The hidden cost of illicit trade: Evaluating the physiological health of smuggled European glass eels (Anguilla anguilla).

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European eel is considered a "critically endangered" species due to its population decline (c.a. 98%) in all European waters, primarily because human activities. The eel life cycle is very complex as it includes long migrations, and although aquaculture can help in repopulation, at this time artificially raised eel larvae have little survival. Therefore, to reverse this situation, in 2007 the EU adopted the Eel Regulation, starting a series of plans to improve the conservation and the recovery of the eel population, which included a ban on importing and exporting eels in all EU countries. However, an illegal global market and food fraud developed, turning Europe into the source of the international illegal eel trade. This study reports the evaluation of the welfare consequences of the illegal transport of European glass eels using their epithelial mucus, one of the most promising tools for studying endangered fish species. We also aimed to determine the recovery time required in controlled conditions before their release to the Ebro River. The results showed that high metabolite and cortisol levels were observed in mucus. In addition, the defensive capacity against possible bacterial infections was also affected due to the transport conditions as bacterial co-culture with mucus analysis indicated. Regarding the recovery time, we analysed the same parameters as for the initial transport point resulting in a stabilisation of stress parameters and an increased defensive capacity from day 3 to the final release, but the best results were observed after 2months under optimal conditions. These findings suggest that transport stress is critical for the survival of glass eels, and the stress and defensive capacities analysis suggests the need for a recovery period prior to release. This approach represents a valuable contribution both for species subject to illegal trade and for the conservation of endangered species.