Mineralised vertebrate tissues provide a record of physiological events such as sexual maturation, growth rate, temperature, stress, and dietary factors. Typically, teeth, scales and otoliths are analysed; Different from bone these skeletal tissues are not remodelled and provide a lasting record 1. Scale reading, pattern analyses of scale ring (circuli) formation to trace the life history of wild Atlantic salmon (Salmo salar), is well established 2. Reduced dietary phosphorus intake reduces scale mineralisation in farmed salmon. 3, 4. This study explores if scale reading can provide current and historic information about growth and mineralisation of the endoskeleton, two important parameters related to dietary phosphorus intake. Vaccination, transport to sea and start of the experiment was recorded, thus creating a known life history record for each salmon. This allowed for experimental events to be matched to the reading of life events in scales. Dietary phosphorus was the single variable that was changed via three experimental diets. Scales were collected at 7, 11.5 and 15 months at sea, and stained for minerals with alizarin red S. Their circuli formation, resorption sites and mineralization patterns were analysed. Scales showed a line of disturbed ring formation and resorption, the latter increasing over time, reflecting seawater transfer (smoltification). Thereafter, scale formation was slower with one circuli in 16.4 days during winter months. During spring, the circuli formation increased to one ring in 6.6 days. Scale mineralisation stopped in salmon fed a low phosphorus diet but growth and circuli formation was undisturbed. Based on mineral staining, the level of dietary phosphorus deficiency can be read on scales, based on both positive and negative, and on differential staining of amorphous calcium phosphate and matured apatite. These observations corroborate independent bone formation and mineralisation mechanisms 3, and suggest that scales can be used for minimal-invasive monitoring of skeletal health of farmed Atlantic salmon.
Clercq et al. (Wed,) studied this question.