Genetic improvement of survival in the seawater phase of Atlantic salmon

Mortality of farmed Atlantic salmon prior to slaughter continues to remain an industry challenge for production and growth in Norway. Approximately 12-14% are lost in the first few months after transfer to seawater. Although the reasons for these mortalities are likely many, sub optimal smolt production has been implicated. In SmoltFieldGen we focused on using different methods of recording smoltification parameters during smoltification and seawater transfer as well as non-invasive growth measurements at sea. The goal was to evaluate these recording methods to improve overall survival and robustness of farmed Atlantic salmon through selective breeding. Specifically, we followed 2000 salmon through (0+) smoltification, 2000 through (1+) smoltififcation and 5500 through seawater transfer and growth at sea. The three groups of salmon came from the same 50 breeding nucleus families MOWI Genetics AS and the genetic link among the animals within and between the groups were established through genotyping. Results: 1) The heritability of ratio of the seawater (SW) and the freshwater (FW) component analyzed by the SmoltVision kit provided by Pharmaq Analytiq lies between 5 and 21% for 0+ salmon and 6 and 20% for 1+ salmon. This shows a genetic potential to genetically improve smoltification status of the salmon. 2) The genetic correlation of the SW/FW ratio between the 0+ and 1+ groups was low and not significantly different from zero, suggesting that this may be due to the differences in the time the parameters were recorded and on two different age groups of salmon. 3) We evaluated pairing a diode frame with a PIT tag reader to non-invasively record growth of fish in a sea cage for eight months and found the diode frame to be highly accurate for estimating the mean growth rate of a group of fish, but less precise for estimating the growth rate of individual fish. This means that if the weight or length of individual fish is needed a few repeated records of each fish is needed. 4) For the fish reared in the sea cage the estimated heritability was 39 and 41% for length and weight, respectively using manual recording and showed high genetic correlation (>0.8) to the diode frame recordings, indicating potential for using diode frames to select for increased growth at sea without invoking stress on the fish. In final smoltification experimentwe collected non-lethal gill and blood biospsies in time series over a six weeks smoltification period and growth over a seven week period in seawater. We scored 18 different welfare indicator traits, blood plasma traits and sampled gut microbiota. There was no significant difference in the sea water growth or on farm welfare indicators between the sampled and controlled fish, indicating that with proper precautions it is possible to return sampled smolt to sea water and link the smoltifciation measurements to growth and survival at sea.

The gene expression of NKA1a freshwater, NKA1b seawater genes where significantly heritable, enabling breeding for optimal synchronised smolt. However, the genetic background is different between 0+ and 1+ smolts, meaning selection must be smolt type specific. In a controlled environment, fish measured for smoltification parameters did not significantly differ in growth or onfarm welfare indicators to control smolt. This makes it possible to link individual smoltification parameters to growth in sea water and help helping producers optimize smolt production. For the first time it is possible for the salmon industry to follow the growth of individual Atlantic salmon at sea though the linking diode frames with pit tag readers. This gives managers better information on the biomass in the pen and the ability to evaluate the effects of different interventions on individual growth of individual fish, ie smoltification parameters.

In farmed Atlantic salmon high survival and robustness in the seawater phase is of critical importance for good growth, good animal welfare and a good economic result. In today?s breeding programs genetic improvement for these traits is sought through selection for growth until harvest and survival in specific disease challenge tests. In this project using fish from 100 families we will record growth and total survival in the entire seawater period as well as specific causes of mortality, and discriminate between fish with optimal and sub-optimal growth. We will use existing and advanced disease diagnostic methods for assigning specific causes of mortality and obtained growth curves of individual through repeat recordings of their size. We will quantify the genetic variation of mortalities during the early period of sea transfer, and estimate the genetic correlation between early survival at sea and smoltification status of their sibs. In addition we will estimate the genetic variation in specific and total survival at sea, and the genetic correlation of survival at sea with survival from controlled challenge tests with specific diseases. Furthermore, based on the growth curves of individuals we will estimate the genetic variation in healthy and subclinical growth, and the genetic correlation of these traits with survival and smoltification status. A genome wide association study will be undertaken for the studied traits with genotypes generated from a 57K SNP array. Finally, we aim to link gut microbiota data to survival and growth of healthy and sub-clinical fish. The project will increase our limited knowledge on the genetic variation in survival at sea, and thus contribute to a more effective selection for increased survival in the seawater period.