Selection for improved feed efficiency in Atlantic salmon

Feed account for around 50% of the total cost in production of farmed Atlantic salmon. Improved feed efficiency ratio (FER, wet gain/feed intake) will reduce feed costs, increase resource efficiency of feed ingredients and reduce nutrient effluent to the environment. Direct selection for improved FER requires either individual records for growth rate and feed intake on large number of fish for which there is no technology available, or average growth rate and feed intake records of many families reared in separate tanks or cages which is very costly to obtain. Indirect selection for improved FER may be possible through selection for traits that are favourably genetic correlated to FER, e.g. increased growth rate and reduced body fat deposition, or traits in the energy budget like increased digestibility of the main nutrients (protein, fat and energy) in the feed. However, no estimate is available for the effect of a reduced body fat on FER or of genetic variation in digestibility of the main nutrients. Unfortunately, the most important experiment in the entire project from which we should obtain an estimate of the latter had to be terminated due to the development of cataract on the fish selected for this purpose. In this project we addressed some of these topics that can be summarized in seven points as follows: (1) The daily ?not eaten? feed pellets given into tanks with no fish can be recorded with high accuracy by collecting and counting their number on a grate outside each of the tank, and that this number of pellets provide a more accurate estimate of the ?not eaten? feed than the dry matter of these pellets. (2) In an experimental set-up with single fish in separate tanks the fish used many days to be adjusted to the tank environment and grew slower as compared to the mean growth rate of several other fish in a common tank. Negative correlation between growth rate of single fish in separate tanks in a period as compared to when the same fish in a following up period were reared together in the same tank. This may be caused by compensatory growth of the fish in the following up period and should have been followed up in an additional growth period in which the effect of the compensatory growth was absent or minimal. (3) Fish fed a diet with relative high energy ratio between protein/fat (HP) showed favourable trait characteristics (growth, fillet yield, fillet fat, visceral fat) as compared to the fish fed a lower energy ratio between protein/fat (LP), and this strongly indicates that salmon need a feed with relatively high protein content to show its genetic potential for growth and with a favorable effects also on other traits. (4) We found a significant genotype by diet (HP vs. LP) interaction for growth. This may have a significant effect on the genetic gain for growth depending on the protein/fat ratio of the diet given to the breeding nucleus fish today as compared to in the future. What is the optimal protein/fat ratio of the diet will be determined by the relative price of protein and fat in the feed and the fish genetic potential for growth and body fat deposition. (5) Energy in the fillet accounts for only 60 % of the energy in the whole body, Energy in the whole body must therefore be taken into account when selecting for improved feed efficiency. (6) The estimated heritability for the digestibility coefficients of fat in the feed was 0.13 ± 0.04, while those for protein and energy were not significantly different from zero. As the digestibility coefficients of the feed nutrients may be affected by the feed intake, it remains to be answered if selection for improved digestibility of fat will result in a favorable correlated response in feed efficiency. (7) To detect a true significant difference in feed efficiency between two groups of fish with high and low genetic potential for body fat deposition, a minimum of 3 to 6 replicated tanks per experimental group is required, the lowest figure when a combined sib and within family selection is applied when establishing the two high and low body fat groups, and the highest figure when applying sib selection only.

Feed account for around 50% of the total cost in production of farmed Atlantic salmon. Due to lack of technology to obtain individual feed intake records on large numbers of fish, indirect selection through increased growth rate, reduced body fat and increased digestibility of fat in the feed are the options to obtained improved feed efficiency through selection. What is to be gained by reducing body fat remains to be answered. As energy in salmon fillet accounts for only 60% of the energy in the whole body, energy in the whole body must be considered when selecting for improved feed efficiency. The daily not-eaten feed given to single fish in separate tanks can be recorded by counting the pellets on a grate outside the tanks. However, single fish in tanks used many days to adjust to the tank environment and grew slower compared to several fish in a tank. Fish fed high protein feed showed favourable trait characteristics compared to fish that received low protein feed.

In production of Atlantic salmon feed is the most important cost item with 50% of the total cost. Improved feed efficiency (gain/feed) will reduce feed costs, increase resource efficiency of feed ingredients and reduce nutrient effluent to the environment. This is of great importance for the future growth and sustainability of Atlantic salmon aquaculture, since marine sources of protein in the salmon feed represent a limited resource. Direct selection for improved feed efficiency is not possible due to the lack of technology to obtain individual feed intake records for a large number of fish. Today genetic improvement of feed efficiency is obtained through selection for traits (e.g. growth rate, filet fat) correlated to feed efficiency. However, results from livestock species indicate that such a strategy utilize a relatively small fraction of the genetic variation in feed efficiency. This project will investigate to what degree feed efficiency can be improved through indirect selection for traits possibly correlated to feed efficiency (growth, body fat, digestibility of nutrients, energy loss during feed deprivation). We will also study the effect of two diets with different protein/energy ratios on growth and body composition on feed efficiency. In addition, we will investigate if individual fish can grow well in separate small tanks (500 L) and if we can obtain reliable feed intake records for these animals; and if so, conduct such a study with a limited number of fish to obtain a first gross estimate of the magnitude of genetic variation in feed efficiency. This could facilitate obtaining individual feed intake and thus feed efficiency records for a large numbers of fish, which not only could facilitate direct selection for improved feed efficiency, but also pave the way for marker-assisted and genomic selection for increased feed efficiency.