Skeletal anomalies and shape variation in diploid and triploid Atlantic salmon (Salmo salar L.) fed different diets

Abstract

Usage of triploid Atlantic salmon (Salmo salar L.) have been of interest for salmon farmers to mitigate interbreeding between farmed and wild population, and to prevent early sexual maturation. Triploids tend to be more prone to develop skeletal deformities than diploids. This may result from inadequate rearing temperatures and/or diet formulation (i.e. low dietary phosphorus). The main objective of this study was to examine for differences in the occurrence of skeletal anomalies between diploid (2n) and triploid (3n) Atlantic salmon (Salmo salar L.) smolt fed a commercial high-protein phosphorus-rich fishmeal-based diet (56-60% protein; ca. 18g phosphorus kg-1; STD) and an experimental diet where 45% of fishmeal was replaced with hydrolyzed proteins (EXP). Shape analysis was applied to see if any group differed morphological relative to the control group (undeformed 2n STD). Diploids and triploids were divided into twelve tanks (initially 3000 fish per tank; tank biomass ca. 620 g) and were fed either STD or EXP diet (3x2n STD, 3x2n EXP, 3x3n STD and 3x3n EXP) from start-feeding until parr-smolt transformation. At the end of the feeding experiment, the fish were euthanized with an overdosed of anaesthetic (Benzocaine, 120 mg L1) and then stored frozen (-20°C). A total of 594 (2n STD, 123; 2n EXP, 171; 3n STD, 138; 3n EXP, 162) post-smolt were thawed and stretched before being measured, visual inspected and x-rayed. Five fish were placed on a digital plate at a time for each picture. ImageJ was used to analyze x-ray pictures for skeletal anomalies and to plot xy-coordinates for shape analysis. Shape analysis was carried out in the statistical software R with the package “geomorph”. Both diploids and triploids on the EXP diet had a significant higher incident of spinal deformities (diploid, 11.77 ± 4.22; triploid, 24.58 ± 0.99) compared to the groups on STD diet (diploid, 5.83 ± 1.01; triploid, 22.12 ± 2.47). Triploids had slightly but significantly fewer vertebrae (STD diet, 57.82 ± 0.45; EXP diet 57.78 ± 0.42) than diploids (STD diet, 58.03 ± 0.43; EXP diet, 58.00 ± 0.48). The cranial and caudal trunk (R1 and R2) was the most affected area with vertebral deformities amongst all groups. Five main axes of morphometry explained more than 5% of the shape variation, and these accounted for 73.3% of the total variability among groups. Shape analysis did not reveal any major shape differences between diploids and triploids in both diets, except for a slight bend in the cranial trunk and elongation of the caudal trunk region in triploids, that could be caused by a ploidy effect or underlying deformities in that region. Overall, the incidents of skeletal deformities were reduced compared to previous studies done on triploid Atlantic salmon (Diploids 20+%, Triploids 40+%), and could be a result of a combination of low rearing temperature and phosphorus-rich diets. The EXP diet with hydrolyzed protein will potentially increase the incidents of skeletal deformities compared to phosphorus-rich standard diet.