Growth, feed utilization, health and biometric parameters in Atlantic salmon (Salmo salar L.) - Influence of dietary protein-to-lipid ratio and body fat status

Field: Agricultural and Biological Sciences (Life Sciences)

Today, commercial diets for large farmed Atlantic salmon (Salmo salar L.) commonly contain 30-35 % protein and 35-39 % lipids, i.e. a ratio of protein-to-lipids below 1. Such energy dense diets have generally been shown to improve feed utilization and growth. However, reducing the dietary protein-to-lipid ratio may lead to increased deposition of fat in the muscle and visceral cavity. There is evidence that high levels of body lipids may reduce feed intake and growth in salmonids, and is often referred to as lipostatic regulation. Thus, there is a risk of lowered growth prior to and during periods with increased feed intake and high fat accumulation when feeding high-fat diets. The present thesis test the hypothesis that increased dietary protein-to-lipid ratio, and the possible involvement of lipostatic regulation on body fat levels can be utilized to significantly improve fitness-related traits including growth, survival and nutrient deposition of farmed Atlantic salmon.\n \nPaper I describes the effects of isoenergetic diets with different protein-to-lipid (P/L) ratio on growth, feed intake, feed conversion, biometrics, nutrient retention and deposition in S1 Atlantic salmon post-smolt. The study was conducted during the early seawater phase from April to September. Significantly lower muscle fat, whole body lipid, and energy level was observed in the post-smolt fed high compared to low P/L ratio in July, approximately three months after the trial started. Reduced level of muscle fat/body fat in July significantly improved feed intake, growth and weight gain compared to fish fed low P/L ratio from July to September. The high P/L ratio group had also a significantly lower feed conversion ratio based on gutted weight (FCRg). In line with this, the visceral somatic index (VSI) of the group fed the high dietary P/L ratio was relatively stable during the experiment, whereas the VSI of the group fed the low dietary P/L ratio increased gradually, resulting in significantly higher VSI at the end of the trial. The increased protein content in the high P/L ratio diets was efficiently utilized for growth and weight gain, assessed by nutrient retention, particularly in the last period of the trial. \n\nIn Paper II, low and high P/L ratios and restricted ration (~ 50%) of the high P/L diet were used to alter the lipid deposition prior to the autumn in large salmon. In this study, a clear treatment effect on body fat was observed after three months (May - August). The salmon fed low P/L ratio had higher fat content than those fed high P/L ratio, both in muscle (16.4 vs. 13.2 %) and viscera (39 vs. 29 %). Restricting the ration of the high P/L diet to 50 % further reduced the fat content to 11.3 % in muscle and 27 % in the viscera. Tagged individuals from the groups with different lipid content were restocked and mixed in the same pens, and then fed the same diet for seven months (August – March). As in Paper I, reducing the level of muscle fat prior to autumn significantly increased growth and weight gain from August to October. In other words, the weight gain was the highest for the restricted group, intermediate for the high P/L ratio and the lowest for the low P/L ratio group. In October, after two months of feeding a common diet, the muscle fat content was similar in fish from all three groups, whereas the differences in visceral fat content disappeared after four months (December). Although the differences in body weight, length and lipid content between the groups had been offset, the high P/L ratio and restricted group showed a significantly increased growth compared to the low P/L-ratio group in the latter stages of the trial (December – March), resulting in an overall weight gain difference of up to 1 kg. The high P/L ratio group had significantly higher final body weight, whereas the restricted group ended up with a numerically higher final body weight than in the low P/L ratio group. In addition, the variation in body weight and shape was significantly higher in the low P/L ratio group. \n\nThe results from Paper I and II demonstrate that in early autumn, the salmon is able to replenish lipid stores rapidly after dietary lipid restriction and that energy intake and storage is of high priority. In Paper I, unlike Paper II, low and high P/L ratio were fed throughout the trial and it is therefore not possible to isolate the direct effect of diet on growth from the indirect effect caused by different body fat accumulation. Hence, paper I may also indicate that dietary P/L-ratio of 1.12 (DP/DE of 15.2 g MJ kg-1) was below the P/L ratio for optimal growth during the early seawater phase for S1 salmon. This result is in line with previous studies, although now verified using isoenergetic diets under ambient environmental conditions.\n\nPaper III describes the effects of increased dietary P/L ratio for S0 salmon on mortality rates, biometric and quality related parameters during the entire grow-out period in sea, within the SAV3 endemic zone. The low P/L group was fed a typical standard diet with 35% protein and 35% fat (P/L: 1), versus the high P/L group that was fed a diet with 47% protein and 24% fat content (P/L: 2). During the first summer at sea, a co-infection of SAV3 and PRV was detected and a natural PD outbreak was observed. The increased dietary P/L ratio improved survival during the natural outbreak of PD. In addition to diet, body weight and delousing mortality (induced stress) prior to the PD outbreak were also found to contribute significantly to explain the observed variation in PD associated mortality. The high P/L group had a mean mortality rate of 1.9 %, whereas the low P/L group had a mean mortality rate of 3.7 %. Subsequent to the PD outbreak, a large amount of fish failed to grow and caused an accumulation of runts (severely thin diseased fish). At the end of the trial, a significantly lower amount of runt fish was detected among fish fed high P/L ratio (12 vs. 21%) and among large compared to small body weight groups (11 vs. 20%) prior to PD.\n\nIn Paper IV, an event of sudden mortality of large (2.5 kg) seemingly healthy farmed salmon during the winter period in northern Norway is reported. The experimental fish were reared in four net-pens and two dietary treatments were established; a high or low P/L ratio diets. An increased mortality (of 6 and 10%) was only observed within the two net-pens receiving the high P/L ratio experimental diets, following an abrupt reduction in dietary P/L ratio (increase in dietary fat level) six weeks earlier. The moribund/dying fish had significantly higher lipid content in the liver, altered hepatic fatty acid composition, and increased levels of ALT and AP in the blood plasma compared to non-dying fish, indicating impaired hepatic function. A possible hypothesis involving reduced recruitment of fat cells in high P/L-salmon is presented.\n\nTaken together, the results from this thesis show that alterations in dietary protein-to-lipid ratio have profound potential effects on growth, lipid deposition, nutrient retention and health of farmed Atlantic salmon. The results obtained during this work related to fat deposition and subsequent growth may be crucial knowledge when developing new dietary concepts in semi-closed and closed RAS production units, where water temperature and photoperiod can be manipulated.