The fish farm of origin is assigned by the element profile of Atlantic salmon (Salmo salar L.) scales in a simulated escape event

The Norwegian aquaculture management authorities require that the sea farm of origin for escaped salmon can be identified. As a contribution to develop a reliable and feasible method for tracing escaped salmon to their sea cage of origin the proof-of-concept of the case-based red alert method 'FarmSalmTrack' is demonstrated through a simulated escape event. The 'FarmSalmTrack' method uses the trace element distribution in the scale hydroxyapatite, HAP-layer, to discriminate between groups of salmon. The case-based approach can primarily be used for larger escapes and preferably where the escaped fish are caught relatively soon after the time of escape. The method is dependent on baseline data being available from all sea farm cages potentially responsible for the escape. Classification of the escapees towards the baseline material collected from nearby sea cages holding salmon of same age and size is done in a three-step process; (1) analysis of the Sr/Ba ratio which reveals if seawater or salt are added to the hatchery tanks, (2) analysis of element profiles in the juvenile section of the scale and (3) analysis of trace element profile of the scale HAP-layer formed in seawater. Step (1) reduced the number of suspected sea cages by one half, step (2) identified the escapees' hatchery of origin and step (3) identified the sea cage of origin. The classification was based on the concentration of the elements B, Ba, Li, Mg, Mn, Na, S, Sr, U and Zn along selected sclerites. Barium was the most important element for correct classification using the elemental profile of the juvenile section of the scale, while Zn showed the strongest influence on the classification using the seawater section of the scale. Improved understanding of the incorporation of non-essential life elements and essential life elements into the bio-mineralised structures of the fish scale HAP-layer would be highly beneficial for further development of the 'FarmSalmTrack-method'.