Development and detection of phosphorus deficiency in Atlantic salmon, Salmo salar L., parr and post-smolts

A pathological condition was induced in Atlantic salmon, Salmo salar L., by feeding a diet severely deficient in phosphorus. Parallel results were obtained in two consecutive experiments, one in fresh water and one in sea water. The deficiency condition was first manifest as a reduction in whole-body calcium (Ca) and phosphorus (P) levels and the development of abnormally soft and malformed bones. In later stages, growth was severely impaired and there was elevated mortality compared with fish fed a diet with sufficient phosphorus. Two semipurified casein-based test diets were used in the experiments. The low-P diet contained only the P from casein, 3.5 g kg(-1) or 0.35%, whereas the reference diet was supplemented with P and contained 9.0 g kg(-1) or 0.9%. The diets were fed to salmon parr in fresh water (initial weight 4.7 g) and to post-smolts in sea water (initial weight 113 g) for 12 and 15 weeks, respectively. In both experiments, the fish grew adequately for a prolonged period, whereas the whole-body Ca and P levels declined almost immediately. Within a few weeks, the whole-body P content was reduced to approximate to 65% of the initial value, and Ca to 40%. The reductions in mineral content were most severe in the mineral-rich tissues, bones and scales. On gross examination, all bony structures were abnormally soft. The ribs were wrinkly, and the spine displayed scoliotic changes. These changes were correlated to the reduction in bone mineralization, and were manifest at the time when reductions in growth rate could be noted. After the deficient fish were fed adequate levels of P for 9 weeks, their whole-body mineral levels returned to normal. The bone lesions did not heal completely during the same period of time.