Feasibility of using rare earth elements (REEs) to mark and identify escaped farmed Atlantic salmon Salmo salar L.

A series of rare earth elements (REEs) were evaluated as potential markers in scales of Atlantic salmon 1(+) and 0(+) smolt by addition of these elements to the feed. Chlorides of 5 REEs were tested, alone or in combination: lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd) and dysprosium (Dy). The labelled-diets contained 125 or 250 mg REE kg(-1) feed and were administered for up to 10 weeks. Fish scales were collected and analysed by inductively coupled plasma-mass spectrometry (ICP-MS) before start, 5 and 10 weeks after labelling started and finally 2 or 4 months after marking. The results demonstrated that the five tested elements were clearly incorporated into the scales of the fish fed the supplemented diets. Uptake and concentrations increased gradually during the feeding period, and no principal differences were found between the two smolt types. Combining 2 REEs in the same feed did not affect the incorporation of either of them. After a 'dilution'-period of 2-4 months the concentrations of all markers (except La) in the scales of the treated-fish were still significantly higher than those of the untreated-fish. The background concentrations of the tested markers and some other elements were also measured and found to give interesting information.