed intake, growth, body lipid and energy retention, and these effects accelerated with increasing MCT level. Also chyme dry matter decreased with increasing MCT level which also seemed to stimulate pancreatic and bile secretion. MCT did not significantly affect condition factor, dress out percentage, relative liver size or pH in gastrointestinal contents. The only significant effects of dietary cysteine supplementation were increased enzyme activities in chyme of the pyloric region with peak values for the intermediate cysteine level. The results elucidate the importance of diet composition for digestive functions, production efficiency and composition quality.     

Potential of three new krill products for seabream larval production

The objective of this study was to evaluate three products derived from krill as sources of essential fatty acids, protein and, particularly, phospholipids in microdiets for larval gilthead seabream (Sparus aurata). Their effect on larval performance, biochemical composition and histological development was investigated. The addition of krill phospholipids, rich in highly unsaturated fatty acids, improved larval sea bream growth in terms of weight and length, enhanced hepatic utilization of dietary lipids and reduced the incidence of enterocyte injuries. These results confirm the higher nutritional value of marine phospholipids for the early development of marine fish larvae in comparison with soybean phospholipids.     

Absorption of glucose, amino acids, and dipeptides by the intestines of Atlantic salmon (Salmo salar L.)

Rates of glucose, amino acid and dipeptide absorption by the intestine of Atlantic salmon (Salmo salar L.) were measured in vitro at 10 °C as functions of concentration and region using intact tissues (everted sleeve method). Salmon (weight range 300–1300 g) fed a commercial, extruded salmon feed, were kept at 12–13 °C in freshwater. Maximum rates (V _max) of glucose and dipeptide transport were low compared to most amino acid V _max values. There was a declining proximal-to-distal gradient of absorption along the post-gastric intestinal tract. A saturable component of absorption was not evident for proline and glycyl-proline in the distal intestine, and glycyl-sarcosine in any region. `Apparent diffusion', which may include low affinity, high capacity carrier systems and carrier-independent influx, appears to account for the majority of total uptake at higher concentrations of amino acids and dipeptides. There was competition between the dipeptides for transporter sites in the pyloric ceca and mid intestine, suggesting a common carrier. There was also indication of hydrolysis of these dipeptides by brush border membrane enzymes.     

The effect of low inclusion levels of Antarctic krill (Euphausia superba) meal on growth performance,apparent digestibility and slaughter quality of Atlantic salmon (Salmo salar)

Two trials with Atlantic salmon (Salmo salar) were conducted to evaluate the potential of krill meal to improve feed intake. In the first experiment, after transfer to sea water, salmon smolts were fed diets added 75 or 150 g kg^?1 Antarctic krill meal in substitution for fish meal for 13 weeks. The apparent digestibility coefficient for crude protein and the majority of the amino acids was significantly lower in the feeds added krill meal (around 83.5%) than in the control diet (84.9%), whereas the digestibility of crude lipids, dry matter and energy was not significantly different among the three diets. Krill meal addition resulted in higher feed intake, which led to higher growth rates and final body weights. In the second experiment, large salmon were fed a diet containing 100 g kg^?1 krill meal for 6 weeks before slaughter. Their feed intake and growth performance were assessed, and fillet and visceral fat contents were measured. Salmon fed the 100 g kg^?1 krill meal diet tended to eat more, resulting in significantly increased growth rates, when compared to control fish. Fish fed krill meal also had a significantly lower condition factor.