Chemical marking of European glass eels Anguilla anguilla with alizarin red S and in combination with strontium: in situ evaluation of short-term salinity effects on survival and efficient mass-marking.

This study presents a new chemical double-marking technique for European glass eels Anguilla anguilla by combing alizarin red S (ARS) and strontium chloride hexahydrate (Sr). Marked eels (double marked with ARS and Sr, but also single marked with ARS) were exposed in situ to brackish water (15 g l-1 artificial sea salt) for 14 days and did not exhibit increased mortalities compared with unmarked eels. Indeed, no mortality occurred in a marked group during the experiments. Moreover, an efficient mass-marking approach with low handling effort for both single ARS and double ARS-Sr techniques is described and was proven to be practicable for large-scale stocking programmes.