| Summary: | Skeletal anomalies are found in farmed Atlantic salmon with triploids generally showing higher prevalence than diploids.The use of high temperature regimes in the hatchery phase of Atlantic salmon to accelerate growth has been reported asa critical factor affecting normal skeletal development. One skeletal anomaly, lower jaw deformity (LJD) is a downwardcurvature of the lower jaw that has a negative impact on production efficiency through product downgrading, and animalwelfare via higher susceptibility to diseases and mortality. The causes of LJD are unknown, although it is more commonin triploid fish. LJD may be triggered by environmental, nutritional or genetic factors in association with physiologicalcharacteristics of triploids. In order to assess the effect of ploidy and temperature on the prevalence of skeletal anomalies and to understand molecularmechanisms underlying the onset of LJD, diploid and triploid Atlantic salmon embryos were collected from a commercialhatchery and held and reared separately for nine months. After incubation at 7.8 0.2C, the experiment was divided intotwo phases: early rearing and a temperature experiment. Early rearing was from hatch to 8g with diploid and triploidfish held under the same environmental conditions (at 8.4 0.2C to gradually reach 14.2 0.1C three weeks afterfirst feeding). The temperature experiment was from 8 to 60g with diploid and triploid fish exposed to two differenttemperature regimes: Standard 14C and High 18C. Fish were exposed to 24h photoperiod from first feeding onward andfed commercial diets. During early rearing, ploidy had no effect on mortality and growth. Opercular anomalies, predominantly shortness of theoperculum, were the most common (88.3 9.7% diploids and 83.3 7.6% triploids) with no significant effect of ploidy.A short lower jaw was the second most common anomaly (10.8 3.4% diploids and 11.7 7.5% triploids), again with noeffect of ploidy. LJD prevalence was low (1.7 2.4%) and was first observed at ~7.5g. During the temperature experiment, ploidy significantly affected mortality which was higher in triploids, but did notaffect growth. Temperature had no effect on mortality but significantly affected specific growth rate which was higher infish exposed to the higher temperature. In triploids, LJD increased from 1.7 2.4% (at 8g) up to a maximum of 10.7 2.5% (at60g) and short jaw decreased from 11.7 7.5% to 2.7 2.5% over the same period, irrespective of temperature.This confirmed that LJD is a ploidy-related anomaly and suggests a possible relationship between short jaw and LJDdevelopment in triploids. The prevalence of other abnormality categories was similar to that in the early rearing phase. Preliminary molecular studies, using qPCR of genes involved with bone and cartilage formation and transcriptome analysis,identified an association of LJD and changed expression of genes linked with cartilage formation.
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