Resorption of scales in Atlantic salmon (Salmo salar) during its anadromous migration: a quantitative study
Resorption processes in the scales of Atlantic salmon, during its anadromous migration are known since 1935, but have never been quantified. In fact, the ellipsoid form of the ascending salmon scale resorbs into an irregular highly deformed shape in spawning salmon. The aim of this study is to quantify the changes of both anterior and posterior scale areas using image analysis techniques. Salmon scales were sampled taking into account the sex and sea age of fish at two stages of migration, viz at the beginning of the upstream migration in rivers and at the spawning areas. Analyses confirmed that scales are highly resorbed during the anadromous migration and that the resorption is caused by the osteoclastic activity. Morphometric analyses showed that the scale posterior area eroded significantly more intensely than the anterior one. The reason behind the higher erosion in the posterior surface is probably the fact that this area is closer to the circulatory system. Whatever the sex, scales eroded because they ensure a source of minerals used during sexual maturation. However, scales of males eroded significantly more than those of females. Therefore, scale quantification could well be used to discriminate males from females especially after spawning. The time spent at sea seemed to play a minor role on scale modifications during the upstream migration in females. In the light of these quantifications, the use of eroded scales could constitute an accurate tool to improve the knowledge and the management of wild populations.