The development and implementation of novel production methods and technology, and treatments such as mechanical sea lice treatments, requires robust fish that can cope with current and future rearing technologies and changing environments. As of today, it is not known whether salmon that perform well in the traditional production systems are also those that will thrive best under novel conditions. To approach this question, the current project has studied how salmon from the same families perform under either normal or more stressful rearing conditions in both freshwater and seawater. Monitoring of environment, welfare, stress physiology and performance, as well as underlying genetic traits, have been employed to facilitate the identification of biological mechanisms underlying variable ability to adapt to new, potentially stressful environments.
The first year a pilot study comparing two family groups under hypercapnia or hypoxia, against standard control, showed clear negative effect on growth of especially the hypercapnia treatment, and analysis of blood plasma and brain samples showed that exposure to hypercapnic water increased baseline plasma cortisol, compromised stress responsiveness of brainstem serotonergic activity and suppressed growth. However, no effects on morphological OWIs were detected.
The main experiment with 50 salmon families (90 fish/family) began summer 2018. Hypercapnia was chosen as a stressor in the freshwater stage, and reduced access to surface as sea water stressor. The fish were transferred to sea in November 2018 and slaughtered in February 2020. The fishes were measured and weighed 4 times both in the fresh water and sea water period. At slaughter we took photo of all fish and tissue samples for DNA analysis. There were clear treatment effects both in fresh and sea water and different responses between families indicated heritable variability in stress tolerance. However, heritability calculations og genetic parameters for weight and survival showed small and insignificant GxE interactions, and very little rearranging of the families showing that stress tolerance had low heritability. This is an important result for the breeding companies, that confirms that earlier selections of breeders had been correct also for A. salmon selected from stressful environments. From September 2019 all fish will be kept in common garden, and the sea water stressed families showed some compensatory growth, but were still significantly smaller at slaughter.
We have also studied if startle response from an acute stressor and cortisol accumulated in fish scales as an indicator of cumulative release of cortisol can be used as welfare indicators. The startle response showed to be very fast for the first responders (<0.1s). We found no effect on startle response of treatment, but significant individual variation within groups. Therefore it was challenging to measure accurately and interpret, and conclude that startle response was not useful as an operational welfare indicator. We found no difference in scale cortisol level between different treatments, or between normal and poor performing fish in the main experiment, but scale sample from large fish showed on average more than 150 times higher cortisol concentration in the scales compared to scales from smolt. We therefore need more studies to understand accumulation of cortisol in salmon scales and how it is related to stress level.
There are indications that a genetically determined association between stress responsiveness and heart failure underlies the recent increase in acute mortality of large pre-harvest fish during stress exposure (e.g. delousing).
The third WP in the project had the main aim o study the genetic basis for acute mortality during de-lousing. For this, five different salmon farms with different genetic strains and lines were visited at a day of delousing. From all farms, 20 deceased individuals, and 20 surviving individuals of similar physical appearance were sampled for DNA extraction and apparent indicators of acute heart failure, i.e. lack of heart beat in incapacitated fish. To identify genetic variation that might underlie acute non-specific mortality, a genome wide association study (GWAS) was performed in collaboration with Benchmark Genetics Norway. However, it was too few individuals to inform on genetic variants (e.g. QTLs) associated with cardiac arrest and mortality, but it will allow for analysis of allele frequencies between mortalities and controls for candidate genes (e.g. Linked to heart disease in humans).
At all samplings several morphometric measurements of the heart were performed and linked to mortality risk. These results are not yet published except for data from one sampling where there was a CMS outbreak allowing for specifically linking heart morphology to CMS mortality.
Prosjektet har vist at kronisk stress påvirker alle familier negativt og hyperkapnia kan føre til at fisken ikke mestrer akutt tilleggsstress. Arvbarhetsberegninger viste at det ble lite rerangering av familiene under ulike miljø og at stresstoleranse var lite arvbart. Dette var et viktig resultat for avlsselskapene som bekrefter at tidliger utvalg av avlsfisk har vært riktige.
Morfologiske og molekylære indikatorer for dødelighetsrisiko identifisert i dette prosjektet er lovende indikatorer for bruk i fremtidige risikovurderinger fiskehelsepersonell av planlagte trengings- og avlusningsoperasjoner.
Prosjektet har også generert viktige data og hypoteser for andre nystartede prosjekter som skal belyse hjertehelse og smoltkvalitet.
Prosjektdeltakerne har fått ny kunnskap innen fiskens nevrobiologi, stressfysiologi og genregulering, betydningen av ulike nivå velferdsindikatorer, , kortisol i fiskeskjell, hjertemorfologi og -genomikk, individuelle vekstmønster.
The rapid development of salmon aquaculture incurs renewed focus on production biology and fish welfare. E.g. mortalities caused by delousing procedures has made a solid leap with the transition from traditional delousing in sea cages into treatment in system which relies on crowding and pumping the fish. This, together with the development and implementation of novel production methods such as recirculating systems, rearing at exposed localities, and alternative cage designs, requires robust fish that are able to cope with current and future rearing technologies and changing environments. As of today it is not known whether lines and families that perform well in the traditional production systems are also those that will thrive best under novel conditions. To approach this question the current project will study how an agnate pedigreed family material is responding when reared under different conditions in both freshwater (normal, repeated acute stress, hypercapnia) and saltwater (traditional vs submersible cages). Close monitoring of behavioural and physiological phenotypes as well as underlying genetic traits will be employed to facilitate the identification of biological mechanisms underlying variable ability to adapt to new, potentially stressful environments. Previously established models and novel welfare indicators for animal welfare monitoring will be further developed and verified by neurobiological studies, in order to support the assessment of stress coping ability and ascertain fish welfare under novel conditions. Moreover, there are indications that a genetically determined association between stress responsiveness and heart failure underlies the recent increase in acute mortality of large pre-harvest fish during stress exposure (e.g. delousing). A genome wide association analysis together with physiological studies will be performed in an attempt to explain the mechanism behind such hitherto unexplained acute mortality and production loss.