Preliminary study for phenotypic description of vertebral abnormalities in triploid trout subjected to prolonged deficiency in phosphorus

Summary In trout farmed in freshwater, the use of low phosphorus diet and other concurrent changes in food formulations could affect the normal growth of the skeleton and enhance the occurrence of vertebral abnormalities. Hence, it is important to refine diagnostic of malformations to quickly detect signs of P deficiency during production. Two practical diets consisting of P‐deficient (0.5% total P) and P‐sufficient (0.9% total P) were formulated. Experiments were conducted on all‐female juvenile triploid rainbow trout (initial mass ~60 g) until fish fed continuously with P‐sufficient diet reached <850 g (241 days; 12°C; 18:6 D:L). The choice of using practical diets as well as triploid trout were made in order to adapt to the context of production. Early P‐deficient and P‐sufficient trout were fed continuously (starting at day 0 for 241 days) with the respective diets while late P‐deficient trout only started feeding on P‐deficient diet at day 134 of the experiment. Growth performance were used to assess P deficiency and induced vertebral abnormalities were count and described using the classification suggested by Witten et al. ([Witten, P. E., 2009]) for Atlantic salmon. Our results showed that P deficient diet impaired vertebral mineralization and growth performance. The P deficient vertebrae showed increasing evidence with time of compression and reduction of intervertebral spaces (vertebrae showing pronounced biconcave end plates ‐ type 11 have evolved into platyspondyly; type 1, 2, 3 and 4 ). As P deficiency seems to affect all vertebrae along the vertebral column, examination of the caudal region alone could enable to increase the resolution of x‐rays and minimize time‐consuming efforts for analyses. The dimensions of rearing tanks might be responsible of high frequency of post‐cranial vertebrae precluding conclusion on the intensity of the effect of P deficiency.